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

Posted By georgepds, Jul 25, 2013 at 5:17 PM

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1. #1

### georgepds Burning Hunk

Nov 25, 2012
195
39
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.

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/

2. #2

### Redbarn Burning Hunk

Jul 30, 2012
234
91
Loc:
SE PA
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.

3. #3

### bmblank Minister of Fire

Jan 17, 2013
698
257
Loc:
Michigan
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.

4. #4

### Circus Member

Jan 11, 2013
216
23
Loc:
EC Wisconsin
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.

5. #5

### georgepds Burning Hunk

Nov 25, 2012
195
39

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

6. #6

### georgepds Burning Hunk

Nov 25, 2012
195
39
Works well for a day...but not for a season...
And I want to avoid water because of potential freezing

7. #7

### bmblank Minister of Fire

Jan 17, 2013
698
257
Loc:
Michigan
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.

8. #8

Nov 25, 2012
195
39

9. #9

### HDRock Minister of Fire

Oct 25, 2012
2,239
859
Loc:
Grand Blanc, Mi
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

10. #10

### woodgeek Minister of Fire

Jan 27, 2008
3,538
609
Loc:
SE PA
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?

11. #11

### Where2 Feeling the Heat

Feb 3, 2013
257
43
Loc:
South Florida
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.

12. #12

### georgepds Burning Hunk

Nov 25, 2012
195
39
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

woodgeek likes this.
13. #13

### georgepds Burning Hunk

Nov 25, 2012
195
39
Can you please tell me how you calculate the 50 year payback?

14. #14

### woodgeek Minister of Fire

Jan 27, 2008
3,538
609
Loc:
SE PA
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.

15. #15

### georgepds Burning Hunk

Nov 25, 2012
195
39
[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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16. #16

### Circus Member

Jan 11, 2013
216
23
Loc:
EC Wisconsin
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.

17. #17

### woodgeek Minister of Fire

Jan 27, 2008
3,538
609
Loc:
SE PA
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?

18. #18

### Circus Member

Jan 11, 2013
216
23
Loc:
EC Wisconsin
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.

19. #19

### woodgeek Minister of Fire

Jan 27, 2008
3,538
609
Loc:
SE PA
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.

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.

20. #20

### Circus Member

Jan 11, 2013
216
23
Loc:
EC Wisconsin
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.

21. #21

### georgepds Burning Hunk

Nov 25, 2012
195
39

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, almost 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

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?

22. #22

### georgepds Burning Hunk

Nov 25, 2012
195
39
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,"

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.

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

23. #23

### georgepds Burning Hunk

Nov 25, 2012
195
39
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.

24. #24

### georgepds Burning Hunk

Nov 25, 2012
195
39
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.

25. #25

### Circus Member

Jan 11, 2013
216
23
Loc:
EC Wisconsin
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.