# Some rough numbers on how to get to a net-zero energy lifestyle



## georgepds (Jul 25, 2013)

Some rough numbers on how to get to a net-zero energy lifestyle

I started thinking a bit about heating with PV electric. I produce more energy than I use with my 4.6 kw PV array. Here in Massachusetts the electric company is happy to credit you for the net export, but they’ll never send you a check … so what to do with all that energy. I thought why not heat the house, or get an electric car

First I needed to estimate how much heat I need. I did this using the rough approximation that I use three chords/ year, and then used the high heat value for a chord of wood (which ranges from 15 to 25 MBT) : 3 chords of wood, at 25 MBTU/chord , is 75 MBTU.

I then estimated the how much electricity I’d need to generate that heat, using a heat pump : If I use a heat pump with a HSPF of 12 ( this is the value for the Fujitsu RLS2 I’m installing) , I require ~6 MWh

This got me to thinking how much energy I’d have to generate to cover the other big energy use : car 

Electric cars get ~3.3 miles/kwh , or 0.3 kwh/mile.. to go 20k miles in a year would require about ~6 MWh 

So I’m starting to get a feel for the partition of energy between normal electric, heating, and auto. Each requires roughly 6 MWh/year

House : ---500 kwh/month ----------12 months ----------~6 MWh /year
Heat : ----75 MBTU/year-------------- HSPF 12 wh/BTU ---~6 MWh/year
Car : -----0.3 kwh/mile ----------------20 k miles /year----- ~ 6 MWh/year

This shows I lead a 18 MWh/year life.. but I could probably get by on half ( I use only ~250 kwh/month, still like to heat with wood, and , when I stop working, should only drive about 10 k miles /year) .

Finally, to estimate how much panel I’d need, I use the PV watt calculator. 

PV watts : http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/US/Massachusetts/Boston.html

The default values yield 1.2 MWH/ year for every 1 kw installed. I actually get a bit better , mostly because the DC to AC conversion is higher with the enphase micro inverters I use. I get about 1.4 MWH/ year for every kw installed ( based on measurements over the past year) .. To cover all my needs I’d have to have a 6.4 kw installed to produce 9 MWh. I’ve got 4.6 kw up on the roof, so I need another 1.8 kw to get to an ~ net zero lifestyle.

A note on cost: After state and federal rebates it cost me ~$2k per kw of PV installed.. so it would be about $12.8k total to get that 6.4 kw. These figures are large, and don’t include the capital cost of a heat pump (~$4k) or car (~$30k) , but they are small compared to what people pay for a house (~$500 k) 

A note on solar thermal panels: I like the air heat panels. There is no water to freeze, and the only moving part is the fan. Each panel gives ~15kBTU/day. But, they only work in the winter. The heat captured in the summer cannot be stored. The net metered solar PV lets me bank the excess in the summer, and use it in the winter. I can put some numbers on it, a solar panel with a heat pump has a net conversion efficiency of 52 % ( 15% for the panel 3.5 for the heat pump COP). The solar heat panel has a ~100% efficiency… but the duty cycle is different. The solar heat panel can only use the energy produced by the sun during the winter, roughly 1/3 of the net energy falling on the panel per year . This gives a net useful production of ~33%. On a best use of space basis, you’re better off using solar PV and a heat pump than using a solar heat panel ( you capture roughly half the energy falling on that space in a year, compared to only one third for the solar air panel) 

Solar air panels: http://www.altestore.com/store/Solar-Air-Heaters/c469/


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## Redbarn (Jul 26, 2013)

Very interesting. You got me doing similar calcs on House + heat but no car.
I've been a Solar Wannabe for a while now but as solar components prices are still falling, procrastination is paying off.


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## bmblank (Jul 26, 2013)

You mention solar air and the inability to store heat, but what about solar hydronic? Storable heat as well as the added benefit of heating domestic water. Hot air is not used in the summer months, but hot water is used year around.


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## Circus (Jul 26, 2013)

georgepds said:


> On a best use of space basis, you’re better off using solar PV and a heat pump than using a solar heat panel ( you capture roughly half the energy falling on that space in a year


 
There isn't an "either or". SHA occupies the south wall while PVs occupy the roof. SHA will cost a DIYer about $100 per 4x8 panel. It is tough (emotionally) cutting large holes into your house and SHA seldom use off the shelf hardware. One off designs don't matter to a DIYer but they drive contractors crazy, raising the cost.


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## georgepds (Jul 27, 2013)

Circus said:


> There isn't an "either or". SHA occupies the south wall while PVs occupy the roof. SHA will cost a DIYer about $100 per 4x8 panel. It is tough (emotionally) cutting large holes into your house and SHA seldom use off the shelf hardware. One off designs don't matter to a DIYer but they drive contractors crazy, raising the cost.





I agree.  I'm now thinking about doing just that..but with commercial panels...studying Nick Pine's experiments there's a lot that can go wrong with D I Y solar thermal...  And then there's the WAF


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## georgepds (Jul 27, 2013)

bmblank said:


> You mention solar air and the inability to store heat, but what about solar hydronic? Storable heat as well as the added benefit of heating domestic water. Hot air is not used in the summer months, but hot water is used year around.


Works well for a day...but not for a season...
And I want to avoid water because of potential freezing


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## bmblank (Jul 27, 2013)

Have you checked out a drain back system? Water drains back leaving the panels dry when it's cold. Heats water when the sun's out, year around, day in, day out. I don't understand the difference between that and heating air when the sun hits.


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## georgepds (Jul 28, 2013)

bmblank said:


> Have you checked out a drain back system? Water drains back leaving the panels dry when it's cold. Heats water when the sun's out, year around, day in, day out. I don't understand the difference between that and heating air when the sun hits.[/quote
> 
> 
> Yes.  .but the air can never freeze..if there is a problem in the drain back..you can get freezing
> ...


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## HDRock (Jul 28, 2013)

The problem is , big money is keeping zero energy lifestyle, out of reach , It is getting better but, Still a long way from what it could, should be


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## woodgeek (Jul 28, 2013)

I think you'll find the commercial solar airheaters have a roughly 50 year simple payback.  

Stated another way, the cost per BTU amortized over a 10-20 yr period is well above the cost of your PV/MSHP combo (or wood, for that matter).   And as you say, the PV pays all year round.

I realize this is a thought experiment, but where would you put another 8 kW of panels?


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## Where2 (Jul 28, 2013)

HDRock said:


> The problem is , big money is keeping zero energy lifestyle, out of reach , It is getting better but, Still a long way from what it could, should be


 
I said that about PV in the spring of 2011, when I was taking an "Intro to Solar Energy" class at the local community college. Six months later, I bought 4400W of panels. Now that my 4400W array is simply awaiting an electric meter swap, I've started plans for the remaining PV necessary to take my all electric house to ~ net-zero.


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## georgepds (Jul 28, 2013)

Well..I've got 4.5 K W up on the roof...I figure another 2.5 upon the shed for a total of 7  K W

That should bring in ~9..8 M W H. per year..or 3 M W H for normal use and the other 6   for heat and 0.8 for spare use


B T W. ..this is not theoretical at all...the Fujitsu 12l R LS 2 went in on
Friday works like a charm..this leaves me  only the panels on the shed to meet the goal 

The Fujitsu heat pump has an HSPF of 12


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## georgepds (Jul 28, 2013)

woodgeek said:


> I think you'll find the commercial solar airheaters have a roughly 50 year simple payback.
> 
> Stated another way, the cost per BTU amortized over a 10-20 yr period is well above the cost of your PV/MSHP combo (or wood, for that matter).   And as you say, the PV pays all year round.
> 
> I realize this is a thought experiment, but where would you put another 8 kW of panels?



Can you please tell me how you calculate the 50 year payback?


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## woodgeek (Jul 28, 2013)

the 2-panel air heater on your link has a stated rating of 40 kBTU_thermal/clear sunny day. The single 4x8-ish panel thus has a nominal collection of 20,000 BTU/clear sunny day. This is equivalent to the 6 hour * peak rate daily harvest for any flat plat collector, assuming a 40-ish% eff, I guess. How many clear sunny days do you get during heating season? Let's suppose (optimistically) it is 100 days/year. That one panel will yield 2 Million BTU/season. This is maybe 3% of your demand. In your other thread, you computed the minis give BTUs at $13/ MBTU. Each of those panels will offset $26 per year in heat. And they cost >$1700 delivered. 1700/26 = 65 years simple payback, if you price versus MSHP or wood BTUs.


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## georgepds (Jul 31, 2013)

[quote="woodgeek, post: 1482233, member: 4013.. 
I realize this is a thought experiment, but where would you put another 8 kW of panels?[/quote]

 I ran some numbers.. with the 4.62 kw on the roof I make about 6 MWh a year (according to pvwatts &  measurement) This turns out to be ~ 6 MWh/ year or  72 MBTU using  the 12 HSPF heat pump I've installed. 

I plotted out the monthly and cumulative production and requriements.. it looks like I'm there already.. I can heat with the 4.62 peak kw pv and the 12 HSPF heat pump. I figure I still need another 2.5 kw to produce the 3 MWh needed for the house electricity. The heating requirements come from the deg-day listed at NREL






The monthly results show I produce most of the energy in the summer.. so net metering is crucial if you want to go this way.. This also suggests it's a bad idea to go air solar thermal.. there just is not enough sun around in the winter, and there is no way to store it over the  summer





This curve shows the cumulative production/requirements. The cross over between solar production and heat needs is around June ( if I start counting in January)

I still need about 3 MWh / year for the ordinary electric needs of the house. I can get this if I install another 2.5 kw on the shed

This whole calculation strikes me a pretty remarkable.. Until I did it I did not think that solar pv could be used to heat a house.After rebates I spent $9240 on the PV, and another $3000 on the heat pump.

There is still one caveat.. peak daily requirement for heat is about 485 kbtu/day  in January, or about  20 kbtu/hr... The little heat pump can only put out 16 kbtu/hr max.. so it just does not have enough oompph to heat in January, even if I do have the electricity.. I'd need another 4kbtu/hr source


Looks like I'll have to fire up the woodstove in any event


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## Circus (Aug 2, 2013)

georgepds said:


> This also suggests it's a bad idea to go air solar thermal.. there just is not enough sun around in the winter, and there is no way to store it over the summer​


 
I disagree with all your conclusions. Your method is the most complicated, expensive, unreliable, subsidized and regulated.
I've accomplished 70% of what you've done by using the "bad idea". Pay back for the solar air was less than a year while the solar water took two. Built and installed five years ago, I haven't as much as washed it since. No one dictating what I can do. No body threatening me with contracts.


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## woodgeek (Aug 2, 2013)

Circus said:


> I disagree with all your conclusions. Your method is the most complicated, expensive, unreliable, subsidized and regulated.
> I've accomplished 70% of what you've done by using the "bad idea". Pay back for the solar air was less than a year while the solar water took two. Built and installed five years ago, I haven't as much as washed it since. No one dictating what I can do. No body threatening me with contracts.


 
Glad to hear you had a good experience with DIY solar.  I am sure reasonable ROI can be achieved DIY in many circumstances.  Would you care to share output figures?  What solar fraction is your space heating and DHW?  How much have your efforts reduced your net energy or carbon footprint?


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## Circus (Aug 2, 2013)

woodgeek said:


> What solar fraction is your space heating and DHW? How much have your efforts reduced your net energy or carbon footprint?​


 
SHA doesn't help much in Nov. and Dec. due to clouds. The clouds come from the damp, temperate Gulf of Mexico. SHA helps a lot in Jan. and Feb. due to sun. The sun comes from dry, sub zero Canada. So when the SHA is needed the most, it works the best.
An electric clock spliced in the furnace records run time. 110 sq ft SHA heats 1400 sq. ft.. 2 liter bottles of water stacked against a wall buffers daily temperature swings.
My larger collector heats a downstairs bedroom/bottles up to 100 degrees. At 9pm or so the bedroom door is opened (heating the house). By bedtime the bedroom has cooled to a pleasant temperature. _Solar hot air replaced 50% of the fuel._
The 80 sq ft of SHW supplies all the hot water needed except for Nov. thru Jan. My 6/12 roof isn't steep enough for the winter sun angle. Oh well, people don't need to bath as often (stink so bad) in the winter. _Solar hot water replaced 75% of the fuel._
As for carbon footprint, most PVs never offset the carbon needed to build, pay bribes (certify) and install them. Heat pumps produce the same carbon as coal heat after figuring the 66% line loss from the coal fired power plant.


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## woodgeek (Aug 2, 2013)

Circus said:


> SHA doesn't help much in Nov. and Dec. due to clouds. The clouds come from the damp, temperate Gulf of Mexico. SHA helps a lot in Jan. and Feb. due to sun. The sun comes from dry, sub zero Canada. So when the SHA is needed the most, it works the best.
> An electric clock spliced in the furnace records run time. 110 sq ft SHA heats 1400 sq. ft.. 2 liter bottles of water stacked against a wall buffers daily temperature swings.
> My larger collector heats a downstairs bedroom/bottles up to 100 degrees. At 9pm or so the bedroom door is opened (heating the house). By bedtime the bedroom has cooled to a pleasant temperature. _Solar hot air replaced 50% of the fuel._
> The 80 sq ft of SHW supplies all the hot water needed except for Nov. thru Jan. My 6/12 roof isn't steep enough for the winter sun angle. Oh well, people don't need to bath as often (stink so bad) in the winter. _Solar hot water replaced 75% of the fuel._


 
Using Gary's numbers 130 kBTU/sqft.yr, your SHA would yield 110*0.13 = 14.3 MMBTU/yr. 50% fuel replacement indicates you only use 14 MMBTU fossil energy for heating. Sounds like you have made your home very energy efficient (needing only 29 MMBTU/yr) as well as solar equipped.



Circus said:


> As for carbon footprint, most PVs never offset the carbon needed to build, pay bribes (certify) and install them. Heat pumps produce the same carbon as coal heat after figuring the 66% line loss from the coal fired power plant.


 
The carbon footprint of PV is significantly negative: http://mediamatters.org/research/2013/01/24/myths-and-facts-about-solar-energy/192364

66% losses are not realistic for grid electricity. In the US, the figure is closer to 6.6%. Of course, a thermal coal plant powering a ASHP would have a similar carbon footprint as burning coal in a stoker for heat, due to thermodynamic losses associated with electricity production roughly balancing the thermodynamic gains in COP associate with HP technology. However, at the typical carbon intensities of the US grid (1-1.5 lbs CO2/kWh), an ASHP has a far lower carbon footprint than a coal stoker, and below that of fuel oil or propane in a conventional furnace. And in many markets, you can buy renewable electricity at a small or zero surcharge, allowing the ASHP to have a lower footprint than 50% fraction SHA.


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## Circus (Aug 2, 2013)

woodgeek said:


> Using Gary's numbers 130 kBTU/sqft.yr, your SHA would yield 110*0.13 = 14.3 MMBTU/yr​


 
Hat's off to Gary for figuring out how to measure heat output from a SHA collector. It can't be easy. Beyond me.
I think of a collector as a south facing window during the day and a insulated wall at night when determining the heat produced.


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## georgepds (Aug 4, 2013)

Circus said:


> I disagree with all your conclusions. Your method is the most complicated, expensive, unreliable, subsidized and regulated.
> I've accomplished 70% of what you've done by using the "bad idea". Pay back for the solar air was less than a year while the solar water took two. Built and installed five years ago, I haven't as much as washed it since.....


 

OK... good for you.. glad to hear it.. but How?
How big is your solar thermal?
What did it cost?
What does it look like?
What are the  house BTU requirements per year,per month?
How much does the solar thermal provide?
Exactly how did you achieve one year payback?

"_Solar hot air replaced 50% of the fuel."_

That's great.. how much fuel do you use in a year up in Wisconsin_?_

If we use Woodgeek's numbers , you are using 29 MBTU / year.. that's remarkable for a 1400 ft^2 house up in Wisconsin, you must have really gone to town on the insulation. I live in a 1500 sq foot house with 6" of insulation everywhere, and , allowing for thermal bridging, get only R13 on the walls and ceiling. I mange to use ~75 MBTU_, a_lmost 3x the amount you do, and IIRC, we live in similar Class 5 heating and cooling zones (Wisconsin/ Massachusetts) 

Myself, I don't have a furnace, I use a wood stove... a furnace would be really helpful here.. price of oil notwithstanding

And then again.. it is a question of style.. ". .. 2 liter bottles of water stacked against a wall buffers daily temperature swings."  Some of us live with others who limit this kind of experimentation with their bedroom space.. not that I have any objection to it myself.

But again, that's a style question, you might find it odd, or even impossible,  to live without a furnace



Circus said:


> .. No one dictating what I can do. No body threatening me with contracts.


 
Who is dictating to you and threatening you ?  What kind of contract are we talking about?  And what does this have to do with solar PV, solar thermal?


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## georgepds (Aug 4, 2013)

Circus said:


> ....
> As for carbon footprint, most PVs never offset the carbon needed to build, pay bribes (certify) and install them..


 
That is not correct.. take a look at woodgeek's reference :_NREL also found that a solar PV system can repay its "energy investment in about two years,"_

http://mediamatters.org/research/2013/01/24/myths-and-facts-about-solar-energy/192364




Circus said:


> ....
> Heat pumps produce the same carbon as coal heat after figuring the 66% line loss from the coal fired power plant.


 
That's not correct either. First.. you seem to forget the electricity here is coming from solar PV .. that has exactly zero recurring CO2 production

If you're interested in CO2 associated with energy from the electric company.. Wilson et all Consumer guide to home energy savings notes the following lbs of CO2 per million BTU

Coal ( direct combustion) 210
Electricity ( national average, includes the 66% loss you quote) 460

The heat pump has a HSPF of 12, which corresponds to a seasonally adjusted COP of 3.5.. Dividing the 460/3.5 = 130 lbs of CO2 using the heat pump plugged into the wall .. This is less than that of direct combustion coal.



Circus said:


> ....
> As for carbon footprint, most PVs never offset the carbon needed to build, pay bribes (certify) and install them..


 
Umm.. you going to have to help me here, what bribes?


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## georgepds (Aug 4, 2013)

Circus said:


> Hat's off to Gary for figuring out how to measure heat output from a SHA collector. It can't be easy. Beyond me
> .
> I think of a collector as a south facing window during the day and a insulated wall at night when determining the heat produced.


 
Well, it's not beyond me.. Here is how to do it. Look up the average incident solar radiation per sq meter for a vertical south facing panel in Madison Wi, see NREL

http://rredc.nrel.gov/solar/old_data/nsrdb/1961-1990/redbook/sum2/state.html

Look up the monthly PV power in kwh/m^2 for ., convert to kBTU, multiply times the number of days in the month, and then add up the heating months from November through April , inclusive. Finally, multiply the number by 10 ( a 110 ft^2 collector is a~10 m^2)

What you get is 20 MBTU available per yer ( assuming 100% efficiency and no shading).

Now look up the best efficiency for the best solar thermal air collector and you find 93%, or 18.7 MBTU / year http://en.wikipedia.org/wiki/Solar_thermal_collector#Air

It's remarkable that you can heat your house with so little energy in Wisconsin (~37 MBTU / year).. tell us what you do for insulation and infiltration..

The same wikipedia reference notes "Payback for glazed solar air heating panels can be less than 9–15 years depending on the fuel being replaced.".. which is a different from your 1 year payback claim.


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## georgepds (Aug 4, 2013)

georgepds said:


> ....
> 
> What you get is 20 kBTU available per yer ( assuming 100% efficiency and no shading).
> 
> Now look up the best efficiency for the best solar thermal air collector and you find 93%, or 18.7 kBTU / year....


 
Just to put this number into perspective 1 gal of # 2 diesel contains 139 kbtu of energy.. assuming that furnace of yours is 100% efficient, and uses just as much energy as the solar thermal panel, that means you used 135 gallons of fuel (139/18.7)

If, per chance, you used more than that, you might want to reconsider your figures.


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## Circus (Aug 5, 2013)

georgepds said:


> that means you used 135 gallons of fuel (139/18.7)​


 
That a little pessimistic
That's the problem with sizing SHA, there's so many variables. In my case the 2 story building, with conventional insulation and 130 sq. ft of south facing glazing (some triple glazed), was efficient before any solar was done. iow. Low fuel use baseline.

I like to think my solar motivation was benevolent or financial but it wasn't. I was angry at my LP supplier for charging me $3.50 per gal while LP was $.90 wholesale. $1400 to fill my tank!? Now when the price spikes I'm happy, just like when I fuel my Spitfire next to the Ford Behemoth.

Check the requirements before investing in PV. Green legislation is being undone. Some utilities are discontinuing net metering, ending any surplus carryover and they're raising liability requirements.
. example: "Unnamed corporation" is currently changing past agreements. Corporations are basically evil monopolies and should be avoided.


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## woodgeek (Aug 5, 2013)

Anger can def be a motivation for positive change.  I was angry at my oil boiler for trying to bankrupt me and kill my kids.  Now its scrap.


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## georgepds (Aug 5, 2013)

Circus said:


> ....iow. Low fuel use baseline.
> 
> I like to think my solar motivation was benevolent or financial but it wasn't. I was angry at my LP supplier for charging me $3.50 per gal while LP was $.90 wholesale. $1400 to fill my tank!? ....


 

OK.. so you use propane as a fuel.. how much did you use last winter? How much did you use before you put in solar thermal panels?

If it was 1 tank, than you used 400 gallons, or 36.4 MBTU ( 1 gallon of propane is 91 kBTU), more energy than is possible with that 110 ft^2 of south facing unshaded solar thermal

I think you probably used more.. the 18.7 from the solar , plus the 36.4 from a full tank of propane, adds up to 55 MBTU.. that's low for a normally insulated house is a class 5 cooling/heating zone



Circus said:


> ...example: "Unnamed corporation" is currently changing past agreements. Corporations are basically evil monopolies and should be avoided....


 
What unamed corporation?

How does one post on the internet without dealing with a corporation?


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## woodgeek (Aug 5, 2013)

The cute little car seems to have gotten George's ire up. We have these disputes all the time....which is greener, conservation (turning down the stat) or efficiency (more insulation), DIY solar (low material use and no govt incentive) or net zero PV (greater CO2 reduction than DIY, but more complex/costly)?

Not really an either/or. Just part of a bigger problem, which is IMO all 'mental'. Every positive action we take can also be an excuse or cop-out for not doing something else. If someone turns their stat down to 60°F and puts on a sweater, half of us will scoff (for being uncomfortable for no reason). If someone else improves their house/heater to save the same amount at 70°F, the other half will scoff (for still 'wasting' some energy). If someone does 'all of the above' then they are heroes and we would all respect them.

And then there are the folks buying shiny green toys and driving to the recycling center in hybrid SUVs, and still using more any energy than any of us while being smug.

If you think 'green' is not being widely adopted b/c of cost, then you probably favor the DIY/conservation side. Conversely, I live in a high rent district where everyone could energy retrofit their big ol houses with the money in their sofa cushions, but they don't because they don't know any better or are misinformed, or wrongly equate green with discomfort/deprivation (i.e. conservation). I think we here can all be role models for our different communities.


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## woodgeek (Aug 5, 2013)

As someone under 50, I was a kid during the last 'energy crisis' and spent countless hours designing/daydreaming about how I was going to build a passive solar house in the future to survive some energy apocalypse. I would've gone big for earthships and massive amounts of south facing glass and thermal mass, solar DHW, and riding out some temp swings. And I would do it all DIY and it would work great.

When my dad poured a ton of insulation in the attic in 1980 I was a super fan. 30 years later I realize he should've airsealed that attic first to get bigger savings, airsealed his rim joists to avoid the dehumidifier in the basement, and that asbestos filled vermiculite was not a good insulation to use! That's called progress.

30 years later we have affordable wind utility power, affordable rooftop PV, and can build superinsulated and net-zero houses (with an appropriate amount of passive solar gain) using off the shelf technology. These houses are more comfortable than conventional houses from the 1970s, and use far less energy and CO2 than the passive solar house experiments of the 1970s and 80s. And unlike those earthships, such houses COULD be adopted widely and become a new normal, now that would be real progress.


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## Circus (Aug 5, 2013)

Don't get me wrong, I'd like to buy a grid-tie PV system. But only large enough to break even every month. Energy harvested in the summer might not be savable for winter use.



georgepds said:


> What unamed corporation?​


 
http://northwindre.com/2013-wps-rate-case/


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## StihlHead (Aug 5, 2013)

Circus said:


> Don't get me wrong, I'd like to buy a grid-tie PV system. But only large enough to break even every month. Energy harvested in the summer might not be savable for winter use.


 
Here in the west they only allow you about break-even energy PV panels if you tie to the grid, based on previous use. So it is hard to scale up if you are going to convert to something like an electric HP from oil, LP or wood. The issue with the PV grid connection is that it still requires a grid and an on-demand source of grid tied energy to be available 27/7: meaning oil, NG, coal, wood, nuke or hydro power. Also with any energy grid-tied system, burning oil, NG, coal, or wood on site is always going to be far more efficient than producing electricity and distributing it on the grid and then converting it back to heat, even with a HP. From the perspective of the engineer at the power plants trying to balance the loading of a grid with all these multiple power sources (all these tied mini PV and hydro systems with large electric plants) it can be a royal nightmare. In San Diego we simply used Mexico as a shunt. If there was an excess it went south, and if there was not enough we simply cut them off.

As for solar hot water systems, they do not always leak. Nor do you need water bottles and exotic storage systems. A simple water storage tank is highly effective. I installed many solar systems during the 70s/80s and lived with several different solar water heating systems. Depending on the latitude that you live at, they can produce space heat as well as DHW. I installed many chill chasers that were very effective, year round. The return on them is usually going to me far shorter than PV, they are essentially 'off-grid' and localized and they can be very effective and efficient.

Also if you want to play mind games with this stuff, why not move to a warmer climate that does not require a lot of winter heating and use a PV system for A/C when solar power is available? Or why not live in a cave like a lot of people do in places like Utah, and never need heat or A/C? Also insulate the crap out of wherever you live to reduce the energy needed to heat and cool the place. And in the longer run, what difference will any of this stuff make if we keep reproducing at the current rate? At some point, energy demand will strip energy supply, no matter how efficient, effective, or creative we get.


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## georgepds (Aug 5, 2013)

You know.. I'd like to install solar thermal ( especially solar air, which does not have the freezing problems) , but every time I do the numbers.. they come out low

For someone else's opinion, try here.. http://www.greenbuildingadvisor.com/blogs/dept/musings/solar-thermal-dead

_"Green builders have an emotional connection to solar hot water systems, because they represent a fairly simple technology that's been around for over 100 years. But it's time to admit that a PV array is cheaper and less troublesome than fluid-filled solar collectors on your roof"_

He's mostly interested in cost ,and does list conditions under which solar thermal is appropriate ( none of which apply to those of us who live close to the Canadian border)


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## georgepds (Aug 5, 2013)

Circus said:


> Don't get me wrong, I'd like to buy a grid-tie PV system. But only large enough to break even every month. Energy harvested in the summer might not be savable for winter use.
> 
> http://northwindre.com/2013-wps-rate-case/


 
Thanks , that helps. It appears net metering is dead in Wisconsin . The state government in Wisconsin appears to be filled with wingnuts these days, I'd blame them for passing this rate law as much as the electric utilities who promoted it.

The political environment is a bit different here in Massachusetts (though the law could be changed), and the Net metering support by state officials much stronger


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## georgepds (Aug 5, 2013)

StihlHead said:


> ....Also if you want to play mind games with this stuff, why not move to a warmer climate that does not require a lot of winter heating and use a PV system for A/C when solar power is available? .....


 
If by mind games, you mean theoretical exercise, that's not what this is for me. I'm trying to implement a practical system exploiting the advantage local laws on net metering give. I currently generate about 3 MWh more than I need, and the heat pump will absorb that over production ( in a time shifted manner) . I'm trying to decide if I should install another 2.5 kV of solar to make up for the other 3 MWH to heat the house

As to why not move.. Well I like it here. I spent all last weekend kayaking in the great salt marsh,chasing cormorants, terns and egrets. Sure it's cold in the winter, but the work is good, and the play is better, and with the net metering , as implemented here, I can offset the cost of the cold. I also like the great desserts in Arizona , New Mexico, and Nevada too, and spent quite a lot of time working in them (mostly near Roswell, Barstow and Yuma) .. but I was born on the Atlantic, grew up on the shore,  and feel out of place if I bed more than 1/4 mile from the ocean.

Also.. despite my current fascination with solar PV heat, I like to heat with wood in the winter.. there is little like it, and that, more than anything else, will probably limit the size of the solar PV I install


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## georgepds (Aug 5, 2013)

georgepds said:


> Thanks , that helps. It appears net metering is dead in Wisconsin . The state government in Wisconsin appears to be filled with wingnuts these days, I'd blame them for passing this rate law as much as the electric utilities who promoted it.
> 
> The political environment is a bit different here in Massachusetts (though the law could be changed), and the Net metering support by state officials much stronger


 

The rejection of NM in Wi is a proposal, not the law, Wi allows for net metering

http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=WI03R

_Net Excess Generation: Varies by utility; Generally credited to customer's next bill at retail rate for renewables and avoided-cost rate for non-renewables ...Credit is carried over monthly and reconciled annually, at the avoided-cost rate_

Not sure of this , but I suspect this means that they sell you electricity at a higher price than they credit you for.. not a good thing if you want to heat your house with the energy you generated in the summer using solar PV and a heat pump


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## StihlHead (Aug 6, 2013)

georgepds said:


> For someone else's opinion, try here.. http://www.greenbuildingadvisor.com/blogs/dept/musings/solar-thermal-dead
> 
> _"Green builders have an emotional connection to solar hot water systems, because they represent a fairly simple technology that's been around for over 100 years. But it's time to admit that a PV array is cheaper and less troublesome than fluid-filled solar collectors on your roof"_


 
I have no real attachment or emotional connection to solar hot water (SHW) systems, but I have installed a lot of them. I also have a degree in electrical engineering and some experience with steam power generation and the grid.

I disagree with a lot of his statements and assumptions (of course he makes some counter statements at the end). Particularly the cost differences (SHW is still about half the cost of a PV system, especially if it is going to be on grid), his statements about SHW maintenance issues, and freeze protection issues. I never had a SHW system freeze up, ever. In either the ones that I installed or owned (in Northern California and Oregon, down to 8 deg. F.). Also the maintenance was all of 2 hours a year for me on any system. Cleaning the panels, checking/calking the roof seals, testing the circuits, looking for leaks. There was no need to drain any drain-down systems. Certainly SHW systems are better the farther south you are, and better below the 45th parallel. But they are likely better for most off-grid and even a lot of on grid systems, w/o the need for inverters, a much larger PV panel array, a two-way meter, a grid connection, or a heat pump water heater. Also he fails to mention some of the faults in PV: the fact that they degrade over time and the issues with PV array production quality lately. Also the drawback for either system here in the PWN is the fact that in winter PV and SHW panels are going to be rather ineffective at generating any hot water.

In the future costs may likely come down enough to make a PV system economical just for heating with. Comparing these two systems though, wood space heating is far cheaper for me. Water heating costs me all of $25 a month in electricity. For a SHW system costing $8k, the return would take 27 years to re-coop the cost, so why bother. That is if I were live here for another 27 years, which is highly unlikely. I am not even likely to live that long. The cost of a PV system would be at least twice that, which males payback beyond the likely life of the PV system, or even this house or myself (similar to the SHW system, but far more expensive). Both systems would qualify for rebates and tax incentives here in this area.

And to counter your posted SHW vs. PV web opinion, I offer this one, which I think is far more realistic:

http://www.homepower.com/articles/solar-electricity/design-installation/pv-vs-solar-water-heating


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## StihlHead (Aug 6, 2013)

georgepds said:


> Not sure of this , but I suspect this means that they sell you electricity at a higher price than they credit you for.. not a good thing if you want to heat your house with the energy you generated in the summer using solar PV and a heat pump


 
Yes, any power company is going to pay less for the excess energy than they will charge you for it, and you will also have to pay for transmission and distribution fees for the electricity used (the cost of maintaining the grid), and any added taxes on the electricity in your area. At least that is the way it is here. So you are going to have to generate more electricity in summer than what you use for winter heating.


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## georgepds (Aug 6, 2013)

I'm beginning to see that Massachusetts is a special place, with respect to Solar PV

First capital cost.. you can have PV installed for ~$4/watt here (not the $8 to $10 in the HP article). Next there are the rebates : 30% federal, 15% state tax credit up to $1000, Mass CEC install incentive of $0.40/watt up to 5 kW (or $0.80/watt up to 5 kW if your house assesment is less than the median for your county).. What this does is reduce capital costs, installed , to ~$2/ Watt

Next recurring cost: There is the credit from the electric company.. it 's a full credit , and includes all those other charges that Stihlhead mentioned.. in fact it's so good they divide it into three parts
Net Met Cr Other $0.09543 per kwh
Net Met Cr $0.03539 first 600 kwh
Net Met Cr $0.04018 next 600 kwh
That last one is to reflect the fact that if you used more than 600 kwh in a month, they would charge you a bit more.. so if you export more than 600 kwh / month they will pay you a bit more

Finally, there is the SREC market.. For each MWH you generate, you get a SREC credit you can sell, this is currently ~$240/ credit, and the right to sell credits is guaranteed for 10 years (and at my house each kW installed generates about 1.4 MWH/year)

The net effect is to bring payback period down to ~4 or 5 years.. after that it is all gravy.. My equipment is warrantied to 25 years, with panel degradation guaranteed at less than 10%/ decade.. I use enphase microinverters.. this allows me to monitor each panel separately.. and if one goes bad.. I know which one, and the rest keep on working.. On the whole.. the system will outlive me

So the hitch is the electric compay will be happy to credit you, but will never send you a check. What that means is that most people install to cover what they use (~6 MWH/year or, roughly, 5 kW). But PV finances are so good, it's worth the price even if the electric company does not pay you back ( it increases payback by a factor of two) .. What I'm trying to do here is find other places to use that electric company credit to offset costs that I would have to pay anyways ( heat for example) .. and so long as I'm doing it, I might as well use a heat pump to squeeze the most benefit from the credit


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## semipro (Aug 7, 2013)

georgepds said:


> You know.. I'd like to install solar thermal ( especially solar air, which does not have the freezing problems) , but every time I do the numbers.. they come out low
> 
> For someone else's opinion, try here.. http://www.greenbuildingadvisor.com/blogs/dept/musings/solar-thermal-dead
> 
> ...


 
Check out the simple drainback systems at Gary's site builditsolar.com.  Gary is a regular here.  
Solar thermals still pays if you DIY.


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## jebatty (Aug 11, 2013)

It's a bit of bummer for me on a proposed solar PV install. First, for net metering the power company requires a variety of disconnects and installation procedures which raise the cost, and second, for unshaded solar I need to do a ground mount with concrete footings installed a fair distance from the house connection point, which involves trenching under a road and a long cable run. Installed cost/watt is about $4.70 for 7.95KW before credits. The only credit is the federal tax credit, no state credits or incentives. The plus side is that the net metering law requires the power company to credit/pay me at the full retail rate, a complete off-set to what we use.


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## DevilsBrew (Aug 11, 2013)

semipro said:


> Check out the simple drainback systems at Gary's site builditsolar.com. Gary is a regular here.
> Solar thermals still pays if you DIY.


 
I've visited builditsolar plenty of times. Great site.


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