# Solar DC mini split— thoughts



## EbS-P (Jun 15, 2022)

This came up an another thread. Solar powered mini splits are available.  From my understanding they run on DC but can be grid tied, the mini split not the PV panels. 


peakbagger said:


> One option for AC that i was not aware or until recently is 48 volt minisplit that runs directly off solar panels (no batteries). It adjusts its load on the amount of generation from the panels. That would keep the AC loads from not needing to be shifted to the night period.  https://www.hotspotenergy.com/solar-air-conditioner/ The pricing is pretty close to a regular minisplit (not including the PV panels). It makes sense in a balanced climate area, when the sun is out more AC is needed.


So what’s the advantage  of the solar mini split? let’s say for this discussion you are limited to 4-8 panels.  Would a small grid tied system make more since?   Or would an off grid system be a better choice.  Let’s also assume that going forward one can add the net metering rider and utilize the time of use rate schedule or the standard rate schedule. 

In a world of infinite resources one could pay in cash for a system that was sized to provide 80-100+ % of your usage.  I don’t see that as realistic. 
I could afford today a 10-13k$ install.   

So I want to limit this discussion upfront system costs under 15k$.

It was a hot day yesterday we used 53 kWh. Here is the graph.  I cooled the house off in the morning and set the thermostat to 79.   

It seems to me that load shifting really negates the DC mini split.  And I would be better to go with a small grid tied system and time of use billing (baking during peak times might become a no-no)

Lastly I full expect that our house will add a second and or third EV in the next 10 years.  Do EVs mean net metering is better than a DC mini split? Do they change the decision in anyway?

When I get some time I will run some numbers.


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## peakbagger (Jun 15, 2022)

IMO, its all wrapped up in the utility rate plan a person has. I brought it up with regards to someone who has a time of use rate where they pay a lot more for day time power than night time. Many big loads can be shifted but unless some one is making ice blocks at night to cool their house during the day (which is done on occasion on commercial buildings), then they need to run AC during day time. In this case a 48 volt DC solar minisplit may make sense as it has power from the panels when the sun is out and in theory the sensible heat load is highest. Note latent load does not follow the sun as much but if the house is reasonably tight, once the latent load from humidity is knocked down it tends to stay down with sensible load taking over as the sun heats the house. The device I linked to does have AC backup and my guess is run it during the low rate period off of utility power to keep the humidity low and then run it off the panels during the high rate period during the day.  

I have one rate 24/7 and generate more than I use with no annual reset so I run mine when I need it and not worry.


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## begreen (Jun 15, 2022)

My interest was also piqued by the 48v minisplit system. I like the setup. One gets a high-efficiency mini-split along with the potential to be solar-powered. I am wondering who is making the mini-split for them and whether that manufacturer's other components, like a second wall unit, could be adapted.


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## woodgeek (Jun 15, 2022)

With all due respect, I wonder if envelope improvements could have a better ROI than a DC minisplit.  Ofc, if you got better insulation (and airsealing against latent humidity loads) it would work synergistically with your cold banking TOU strategy.

Do you know your BTU/h°F per sq ft (insulation) and ACH_50 (blower door)?  Can you get this info from a utility survey?

ETA: is your attic thoroughly air-sealed?  The upper stories should NOT be much hotter than the lower if this is the case.


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## EbS-P (Jun 15, 2022)

peakbagger said:


> IMO, its all wrapped up in the utility rate plan a person has. I brought it up with regards to someone who has a time of use rate where they pay a lot more for day time power than night time. Many big loads can be shifted but unless some one is making ice blocks at night to cool their house during the day (which is done on occasion on commercial buildings), then they need to run AC during day time. In this case a 48 volt DC solar minisplit may make sense as it has power from the panels when the sun is out and in theory the sensible heat load is highest. Note latent load does not follow the sun as much but if the house is reasonably tight, once the latent load from humidity is knocked down it tends to stay down with sensible load taking over as the sun heats the house. The device I linked to does have AC backup and my guess is run it during the low rate period off of utility power to keep the humidity low and then run it off the panels during the high rate period during the day.
> 
> I have one rate 24/7 and generate more than I use with no annual reset so I run mine when I need it and not worry.


I think you hit it right on the head. It’s all rate dependent.  And are electric rates predictable for the next 10 years?


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## EbS-P (Jun 15, 2022)

woodgeek said:


> With all due respect, I wonder if envelope improvements could have a better ROI than a DC minisplit.  Ofc, if you got better insulation (and airsealing against latent humidity loads) it would work synergistically with your cold banking TOU strategy.
> 
> Do you know your BTU/h°F per sq ft (insulation) and ACH_50 (blower door)?  Can you get this info from a utility survey?


Very possible and more probable if I didn’t heat with free wood.

Really I can only guess load. The whole house is cooled by a 3 ton unit.  22k BTU serviceable at 95 outdoor and 60% RH indoor.   You can’t pay anyone down here to do a manual J.  We tried.  

Never seen any energy audit do a door test.  I’m leaky for sure.  

First pic is ceiling. Second is north wall on a 90 degree afternoon.  Attic insulation is on my list. But not for it’s ROI but for a way to feel greener without putting up solar.  Right now we have what was r14 in the walls in 1968.  About r14-20 of loose fiberglass in the attic.    

I’m guessing we will see a 10-20% rate increase next time the utilities commission takes it up.


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## EbS-P (Jun 15, 2022)

begreen said:


> My interest was also piqued by the 48v minisplit system. I like the setup. One gets a high-efficiency mini-split along with the potential to be solar-powered. I am wondering who is making the mini-split for them and whether that manufacturer's other components, like a second wall unit, could be adapted.


For me though at least 3-4 months a year it wouldn’t be needed to heat or cool.  Would it be better just to install a grid tied solar, especially if you already have a heatpump?


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## woodgeek (Jun 15, 2022)

First reaction is 60% indoor RH seems high if you are running AC steadily, suggesting a lot of air infiltration.  In the summer, the cool air in the house falls, depressurizing the upper story and sucking heated attic air into the house (often largely around the top plates of interior framed walls, but also many other openings). This suction draws the hot air right through the insulation as if it weren't there.  Adding more insulation doesn't help.

Attic airsealing is very DIY-able (if you can operate up there when it is cool).  Lot's of guides on line.  You poke around under the insulation, looking for openings.  A major clue is where the fiberglass is discolored due to filtering dust in the air for the last few decades of summers.  In a heating climate, I dropped my heating bill by 25% myself in a similar vintage house just airsealing the attic, and then had pros come in and so the stuff I couldn't reach.  And THEN I had more insulation blown in.

I added it up, and found more than 10 sq ft of openings between the attic and the top story.  And that sort of thing is TYPICAL for 1960s construction.  Imagine a 3x3' window between the air in your attic and your house, open 24/7 !!

And BTW, attic airsealing is WAY greener than putting in solar.  And the payback is often like 1 year or less.

Try the FLIR on the top of interior walls... if you see a lot of heat there... you have infiltration.


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## peakbagger (Jun 15, 2022)

EbS-P said:


> For me though at least 3-4 months a year it wouldn’t be needed to heat or cool.  Would it be better just to install a grid tied solar, especially if you already have a heatpump?


Some utilities have really have no grid tied options or the options they have are very poor. Thus a solar powered DC heat pump just cuts house load and keeps the utility out of the picture.


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## begreen (Jun 15, 2022)

EbS-P said:


> Would it be better just to install a grid tied solar, especially if you already have a heatpump?


That's the current system.  This is thinking toward the future. The heat pump is going on 17 yrs old and the solar system is 12 yrs old with a 9 yrs old second array addition.


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## EbS-P (Jun 16, 2022)

begreen said:


> That's the current system.  This is thinking toward the future. The heat pump is going on 17 yrs old and the solar system is 12 yrs old with a 9 yrs old second array addition.



I’m always a bit torn between multiple independent systems and a single integrated system.   I see advantages of both.   I could totally see using old/used panels to power the minisplit.  

But if I were to start from scratch. With a limited budget it seems to me that if you could skip the mini split you could install more capacity or spend that money on batteries.  

Now if I were wanting to add heating and cooling Capacity, and I am considering it. (3 tons works ok 90% if the time.) I could make the argument that the DC minisplit is a good choice.   My issue is I need cooling on the first floor and heat in the basement.    I could.  

I’m also torn torn between saving money and spending it to reduce my carbon footprint. 


woodgeek said:


> First reaction is 60% indoor RH seems high if you are running AC steadily, suggesting a lot of air infiltration.  In the summer, the cool air in the house falls, depressurizing the upper story and sucking heated attic air into the house (often largely around the top plates of interior framed walls, but also many other openings). This suction draws the hot air right through the insulation as if it weren't there.  Adding more insulation doesn't help.
> 
> Attic airsealing is very DIY-able (if you can operate up there when it is cool).  Lot's of guides on line.  You poke around under the insulation, looking for openings.  A major clue is where the fiberglass is discolored due to filtering dust in the air for the last few decades of summers.  In a heating climate, I dropped my heating bill by 25% myself in a similar vintage house just airsealing the attic, and then had pros come in and so the stuff I couldn't reach.  And THEN I had more insulation blown in.
> 
> ...


Humidity is just high here.   3 miles from the ocean.   Most items left outside grow mildew and lots of it.  Basement makes it worse.  Kids and dog means are  doors always opening and closing.  I’m two cans of great stuff into my air sealing project.  Took me 11 years to properly seal an old sewer line hole.  I’m making progress.  Bathroom renovations added a lot of poorly sealed penetrations.  Attic has a lot of junk, and some seasonal storage.   

I don’t think latent load properly accounted for by many hvac contractors.  Lack of maintenance which I’m guilty of too, compounds the issue.  I’m about to go clean my coils my self.   120$ for an hours worth of work is about my rate.  Just had to shop vac out my condensate drain.  What’s plugged with lots of slime.


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## woodgeek (Jun 16, 2022)

I"m telling you that airsealing doesn't work the way most people think.  They put weather stripping on tiny cracks on their windows and think they did something, when the builders hid *square feet* of openings in their framing 50 years ago, on purpose.  And then put insulation over the openings and the rest is history.  Most of that infiltrating air flows down interior wall cavities into places like the basement.

When I finally attic airsealed (for real), while it was a PITA moving all the crap around up there, and working when I could, it was like I got a whole new house energy-wise.  I kept track and it took several foam board panels and sheets of sheet metal, and 20 tubes of caulking to close all the openings.  In a normal 1960 split level house.

The thing is, the indoor coil temp is supposed to be low enough versus CFM to maintain below 50% indoor RH.  60% indoor RH, and a 3-ton not keeping up all the time means either very high air infiltration, or an AC system on its last legs, or both.  What was the SEER on that thing?  Is it really old?  You might make an economic case for replacement with an inverter drive unit, maybe even a ASHP version.


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## Brian26 (Jun 16, 2022)

woodgeek said:


> I"m telling you that airsealing doesn't work the way most people think.  They put weather stripping on tiny cracks on their windows and think they did something, when the builders hid *square feet* of openings in their framing 50 years ago, on purpose.  And then put insulation over the openings and the rest is history.  Most of that infiltrating air flows down interior wall cavities into places like the basement.
> 
> When I finally attic airsealed (for real), while it was a PITA moving all the crap around up there, and working when I could, it was like I got a whole new house energy-wise.  I kept track and it took several foam board panels and sheets of sheet metal, and 20 tubes of caulking to close all the openings.  In a normal 1960 split level house.
> 
> The thing is, the indoor coil temp is supposed to be low enough versus CFM to maintain below 50% indoor RH.  60% indoor RH, and a 3-ton not keeping up all the time means either very high air infiltration, or an AC system on its last legs, or both.  What was the SEER on that thing?  Is it really old?  You might make an economic case for replacement with an inverter drive unit, maybe even a ASHP version.


This is spot on. During  Covid when I was on unemployment for 3 months I spent those months gutting, air sealing and installing mineral wool in all my exterior walls. My house is a 1958 1800 sq ft split level that had some fiberglass insulation in some walls.  Some walls had none. The amount of gaps and holes I found was insane.  Every piece of the exterior homesote sheathing had big gaps between them.  I could feel the air just rushing in. These corners in the picture below were the worst offenders.  The difference when I finished was astounding.



You said you used 53 kwh in one day for cooling? I just checked my Emporia energy monitor and I used 58.5 kwh so far for the entire month of June for cooling. It hasn't been exceptionally warm here but plenty of days in the 80s. I'm on net meter solar here but 53 kwh in New England would cost you close to $13-14 a day.


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## EbS-P (Jun 16, 2022)

woodgeek said:


> I"m telling you that airsealing doesn't work the way most people think.  They put weather stripping on tiny cracks on their windows and think they did something, when the builders hid *square feet* of openings in their framing 50 years ago, on purpose.  And then put insulation over the openings and the rest is history.  Most of that infiltrating air flows down interior wall cavities into places like the basement.
> 
> When I finally attic airsealed (for real), while it was a PITA moving all the crap around up there, and working when I could, it was like I got a whole new house energy-wise.  I kept track and it took several foam board panels and sheets of sheet metal, and 20 tubes of caulking to close all the openings.  In a normal 1960 split level house.
> 
> The thing is, the indoor coil temp is supposed to be low enough versus CFM to maintain below 50% indoor RH.  60% indoor RH, and a 3-ton not keeping up all the time means either very high air infiltration, or an AC system on its last legs, or both.  What was the SEER on that thing?  Is it really old?  You might make an economic case for replacement with an inverter drive unit, maybe even a ASHP version.


No doubt about high air infiltration.    Heatpump is a trane 16 seer package unit that is 12 years old.  It’s got lots of duct leakage in the crawl space. That’s really hurting things too. Not using it much in the winter really increases Breakeven time on a new more efficient unit.  Last time I looked no package units have a SEER greater than 16.  Not sure I could get a new separate evaporator in the crawspace.  

Had the tech out gauges in it last time it was serviced.  It wasn’t hot or cold out but he thought it all checked out ok.  One thing I can’t quite figure out is the low high stage and the variable speed blower.  Which was replaced 3-5 years ago.  The only way to figure out what stage is running is based on fan speed.  And that can be confusing as the fan has an extra speed period so the fan sounds like it has 3 stages of cooling.  If you don’t listen from startup it can be hard to tell if it’s high stage or not.  The high stage is just a bypass solenoid that closes to utilize the second stage.  Compressor and condenser fan never change speed.  


Brian26 said:


> This is spot on. During  Covid when I was on unemployment for 3 months I spent those months gutting, air sealing and installing mineral wool in all my exterior walls. My house is a 1958 1800 sq ft split level that had some fiberglass insulation in some walls.  Some walls had none. The amount of gaps and holes I found was insane.  Every piece of the exterior homesote sheathing had big gaps between them.  I could feel the air just rushing in. These corners in the picture below were the worst offenders.  The difference when I finished was astounding.
> 
> 
> 
> ...


54 kWh was my total usage for the house and car charging.  I only have a whole house monitor.  AC uses 3 kw.  I’m guessing  it didn’t use more than 15 kw but don’t know really.  

Adding insulation down here really has a long pay off period.  It just doesn’t get cold enough. This past winter the high was always above freezing and that’s pretty normal.  Lows probably only drop below freezing 10-15 days a year.   Summers aren’t oppressive triple digits happens but not regularly.  

 I commend you for taking on that project.  I’ll take on the attic  an maybe pay for the crawl space but I’m not pulling drywall.  


But back to topic would you rather install the 48v mini split or a 120/240.


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## Brian26 (Jun 16, 2022)

This company also has dc solar assisted mini splits. They even sell one that can run completely off solar and batteries.

It looks like the units are just rebadged Mideas based off the spec sheet and Toshiba compressor. 






						YMGI's Solar Assisted DC Inverter Single-Zone Wall-Mounted Mini-split models | YMGI Group
					

Information, Specifications, and Downloads for YMGI's Solar Assisted DC Inverter Single-Zone Wall-Mounted Mini-split models




					ymgigroup.com


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## EbS-P (Jun 16, 2022)

Brian26 said:


> This company also has dc solar assisted mini splits. They even sell one that can run completely off solar and batteries.
> 
> It looks like the units are just rebadged Mideas based off the spec sheet and Toshiba compressor.
> 
> ...


Midea was my guess too.   

If forced into time use billing and peak rates are really high I could see batteries as helpful but why not just go with a 240 volt minisplit.  Are the inverter losses something that makes a DC system more efficient?


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## begreen (Jun 16, 2022)

Brian26 said:


> This company also has dc solar assisted mini splits. They even sell one that can run completely off solar and batteries.
> 
> It looks like the units are just rebadged Mideas based off the spec sheet and Toshiba compressor.
> 
> ...


That was my suspicion. Midea is behind many rebranded products.


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## EbS-P (Jun 17, 2022)

woodgeek said:


> I"m telling you that airsealing doesn't work the way most people think.  They put weather stripping on tiny cracks on their windows and think they did something, when the builders hid *square feet* of openings in their framing 50 years ago, on purpose.  And then put insulation over the openings and the rest is history.  Most of that infiltrating air flows down interior wall cavities into places like the basement.
> 
> When I finally attic airsealed (for real), while it was a PITA moving all the crap around up there, and working when I could, it was like I got a whole new house energy-wise.  I kept track and it took several foam board panels and sheets of sheet metal, and 20 tubes of caulking to close all the openings.  In a normal 1960 split level house.
> 
> The thing is, the indoor coil temp is supposed to be low enough versus CFM to maintain below 50% indoor RH.  60% indoor RH, and a 3-ton not keeping up all the time means either very high air infiltration, or an AC system on its last legs, or both.  What was the SEER on that thing?  Is it really old?  You might make an economic case for replacement with an inverter drive unit, maybe even a ASHP version.


I had to look it up but our average July RH is 75%.  That’s a couple points higher than New Orleans.   Average 21 days of precipitation in July to the total of 100mm.     We set the thermostat at 78 if it gets 95 or or hotter we set to 80.  Basement is always cooler. 

Giving this some more thought.  Not having an inverter is much cheaper.   Small inverter is probably 4-600$ and you need two to get 240v ( So car charging is probably out, but if it’s a 4 panel install it really wouldn’t amount to much) .     So if you get a 12k btu unit with 4 panels.  Adding an inverter and battery could make sense for an off grid backup 120 V system.   Enough battery to run a fridge.   

Now if I could dual head this unit I would consider it.  One head upstairs one downstairs.  I’d probably stick with the 12k btu unit.  It’s probably not saving a lot of money.  Say 1kw  for 5 hours a day 180 days a year.  Works out to be $120   a year.  Best case it’s a factor of 2 higher.   

So if you were to get this unit would you choose DC only with batteries, or the AC/DC grid tied unit?  It’s heating efficiency is  not anymore efficient than my 13 year old heatpump.


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## ABMax24 (Jun 17, 2022)

What about the loss in revenue by not having the solar panels grid tied? Surely there will be times when the panels produce more energy than the heat pump can utilize, greatly lowering the ROI on them. If not there will be times the heat pump can't run because there isn't enough energy coming from the solar panels.

If maximizing self consumption is the goal then it seems a grid battery is the most logical option, and covers all devices in the home, not just the heat pump.

I've thought about this myself in relation to an offgrid house. My conclusion has always been it is better to operate on 120/240VAC or 120/208 3 phase AC for the house wiring and all appliances, because of the vastly greater selection of AC appliances and in many cases better operating efficiencies. The larger inverters are more costly to accommodate this design, but in the end overall efficiency and cost seem to be a wash.

My only concern would be DC ripple in the batteries/DC wiring when operating higher wattage devices like a heat pump on a single/split phase inverter. Although this would vary by inverter, and could also be solved to some extent by operating the house on 3 phase AC and balancing the loads as best as possible.


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## EbS-P (Jun 17, 2022)

I think the loss of use for a small array is offset at least partially by not having to purchase an inverter.  I don’t see the advantage of the dc mini split if you have a large array.  Then I agree 120/240 is the way to go.  If you go off grid and have the money to spend the storage solutions are available.  


ABMax24 said:


> What about the loss in revenue by not having the solar panels grid tied? Surely there will be times when the panels produce more energy than the heat pump can utilize, greatly lowering the ROI on them. If not there will be times the heat pump can't run because there isn't enough energy coming from the solar panels.
> 
> If maximizing self consumption is the goal then it seems a grid battery is the most logical option, and covers all devices in the home, not just the heat pump.
> 
> ...


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## ABMax24 (Jun 17, 2022)

EbS-P said:


> I think the loss of use for a small array is offset at least partially by not having to purchase an inverter.  I don’t see the advantage of the dc mini split if you have a large array.  Then I agree 120/240 is the way to go.  If you go off grid and have the money to spend the storage solutions are available.



The cost of the batteries would offset the cost of the inverter though. I know some of the units are marketed as not needing batteries, but I don't accept the premise that these heatpumps are going to be effective when shutting down and starting up every time a cloud goes by, not to mention what it would do to compressor/motor longevity.

An APSystems QS1 inverter is $400 and will run 4 panels for a total output of 1200 watts, that cost would buy 2-3 12 volt batteries. I'd still be going grid tied.


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## EbS-P (Jun 17, 2022)

ABMax24 said:


> The cost of the batteries would offset the cost of the inverter though. I know some of the units are marketed as not needing batteries, but I don't accept the premise that these heatpumps are going to be effective when shutting down and starting up every time a cloud goes by, not to mention what it would do to compressor/motor longevity.


Good points.


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## Poindexter (Jun 21, 2022)

Appreciate this discussion.  I do agree electricity rates kinda trump all here.  But, if there is a split or differential utility rate, day time rates are almost always higher.  Having  a standalone heat pump/AC unit that can run on 48 volts would allow someone in a hot enough climate to just use solar to decrease there grid consumption, even if they are still pulling some juice off the grid to meet their total AC needs.  

The main alternative, it looks like to me, is to store solar in an expensive battery today, so I can run the meter backwards tomorrow when rates are highest.  Or use today's solar to run whatever size AC today and not buy the battery.

So much math and so many variables.

I do advise caution with air sealing alone.  A very tight house, like mine was, without adequate ventilation, is an opportunity for mold.  While I do plan for my next house to be tight like tupperware, the tightness will be tempered with an HRV system (heat recovery ventilation) to get some air turnovers for mold abatement without trying to condition all the air in the back garden.

What works in one zip code for an efficient healthy home may not work one zip code over.


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## woodgeek (Jun 21, 2022)

When was your house built @Poindexter ?  Do you have central AC?  In this case, airsealing would REDUCE indoor humidity.


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## EbS-P (Jun 21, 2022)

More thinking on this topic. Wouldn’t it make more sense if doing a small system, 3-4 panels install, to have this hooked to your heatpump hot water heater. Thermal storage makes the most of the full rated output all year long. We just had two cool but sunny days right around solstice didn’t need much AC. 

If you already have a heatpump this makes no sense. The mini split unit cost is more than the inverter so all that’s left is the automatic disconnect and you will get 100% utilization of your rated output. 

I think it has its place but ROI is probably better spent elsewhere unless live somewhere hot. Much hotter than coastal NC.


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## peakbagger (Jun 21, 2022)

ABMax24 said:


> The cost of the batteries would offset the cost of the inverter though. I know some of the units are marketed as not needing batteries, but I don't accept the premise that these heatpumps are going to be effective when shutting down and starting up every time a cloud goes by, not to mention what it would do to compressor/motor longevity.
> 
> An APSystems QS1 inverter is $400 and will run 4 panels for a total output of 1200 watts, that cost would buy 2-3 12 volt batteries. I'd still be going grid tied.


 
I think there are some misconceptions. PV is not on off when a cloud goes over the sun so the minisplit would probably not turn off, it just may not be cooling.  A mini split is not like a conventional window shaker or even standard home heat pump. A window unit is generally a single speed motor hooked to a compressor. The compressor is either on or off so there is lots of current inrush everytime the motor compressor starts. It runs at 100% until it overshoots the temperature setting and then the compressor turns off. The fans may have multiple speeds but they do not pull much power. A mini split maintains one temp by running the compressor at variable speed adjusting the compressor speed as needed. Even if it does turn off, its a "soft start" where the motor is ramped up slowly so it will last a lot longer than one that cycled on/off. Thus my guess is if the power from the panels is not high enough the fans may still run but the compressor does not. Conventional old style variable speed drives convert the input AC to DC and then converts it to AC waveform. So having a 48 volt DC input may make the drive simpler without a front end AC stage. Industrially most motors are now VF drives and as long as they are built for VF drive use they are lasting longer and far more efficient as they are soft start and run at lower speeds most of the time. 

I think a big selling point of the DC units that do not require battery's is that they are black out and brown out proof, given the issues with the grid in Texas where the legislature is more interested in bailing out the corporations than consumers, black outs and brown outs are likelihood in hot spells. Lot to be said for something that runs for cooling as long as the sun is out. Its not just Texas, CA has issues especially rural areas that get their power shut off to keep from the power lines starting fires.   In both areas AC is not optional, people die especially the elderly when the AC goes out.


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## EbS-P (Jun 21, 2022)

peakbagger said:


> I think there are some misconceptions. PV is not on off when a cloud goes over the sun so the minisplit would probably not turn off, it just may not be cooling.  A mini split is not like a conventional window shaker or even standard home heat pump. A window unit is generally a single speed motor hooked to a compressor. The compressor is either on or off so there is lots of current inrush everytime the motor compressor starts. It runs at 100% until it overshoots the temperature setting and then the compressor turns off. The fans may have multiple speeds but they do not pull much power. A mini split maintains one temp by running the compressor at variable speed adjusting the compressor speed as needed. Even if it does turn off, its a "soft start" where the motor is ramped up slowly so it will last a lot longer than one that cycled on/off. Thus my guess is if the power from the panels is not high enough the fans may still run but the compressor does not. Conventional old style variable speed drives convert the input AC to DC and then converts it to AC waveform. So having a 48 volt DC input may make the drive simpler without a front end AC stage. Industrially most motors are now VF drives and as long as they are built for VF drive use they are lasting longer and far more efficient as they are soft start and run at lower speeds most of the time.
> 
> I think a big selling point of the DC units that do not require battery's is that they are black out and brown out proof, given the issues with the grid in Texas where the legislature is more interested in bailing out the corporations than consumers, black outs and brown outs are likelihood in hot spells. Lot to be said for something that runs for cooling as long as the sun is out. Its not just Texas, CA has issues especially rural areas that get their power shut off to keep from the power lines starting fires.   In both areas AC is not optional, people die especially the elderly when the AC goes out.


I see this system, with a battery and an inverter as equivalent to an expensive stand by generator.   Better in that you are getting some use out of it every day.    After Florence we saw Generac trucks/vans everywhere for about 12-18 months.  Then we didn’t see too many.  Now, almost 4 years later they are back in good numbers.  I doubt it’s all new installs.  But but I could be wrong.  Nothing like paying big bucks for an 8-10 Kw system to only need it for a grand total of 13 hours (in my case) in four years.  Granted after Florence it was a week for everyone and some people it was 2-3 weeks without power.


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## ABMax24 (Jun 21, 2022)

peakbagger said:


> I think there are some misconceptions. PV is not on off when a cloud goes over the sun so the minisplit would probably not turn off, it just may not be cooling.  A mini split is not like a conventional window shaker or even standard home heat pump. A window unit is generally a single speed motor hooked to a compressor. The compressor is either on or off so there is lots of current inrush everytime the motor compressor starts. It runs at 100% until it overshoots the temperature setting and then the compressor turns off. The fans may have multiple speeds but they do not pull much power. A mini split maintains one temp by running the compressor at variable speed adjusting the compressor speed as needed. Even if it does turn off, its a "soft start" where the motor is ramped up slowly so it will last a lot longer than one that cycled on/off. Thus my guess is if the power from the panels is not high enough the fans may still run but the compressor does not. Conventional old style variable speed drives convert the input AC to DC and then converts it to AC waveform. So having a 48 volt DC input may make the drive simpler without a front end AC stage. Industrially most motors are now VF drives and as long as they are built for VF drive use they are lasting longer and far more efficient as they are soft start and run at lower speeds most of the time.
> 
> I think a big selling point of the DC units that do not require battery's is that they are black out and brown out proof, given the issues with the grid in Texas where the legislature is more interested in bailing out the corporations than consumers, black outs and brown outs are likelihood in hot spells. Lot to be said for something that runs for cooling as long as the sun is out. Its not just Texas, CA has issues especially rural areas that get their power shut off to keep from the power lines starting fires.   In both areas AC is not optional, people die especially the elderly when the AC goes out.



It is quite easy though to have solar output reduced by 80% with cloud cover. Can these DC minisplits have a turndown of 5:1, maybe. To do so will be expensive though, motors, VFDs, and compressors all have limits to their min and max operating range.

Regardless I won't be buying such a unit, to me a house scale battery makes a lot more sense.


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## Poindexter (Jun 21, 2022)

woodgeek said:


> When was your house built @Poindexter ?  Do you have central AC?  In this case, airsealing would REDUCE indoor humidity.


1980 build, state of the art at the time, with vapor barrier under the drywall, but no hats on the connection boxes.  I should have bought stock in spray foam products

For Heating I have an oil fired boiler at about 100-110kBTU / hour with hotwater baseboard, and a BK stove at 35k (ish) BTU per hour.  Both get run pretty hard in January.  Keeping the woodstove raging does lower the duty cycle on the oil fired boiler for the rest of the winter.  I do have a spare burner for my new last month boiler, the spare burner is for natural gas.  I do have a gas line in the easement at the front of my lot out by the street,  but the current price on that stuff is, well the local market is disconnected from the lower 48.

For ventilation I have opable windows and a courtesy fan in each of the two bathrooms.

For AC I got jack doodle.  We do have an arc of trees around the house from east to west, but they protect the north side of the house from winter weather.

I didn't really understand vapor barrier until I moved up here (from Chapel Hill, NC) long about 2008.  I have never had to deal with vapor barrier in a house that has both heating and AC, but it looks like a challenge.


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## begreen (Jun 21, 2022)

The idea of air conditioning in Fairbanks, AK does not compute.


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## ABMax24 (Jun 21, 2022)

Poindexter said:


> For AC I got jack doodle.  We do have an arc of trees around the house from east to west, but they protect the north side of the house from winter weather.



Surely those north trees provide some shade to the house on a day like today, with your nearly 22 hours of daylight.

I know that's even an issue for us, with the sun beating on the house for 17+ hours a day, and the nights not cooling off much, it can be difficult to keep the house cool without AC. At this time of year it's not uncommon for our AC to be running consistently until midnight.


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## Poindexter (Jun 24, 2022)

ABMax24 said:


> Surely those north trees provide some shade to the house on a day like today, with your nearly 22 hours of daylight.
> 
> I know that's even an issue for us, with the sun beating on the house for 17+ hours a day, and the nights not cooling off much, it can be difficult to keep the house cool without AC. At this time of year it's not uncommon for our AC to be running consistently until midnight.





begreen said:


> The idea of air conditioning in Fairbanks, AK does not compute.



It is all in what you are used to.  I flew into Seattle once when they were having the worst snowstorm in 25 years, the place was a madhouse.  Because they weren't used to it.  It looked like September in Fairbanks to me, but that doesn't help them; or make it normal for them.  Christmas-ish 2008 I think it was.

I was talking to my Sarasota, FL area dad last weekend, his heat index was +105-110dF and he was aware it was hot.  He gets chilly at +40dF in January down there with like sweaters and pills for his joints and stuff.  So 60-70 degree (F) swing, he feels it.  I am pretty well acclimated to -20dF for good boots and maybe a, what is that new sweater thing they make out of old Led Zeppelin albums?  Fleece, an artificial fleece.  They are everywhere now.  One of those.  But at +70dF, a 90 degree swing, I am miserable and we are looking at +80s all next week. 

The trees to the north of  the house do moderate nighttime household temps by blocking the sun while the sun is north of west and coming around through north to be north of east overnight.  If I had AC and appropriate vapor barrier (I think for cooling I would need a second layer of vapor barrier on the outside surface of the wall, just under the finished surface) I would be shooting for +65dF in the house.  

In the +70s I am drinking 6-7 liters of water daily, and sweating like I just got poached or braised or something.  Given 'the arctic' is warming twice as fast as the rest of the globe, at least many scientists say so, I probably will be looking an AC system in our next house if we stay up here.

Being able to run something like this system PRN in the summer and then maybe use the solar panel output for something else the rest of the year is something to consider pretty seriously.


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## sloeffle (Jun 24, 2022)

begreen said:


> The idea of air conditioning in Fairbanks, AK does not compute.


I'm glad our hotel had it when we visited Denali a few years ago. 

A few day earlier we were bundled up in coats ( mid 50's ), and the folks from Alaska were walking around in shorts. We chatted with a local at a pizza place in Fairbanks and he knew right away we weren't from Alaska.


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## tlc1976 (Jun 24, 2022)

EbS-P said:


> I see this system, with a battery and an inverter as equivalent to an expensive stand by generator.   Better in that you are getting some use out of it every day.    After Florence we saw Generac trucks/vans everywhere for about 12-18 months.  Then we didn’t see too many.  Now, almost 4 years later they are back in good numbers.  I doubt it’s all new installs.  But but I could be wrong.  Nothing like paying big bucks for an 8-10 Kw system to only need it for a grand total of 13 hours (in my case) in four years.  Granted after Florence it was a week for everyone and some people it was 2-3 weeks without power.


Yes and in addition to upfront costs, that standby generator costs a lot in fuel when you do need it. I have an 8kw Generac.

Solar mini split- one time purchase, no fuel required, self contained, and cools more as the sun beats down harder. Yes it’s not perfect (won’t work at night and probably not as good when it’s cloudy). But if I lived in an area that was hot and sunny most of the time, it’d be a no brainer, I’d be getting one. Especially if the place had a history of grid problems. For a long time I’ve wondered why they don’t use absorption fridge technology to do something similar, or maybe they have.


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## peakbagger (Jun 24, 2022)

the Coefficient of performance for a Mini Split is around 3 (I watt of power in for 3 watts of cooling out). Absorption chillers are far lower, for a single effect hot water unit the COP is less than 1 (usually .8). Of course the heat is "free" but you need a lot of it. There was a Swiss individual once that posted that they did sell house hold absorption chillers in Switzerland.  The smallest I have seen in the US is a Yazaki and I think the smallest one is 5 tons. 

I have a 85 ton unit one of my projects. It dwarfs the 500 ton centrifugal sitting next to it.


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## Brian26 (Jun 25, 2022)

peakbagger said:


> the Coefficient of performance for a Mini Split is around 3 (I watt of power in for 3 watts of cooling out). Absorption chillers are far lower, for a single effect hot water unit the COP is less than 1 (usually .8). Of course the heat is "free" but you need a lot of it. There was a Swiss individual once that posted that they did sell house hold absorption chillers in Switzerland.  The smallest I have seen in the US is a Yazaki and I think the smallest one is 5 tons.
> 
> I have a 85 ton unit one of my projects. It dwarfs the 500 ton centrifugal sitting next to it.


I don't think you and others realize how efficient a properly sized modern high seer 38-42.5 seer AHRI rated mini split heat pumps is.  A COP of 3 for cooling is what a good Carrier central air unit was like 30 years ago. My mini split at its minimum compressor speed is rated at a cop of 10.62 cooling at 82 degrees using only 80 watts. I verified all these numbers with my Emporia Vue Energy monitor which is measuring voltage and adjusting for power factor. My observations were perfectly matched up with the below chart.

I have been on net metered solar for 8 years here in CT that covers close to 90% of my yearly electricity usage. I ran some numbers and even if I had to purchase the electricity these high seer mini splits use so little electricity that the savings over a central unit are massive. Most central air units are are around 16 seer.

Here is my yearly monthly electricity load on just both my mini splits that cover 90%+ of my yearly heating and cooling load. . As you can see my downstairs unit carries most of the heating load and my upstairs does most of the cooling. I haven't even used 100 kwhs this month for cooling so far which if I purchased the electricity would cost me around $26.


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## Piney (Jul 15, 2022)

Slightly tangential: as far as ac goes, we are off grid.  We run ac when needed.  We find that when it is so hot we need ac during the day there is enough sun on the panels to let us.  We set the ac to max, continuous run and drive the house a little cooler than we like so that when the early evening comes and the ac is off we are ahead of the game.  We don’t need ac at night no matter how hot the day was because our area has the greatest diurnal temperature swings in North America. It’s always cold at night. We open a window.  I realize this isn’t useful information for those in the humid south but it might give ideas to those at higher elevations.  It’s amazing what a 7 or 9000 btu wall (why waste a window? We cut a hole) machine can do running full tilt, non-stop, on ‘free’ power for hours on end.


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