# Home battery backup systems



## SpaceBus (Jan 25, 2020)

Moderator moved from Going Solar! thread to new thread for discussion  

Total novice here, but I have a few questions. My water heater is rated at 18kw max and I'm trying to see if it were even possible to run it off of a battery bank. Does this means I should get at least an 18kw inverter for those times the water heater calls for max amperage (2 x 40 amp DP breakers on a 200 amp service box)?


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## jetsam (Jan 25, 2020)

SpaceBus said:


> Total novice here, but I have a few questions. My water heater is rated at 18kw max and I'm trying to see if it were even possible to run it off of a battery bank. Does this means I should get at least an 18kw inverter for those times the water heater calls for max amperage (2 x 40 amp DP breakers on a 200 amp service box)?



1) Do you want to go shut off the water heater breakers every time you use the microwave? Of course not, so size the inverter to the total load it may need to supply. When you run the dryer, the boiler and the refrigerator may kick on without warning, etc.  You can reduce this by planning to not use some circuits when off the grid (I shut off my electric dryer breaker so nobody tries to use it, and thus don't  need to include it in my capacity planning).

2) 18kw is crazy for a DHW heater unless it's commercial (or some kind of tankless heater).
Is it a boiler that also does DHW?


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## begreen (Jan 25, 2020)

On demand heater?


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## SpaceBus (Jan 25, 2020)

jetsam said:


> 1) Do you want to go shut off the water heater breakers every time you use the microwave? Of course not, so size the inverter to the total load it may need to supply. When you run the dryer, the boiler and the refrigerator may kick on without warning, etc.  You can reduce this by planning to not use some circuits when off the grid (I shut off my electric dryer breaker so nobody tries to use it, and thus don't  need to include it in my capacity planning).
> 
> 2) 18kw is crazy for a DHW heater unless it's commercial (or some kind of tankless heater).
> Is it a boiler that also does DHW?





begreen said:


> On demand heater?



Yes, it is a small on demand water heater. Usually it doesn't pull max load since I have a 40 gallon tank heated by the cookstove (and already existing rooftop collector in the future).


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## jetsam (Jan 25, 2020)

I've never even seen an electric tankless one. They're almost unheard of around here, though you do rarely see natural gas tankless (mostly as point of use heaters).

I had a natural gas unit decades ago overseas, and it did a great job supplying a small house. (It was located in the bathroom,  which was a bit odd.)


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## SpaceBus (Jan 25, 2020)

jetsam said:


> I've never even seen an electric tankless one. They're almost unheard of around here, though you do rarely see natural gas tankless (mostly as point of use heaters).
> 
> I had a natural gas unit decades ago overseas, and it did a great job supplying a small house. (It was located in the bathroom,  which was a bit odd.)


Mine is an Ecosmart ECO 18. It works very well and modulates the heating elements based on flow and incoming water temperature. In my case the incoming water is 75-140df since it is coming from a 40 gallon tank heated by my cookstove. The water heater is set at 130df and then feeds into my pex manifold.

I have found a 20kw charger inverter, but it does not appear to be UL listed and probably isn't approved for a grid tie in. Most of the 20 kw inverters I've seen are ul approved and grid tie in approved, but require 200 - 1000v feed. That's a heck of a battery bank if I want grid tied with storage.

Sorry to thread jack, just trying to pick the brains of folks that have done this. We didn't want to rely on propane, but I didn't really think about how to feed the water heater from a battery bank.

Edit: I'm better off just running a 7600w 40 amp charger inverter and hope I never need more than 40 amps at the electric water heater. Trying to make a 20kw inverter play nice with a residential setup will be impossible.


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## begreen (Jan 25, 2020)

SpaceBus said:


> Most of the 20 kw inverters I've seen are ul approved and grid tie in approved, but require 200 - 1000v feed. That's a heck of a battery bank if I want grid tied with storage.


Not with a DIY bank based on a Prius, Volt or Tesla battery. The Volt is ideal as it can be broken down into 48v modules. That said, a 40amp load on a battery bank is going to hit it hard.


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## SpaceBus (Jan 25, 2020)

begreen said:


> Not with a DIY bank based on a Prius, Volt or Tesla battery. The Volt is ideal as it can be broken down into 48v modules. That said, a 40amp load on a battery bank is going to hit it hard.


The water heater is nice because it can modulate a single, or all three if needed, 6000 watt heating element multiple times a second. Unless something crazy happens I should never be heating the water from anything less than 55 df. I haven't seen input temps below 75df and I frequently check when my wife is running taps out of curiosity. We keep the house above 70df for the most part, so the tank should always stay warm. Would an intermittent 40 amp hit on a system made up of several lead acid batteries in series and parallel? Would lithium batteries really be worth the huge premium from this perspective? 

The biggest issue I see trying to make a 20kw inverter work is getting a 3phase system to place nice with my residential service, which doesn't seem possible. I would have to go off grid and put a 400 amp panel in my house; or build an outbuilding with a 400 amp panel and feed my house with an interlink between the meter and the house panel from an 80 amp breaker on the 400 amp outbuilding panel. Either way sounds crazy expensive and I'm better off keeping the range boiler above 70df if I want to build a solar system for my house.


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## begreen (Jan 25, 2020)

Used Volt batteries are much more affordable and majorly less maintenance. I wouldn't use lead-acid for more than a 48VDC system and I would use them for high amperage resistance heating of water or air.


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## jetsam (Jan 26, 2020)

SpaceBus said:


> The water heater is nice because it can modulate a single, or all three if needed, 6000 watt heating element multiple times a second. Unless something crazy happens I should never be heating the water from anything less than 55 df. I haven't seen input temps below 75df and I frequently check when my wife is running taps out of curiosity. We keep the house above 70df for the most part, so the tank should always stay warm. Would an intermittent 40 amp hit on a system made up of several lead acid batteries in series and parallel? Would lithium batteries really be worth the huge premium from this perspective?
> 
> The biggest issue I see trying to make a 20kw inverter work is getting a 3phase system to place nice with my residential service, which doesn't seem possible. I would have to go off grid and put a 400 amp panel in my house; or build an outbuilding with a 400 amp panel and feed my house with an interlink between the meter and the house panel from an 80 amp breaker on the 400 amp outbuilding panel. Either way sounds crazy expensive and I'm better off keeping the range boiler above 70df if I want to build a solar system for my house.



I bet your water heater has 4 internal heating elements, and I further bet that each one is either already fused/breakered,  or that at least they are each designed to be replaceable. Either way, it shouldn't be a major project to shut off 2 of them (actually, figure out which leg the circuit board runs off of, and just shut off the opposite double 40, all done).

I don't know if it'd be smart enough to use the 2 elements that have power at times of low draw, though maybe you'll get lucky and the 2 legs the circuit board is on are the "low" heat elements.


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## SpaceBus (Jan 27, 2020)

jetsam said:


> I bet your water heater has 4 internal heating elements, and I further bet that each one is either already fused/breakered,  or that at least they are each designed to be replaceable. Either way, it shouldn't be a major project to shut off 2 of them (actually, figure out which leg the circuit board runs off of, and just shut off the opposite double 40, all done).
> 
> I don't know if it'd be smart enough to use the 2 elements that have power at times of low draw, though maybe you'll get lucky and the 2 legs the circuit board is on are the "low" heat elements.


It won't work if one breaker is switched or disconnected, unfortunately. When we make the jump to solar I'll most likely have a shed/barn that all the equipment lives in and I'll set up a line side tap and just feed my service panel in the house directly with a 20kw inverter.


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## begreen (Jan 27, 2020)

DIY power wall.


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## peakbagger (Jan 27, 2020)

begreen said:


> DIY power wall.




Maybe I am missing something but no mention of a BMS system in the video.  My limited understanding of LI battery packs is Lithium Ion batteries need to have a BMS for each cell. If they don't, bad things can happen during charging like a fire and toxic fumes. 

Most of the discussions on various Solar Power Forums regarding reusing Lithium Ion battery packs seem to end up at what BMS do they use? I haven't seen a lot of agreement and the various BMS suppliers are gladly willing to sell them but not a lot of information on how to make sure they operate reliably enough not to allow a cell to overcharge. The theory is if one cell tries to overcharge, the BMS cuts that cell out of the charge circuit.  That equalizes the string of batteries maximizing state of charge.  There are companies that make lithium ion  retrofits for golf carts that include a BMS so I know its possible.


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## begreen (Jan 27, 2020)

peakbagger said:


> Maybe I am missing something but no mention of a BMS system in the video.  My limited understanding of LI battery packs is Lithium Ion batteries need to have a BMS for each cell. If they don't, bad things can happen during charging like a fire and toxic fumes.
> 
> Most of the discussions on various Solar Power Forums regarding reusing Lithium Ion battery packs seem to end up at what BMS do they use? I haven't seen a lot of agreement and the various BMS suppliers are gladly willing to sell them but not a lot of information on how to make sure they operate reliably enough not to allow a cell to overcharge. The theory is if one cell tries to overcharge, the BMS cuts that cell out of the charge circuit.  That equalizes the string of batteries maximizing state of charge.  There are companies that make lithium ion  retrofits for golf carts that include a BMS so I know its possible.


He has a lot of videos including successful BMS systems and not so successful


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## peakbagger (Jan 28, 2020)

Okay I just want to make sure folks do not think rigging up a battery is as easy as rigging up Lithium Ion cells. In order to get the performance out of them they need a BMS and special charging whch is the tricky part. With the amperage involved, minor mistakes lead to  real large arcs.


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## begreen (Jan 28, 2020)

Indeed, one needs to treat a battery bank of this size with great respect. He's been experimenting and refining his powerwalls from Volt cells for a few years now. I like that he documents his successes and failures well. I think his first series with a Gen 1 battery was over something like 8 or 10 videos.


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## SpaceBus (Jan 28, 2020)

It seems like the Nissan leaf cells work very well, but a quality BMS is essential. From my limited research there aren't any off the shelf units that would work. Perhaps I just haven't found them.


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## semipro (Jan 28, 2020)

begreen said:


> Indeed, one needs to treat a battery bank of this size with great respect. He's been experimenting and refining his powerwalls from Volt cells for a few years now. I like that he documents his successes and failures well. I think his first series with a Gen 1 battery was over something like 8 or 10 videos.


I've been following David Poz for a while and I'm impressed with his work.


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## semipro (Jan 28, 2020)

SpaceBus said:


> It seems like the Nissan leaf cells work very well, but a quality BMS is essential. From my limited research there aren't any off the shelf units that would work. Perhaps I just haven't found them.


I remember one guy, IIRC somewhere in Scandanavia,  that installed old Leaf batteries in an old server rack with a BMS for backup storage. It seemed like a nice setup. 
 I'll try to find the URL later.


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## SpaceBus (Jan 28, 2020)

semipro said:


> I remember one guy, IIRC somewhere in Scandanavia,  that installed old Leaf batteries in an old server rack with a BMS for backup storage. It seemed like a nice setup.
> I'll try to find the URL later.


The Nissan cells seem by far the most affordable and scalable. I saw a lot for 50 individual modules going for about $3k while looking into the viability of repurposed automotive batteries. Unfortunately I didn't save it, but I did see a lot of 98 modules for a decent price, but it's gone now. Sometimes the rain and clouds can hang around for a week or more here in Maine, so if I designed a system to carry the house for 7-10 days then I'm basically looking at a battery bank that could support my on demand water heater without any preheated water. Well, it would run for a few minutes anyway, but I didn't design the system to run without preheated water.


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## peakbagger (Jan 28, 2020)

The early Leaf batteries have the reputation that they have lost a lot of capacity. Nissan pushed them hard to get more range and the trade off was steeper degradation. Still adequate for home use but not so good for cars. 

I have been keeping an eye out for a packaged BMS solution but to date the only ones I find are science experiments. I think it comes down to liability. A big company can cover liability but to a little guy I expect the cost for liability insurance to sell a packaged BMS would be steep as the potential risk is high if a pack lights up and burns a house down. There are couple of vendors on Ebay that sell battery packs with a big warning that a proper BMS is needed. When you look through their other offerings they sort of sell BMS components but info is lacking.


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## begreen (Jan 28, 2020)

Here is a cool video of the final resurection of a Smart For Two battery. Note that in the middle of the video he talks a bit about his home running on the Volt system. This is everything including hot water, refrigeration etc. on a 6kW inverter. They have tripped the breaker a couple times when the HW htr was running and then another heavy load was turned on.

Also, look at the comprehensive links provided below the video. Very helpful. Here is the BMS he used. 




__





						Smart BMS of Power Battery – LLT POWER ELECTRONIC
					





					www.lithiumbatterypcb.com


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## Starbrightsteve (Jan 28, 2020)

With respect to the original post: if you are adding backup power to a solar system, I don’t think an electric resistance device of any kind ought to be connected to the  inverter /battery. Our solar backup will handle the refrigerator, freezer, basement sump pump, and a few LED lights and the stereo/radio. If we really need a shower we can use our camper.


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## maple1 (Jan 28, 2020)

It would seem to me with everything being talked about here it might be worth ditching the on demand for an ordinary 4500w tank heater. Or maybe even better, a heat pump heater.


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## begreen (Jan 30, 2020)

maple1 said:


> It would seem to me with everything being talked about here it might be worth ditching the on demand for an ordinary 4500w tank heater. Or maybe even better, a heat pump heater.


Agreed. This makes more sense and the resistance heater in that case could be switched off, say during an extended stormy period. As noted above, Poz ran his house all summer on his 6kW inverter. This could also be done with a Tesla PowerWall if one didn't wnat to DIY.


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## maple1 (Jan 30, 2020)

Yes. And I just threw 4500 in there because that's what mine is. Can certainly go less, and still meet needs. I think the next common step down would be 3000w? Not sure.


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## SpaceBus (Jan 30, 2020)

It would be much easier to insulate my current range boiler and add in some kind of indirect heating with a heat pump. I've been watching the air to water heat pumps since leaning about them on this forum. I'd like to keep the wood cook stove integration as well as the rooftop SWH. In the future we want to add a second 40 gallon range boiler in parallel with the current setup to double capacity for a planned large bathtub. I'm also considering having a more robust water coil made for the cookstove, the current coil is just a U shape and I'd like at least one loop.


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## DBoon (Feb 1, 2020)

Why not have the output of the range boiler feed a HPWH? If the cookstove is running, the range boiler water is warm (fall, winter, spring) and if it is not running (summer?) then the range boiler water is cold. Then, the HPWH just picks up the difference. 

I will do something similar with my geothermal heat pump installation. That stores water in a 180 gallon tank, and that tank has a heat exchanger coil in the top. I will run the cold water inlet to the HPWH through the top heat exchanger coil in the 180 gallon geothermal storage tank. When that tank is warm, it pre-heats the water. When not, the HPWH has to do most of the heating.


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## SpaceBus (Feb 1, 2020)

DBoon said:


> Why not have the output of the range boiler feed a HPWH? If the cookstove is running, the range boiler water is warm (fall, winter, spring) and if it is not running (summer?) then the range boiler water is cold. Then, the HPWH just picks up the difference.
> 
> I will do something similar with my geothermal heat pump installation. That stores water in a 180 gallon tank, and that tank has a heat exchanger coil in the top. I will run the cold water inlet to the HPWH through the top heat exchanger coil in the 180 gallon geothermal storage tank. When that tank is warm, it pre-heats the water. When not, the HPWH has to do most of the heating.


I've seen air to water heat pumps set up on this principle, albeit with a smaller tank. I still look longingly at the American Solartechnic unit. Right now I have the outlet of the range boiler feeding the on demand water heater. We don't have a good space in the house for even a 50 gallon HPWH. I guess the builder assumed the rooftop solar collector would be shouldering most of the work for the original 50 gallon resistance heater. I've even considered adding two heating elements to the range boiler to make it a bit of a hybrid. I could also still add a HX coil inside the tank (two open inlets on the side) or make an indirect setup with a water to water HX that is external to the tank (how I plan to integrate the SHW). 
Sorry, kind of rambling, but trying to touch on all of your points. I think long term I'll end up with an air to water HP minisplit to replace my electric baseboard heaters which should also be able to pick up any slack the cookstove or SHW can't handle. The tankless water heater doesn't use much electricity now. The first month I had it installed we used just over 300 KWh for the whole billing cycle, my last bill was $55.


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## maple1 (Feb 1, 2020)

It's not really the overall KWH it uses, it's the huge load it puts on your electric system when it does heat.

Or at least it would be for me. If it fits for you, well, that's good.


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## SpaceBus (Feb 1, 2020)

maple1 said:


> It's not really the overall KWH it uses, it's the huge load it puts on your electric system when it does heat.
> 
> Or at least it would be for me. If it fits for you, well, that's good.


Indeed, this is why I'm looking at a heat pump for the long run. When I installed the current setup, our old water heater elements had rusted away to almost nothing and there was no filtration from the well. It's not perfect yet, but I'm working on it. Hopefully the air to water HP technology will come down in price a bit.
What is the max output of your HP/hot water system? We have a small house, 1,200 sqft, so I'm thinking a 28-32k btu unit would be able to carry the house if necessary and also manage DHW. I'm hoping to not need 180 gallons of storage!


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## DBoon (Feb 2, 2020)

SpaceBus said:


> What is the max output of your HP/hot water system? We have a small house, 1,200 sqft, so I'm thinking a 28-32k btu unit would be able to carry the house if necessary and also manage DHW. I'm hoping to not need 180 gallons of storage!



I bought the smallest that I could find that would provide >120 degree water (my GeoStar is rated up to 140 degrees, though I would like to avoid temperatures above 120 degrees if possible). The rating is 3 tons (36,000 BTU) - twice what I need at 0 degrees F. You don't need a 180 gallon tank - a 2.5 or 3 ton GSHP will keep up nicely to DHW needs (and your house heating needs as well).  I admittedly way oversized my storage tank because:

I wanted to reduce the short cycling cycle of the GSHP as much as possible
I wanted to have the ability to store 4 to 5 hours of heat for my house in the event of a power outage
I wanted to have the ability to benefit from future utility programs in which they would pay me to eliminate high wattage electric loads during certain times of the day (I believe that this will happen in NY state in the next 5 years). 
This tank had both the upper and lower heat exchanger coils, and I thought at one time I would want both to integrate solar hot water panels into the system (I now have no plans to do so). 
Once I purchased a 120 gallon I thought I may as well get the 180 gallon. 
The 180 gallon was the biggest that would fit through a 36" door opening. 
I recall there being a lot of choices in 80 gallon tanks with an upper heat-exchanger. That's what I would recommend for a typical install. Frankly, if I were doing this again, that's probably what I would have gone with. 

Keep in mind that my GSHP is outdoor-reset controlled, so if the outside temperature is 0 degrees the 180 gallon tank (that provides hot water to the radiant floors) will be at about 123 degrees F. At 32 degrees outdoor temperature, the water is more like 100 degrees, and at 50 degrees outdoor temperature the tank temperature is only about 90 degrees. So this is a pre-heater only - you need something else to boost the water temperature for DHW use after that.

But this might be a nice design to feed your on-demand electric water heater if you had a GSHP for home heating. It's a lot cheaper than the de-superheater options for making DHW, and it has a lot more capacity to help with the DHW loads. The nice thing about my tank is that the upper heat-exchanger coil that I'll route my cold water inlet through holds a lot of water (I recall about 9 gallons). So there is a lot of capacity there. And if you are only drawing 1.5 or 2 gallons a minute of hot water, then it should be pretty much infinite capacity at tank temperature. 

In the summer, I'll have no pre-heating of the cold-water inlet since I'll have no call for house heat through the outdoor reset controller on the GSHP. That's ok - I want the HPWH to work harder in the summer and cool/dehumidify the basement.


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## SpaceBus (Feb 2, 2020)

DBoon said:


> I bought the smallest that I could find that would provide >120 degree water (my GeoStar is rated up to 140 degrees, though I would like to avoid temperatures above 120 degrees if possible). The rating is 3 tons (36,000 BTU) - twice what I need at 0 degrees F. You don't need a 180 gallon tank - a 2.5 or 3 ton GSHP will keep up nicely to DHW needs (and your house heating needs as well).  I admittedly way oversized my storage tank because:
> 
> I wanted to reduce the short cycling cycle of the GSHP as much as possible
> I wanted to have the ability to store 4 to 5 hours of heat for my house in the event of a power outage
> ...


My main issue is room for a large tank. Even 80 gallons might be too big. The old water heater was in a utility room that has been repurposed. The 40 gallon range boiler is about the tallest tank I can fit in the current space. A horizontal tank would work and something I have considered. Where do I find these electric hydronic storage tanks?


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## DBoon (Feb 2, 2020)

Just search for geothermal storage tanks or geothermal buffer tanks. 

This company offers smaller sizes with an internal heat exchanger https://www.123zeroenergy.com/pdf-files/solarstor-specs.pdf 

Mine is a Stiebel Eltron https://www.stiebel-eltron-usa.com/sites/default/files/pdf/brochure-indirect-dhw-storage-tanks.pdf

Looks like my upper heat exchanger is only 2.5 gallons. Still, it is rated for basically an infinite continuous draw at whatever gallons/minute that I can practically pull through the tank.


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## SpaceBus (Feb 3, 2020)

DBoon said:


> Just search for geothermal storage tanks or geothermal buffer tanks.
> 
> This company offers smaller sizes with an internal heat exchanger https://www.123zeroenergy.com/pdf-files/solarstor-specs.pdf
> 
> ...


Thanks for the link, I like the 38 gallon tank, and it will fit in my space. I have artificially limited my plumbing inlet to 2.2 gpm so that the tub won't outflow the on demand water heater. The rest of my faucets are less than 1.5 gpm anyway. Thanks for your input.


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## woodgeek (Feb 22, 2020)

Ugh.  I am not a fan of electric on demand heaters at all.  I like a big, well insulated, HP heated tank.  I don't even worry about HW backup, bc the tank stays warm for days and days (and I just ration usage, i.e. navy showers).

As for a battery bank, buy an EV, and get a big (1500W) sine wave inverter for ~$300, and hook it up to the 12V accessory system.  Most EVs can put out 1300W continuous and 2000W peak into 12V.  If its a pure EV (like my old Bolt), you can run all the small (120V) loads in the house for several days, if its a PHEV (like my current Volt) a tank of gas will keep it going for a week or more.

I made up a suicide plug with two hot tie for a dryer plug, which drives the two hots together (shutting off all 240V appliances) and backfeed my house with this rig (I have an interlock on the grid feed).  Between the woodstove, the hot water tank, and a small propane camping stove, I am all set with heat, HW, food and power for several days.  For <$500 total, and unlike my neighbors genny, my rig is silent and I am not slinging big gerry cans of gasoline 2X per day.


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## SpaceBus (Feb 26, 2020)

__





						There's a Simple Way to Store Renewable Energy, And We Already Have The Technology
					

The effect that fossil fuels are having on the climate emergency is driving an international push to use low-carbon sources of energy.




					www.sciencealert.com
				




Perhaps someday this will be scalable.


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## peakbagger (Feb 26, 2020)

I had two projects that were approved for a subsidy for a flow battery, unfortunately both parties decided not to do the projects and returned the rebate. They are pretty slick, there is a fuel cell like device that converts a charged electrolyte from a large tank to electricity, the discharged electrolyte goes to a second large tank. When power is cheap, the device reverses the flow and charges the electrolyte back to the charged tank. The electrolyte base  is reusable for long time (years) and this version was realtively non toxic and didnt need strategic materials. The device does need some basic annual maintenance.  I think it could be scaled sown somewhat but doubt it would get down to home size.


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## begreen (Feb 27, 2020)

They provide a working example at Newcastle University.








						Newcastle University connects first grid-scale pumped heat energy storage system
					

World-first in grid-scale pumped heat energy storage places UK at forefront of energy storage R&D, team claims




					www.theengineer.co.uk


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## woodgeek (Feb 29, 2020)

This is really cool (no pun intended).  I might have the students at my uni do a design project on it next year!


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## SpaceBus (Feb 29, 2020)

Air-to-Water Heat Pump Retrofit - GreenBuildingAdvisor
					

Lessons from a mechanical engineer on replacing a propane boiler with an air-to-water heat pump system.




					www.greenbuildingadvisor.com
				




I'll probably do something very similar to this in a few years. This will make my non wood heat capable of functioning on solar. Currently the "primary" heat is electric baseboard. The above setup should easily heat and cool my 1200 sqft house, provide DHW, and make it work with my existing range boiler/woof cookstove DHW.


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