Category Archives: solar

Simple Solar PV math

First: assumptions:

1. Price of grid electricity. In MA, it can be kinda expensive. And let’s assume you are doing GREEN UP or NEW ENGLAND WIND FUND to make you 100% renewables. Let’s guesstimate $0.20. That might be low, but whatever. It will also increase at rate of inflation. say 3%.

2. Price of solar panels installed. I think it might be even cheaper than this now, but let’s say it’s $4000/KW peak (what the panels are rated)

3. Output of said panels in an average year. I believe in New England, assuming maybe 90% sun (maybe w/ microinverters) and roughly S facing, you can assume 1200 KWh/KW peak. So if you install 1KW of panels, you will get 1200 KWh/year.

4. Borrowing money at 5% for 30 years.

5. Panels will pretty much work with no maintenance (maybe a new inverter) for 30 years. They have a warranty nearly that long. And likely for many more. But we can ignore that.

Second: Calculation:

$4000 at 5% fixed 30 years is $258/year
… EXCEL: = 12 * PMT(5/1200,30*12,4000)

And so that is

$258 / 1200 KW = $0.215/KWh for that electricity in year one

Thirdly: what does that mean exactly?

We are basically at “GRID PARITY” pricing with PVs here in New England from day zero and year one, and…
1. Even assuming NO TAX BREAKS, which there actually are.
2. And things will just keep getting better and better as inflation happens. Even assuming you get costs of living increases at your job which help you keep pace with the equally increasing fossil fuel prices, with the solar, you are locked in to 2013 prices for 30 (or more) years!

Forthly: Comments and Conclusion

One complication is that people move every 7 years I think I’ve heard. So the problem there is that the buyer of your solarized home will not understand all of this wonderful stuff, and solar PVs will be even cheaper 7 years from now, so when you sell, you won’t be able to sell the house for much more with the panels. Maybe a tiny bit. Maybe. And you will still have your extra 5% loan for the PVs to pay off.

Which is why I still think it might be most conservative to do GREEN UP (or similar) or NEW ENGLAND WIND FUND and get your 100% renewables that way.

And buy a Prius when it is time to buy a new car. And eat less meat. These 2 have been shown (calculations again!) to contribute as much to reducing CO2 as solar panels do. And for many situations they also cost less! And they aren’t attached to the house, so they can come with you when you move.

On the other hand, there are many reasons to do things in life besides money. Most of us live in houses, buy cars, and build kitchens… all far fancier than we NEED. So then… so WHAT if you add some fancy solar panels to the house. If you can afford it, then go for it. They are cool. And the kids will learn a lot from it.

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Our oil problem

“Our oil problem is not that “we’re running out.” Our oil problem is that we’re producing so much of the stuff that we are changing the planet’s climate.”
— David Frum, ‘Peak oil’ doomsayers proved wrong
LINK

True on climate, but the commenters at the article have it right:

“[W]e are extraordinarily blessed with a moment of respite to temporarily postpone the extremely difficult economic environment brought on by the decline of abundant oil….but it is only temporary, and we would be wise to use this moment to prepare ourselves.”

My comments: Keep bringing on the bikes, insulation, and solar. :-)

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Some day houses for sale will have MPG stickers on them

There is sorta such a thing today.  It’s called HERS.  100 is a “normal” house.  0 is a “net zero energy” house.  And negative means you have even more PVs than you need.  Nice.  Someday websites like realtor.com, redfin.com, etc. will let you search on such things.

The best house I’ve seen is this -33 HERS of Carolyn and Kyle Cave in Hadley, MA.   It’s also nice to know what a house is pre-PV to get an idea of how efficient the house and it’s occupants are.  Oh, and a house in Maynard MA is -8 HERS.

Anyway, good work Caves!  Your house follows the important rule of thumb I now encourage people to use — Build (or pick)  your house with a lot of good roof space for PVs.  Small footprint houses like ours are a little more efficient, but we don’t have nearly as much room for PVs.  Dumb.

(Oh, and our house is nearly 0 HERS.  I am not exactly sure what — I forgot to ask for the pre-PV score and if I recall correctly the PV offset used the wrong KW total.)

(Oh, and read about the limitations of HERS at the link at top…)

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To figure out how much electricity/energy we use each month…

Like many/most people who have grid-tied PV solar panels…. to figure out how much electricity/energy we use each month I have to do some math.  That’s because the smart meter doesn’t know how much electricity we use directly from the panels.  Some electrons never even hits the electric co’s meter, which can only show numbers for 2 things: (1) the extra KWh flowing out and (2) the extra KWh we need that is coming in (at nights, clouds and cold cold weather)

That’s not enough.  I also have to read (3) the total produced by our PV solar panels.  And then do some math.  The pain in this is that since one’s electricity bill is usually not calendar month, and our smart meter isn’t being read automatically by any device, I have to remember to “read the meter” near the beginning/end of the month.  I can’t use numbers on my electricity bill.

Algebra:

The basic idea:  IN KWh = OUT KWh

(2) ELECTRIC CO METER IN (FROM GRID) +  (3) PV PRODUCED = (1) ELECTRIC CO METER OUT (TO GRID) + X (USED BY HOUSE)

Solve for X and I’ve got it.

Additionally I think it makes sense to divide by the number of occupants in your house before comparing with your friends.  And maybe adjust by things like HDD and CDD (heating degree days and cooling degree days) if they live in a different part of the Earth.

So that’s KWh used per person per month.   We have averaged under 800 KWh per month year round on average for 28 months.  And we have 4 people here.  With 8 living with us for 10(?) months in 2011.

So under 200 KWh per person per month.  For everything, including heating and AC, cooking, lawn mowing and 2 home offices.

It’s hard to compare to most people in the northeast because most people don’t know their grand total since almost no one heats their house with electricity (geothermal or air-source heat pumps) like we do.  So they’d need to add up their gas/propane/wood/oil BTUs used and convert to KWhs.

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What we pay for electricity

Our 6/15/2012 electricity bill was $15.86 even though we made approximately 200 KWh more than we used. That’s because we don’t have net metering here yet. (Yet, as in… I assume some day there will be.)

And it looks like we’ve paid approximately $630 for electricity in the last year. If you pretend we have net metering, then that would have been roughly $630 / (11,000 – 8,400) KWh (used vs generated) = 24 cents/KWh. A bit expensive, but not outrageous for Massachusetts.

What this $630/yr makes me wonder is what it would cost to add a smaller PV array with batteries that is off-grid to power us at night so we’d have even closer to $0 electricity bills.

Eh, money probably much better spent on something practical for the family. Like a pool. Or our kids’ school tuition.

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Hot solar panels

Today was hot. I think like 95F and mostly sunny and humid.

Anyway, the PV (solar electric) panels didn’t like it. They made 31.9 KWh vs 35.8 a few days ago (with what looks like a similar mix of sun and clouds). It was probably 75 or 80 that day?

So that’s 11% worse performance right there.

35.8 – 31.9 = 3.9 and

3.9/35.8= 11%

And compare that 35.8 to a cold day and that’s another 10% I am sure!

In my Enphase history I see plenty of sunny days above 42 KWh, so a day with a few clouds could easily still be 40 KWh.

Anyway, just sayin’. Our roof mount panels have a fair bit of a space below them. I bet if they were quite flush mount the heat would be even harder on them.

Some day I will rig a big cheap LASKO window fan up on a ladder and compare the output of the cooled-off row via the Enphase page.

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Our PECs: site vs source (primary) energy

Another good way to weigh one’s home energy usage is per person. (not arbitrary per sqft or per HDD, etc.). PEC=Primary Energy Consumption. Makes sense if we are trying to be “green”.

That’s what Marc Rosenbaum is proposing here.

If I am understanding the gist of what he is saying it is as follows as applies to us:

1. Here in MA/New England, power plants are about 33% efficient. They waste 2/3 of the energy (mostly fossil fuels) in making a unit of electricity. The exact number Marc uses is 1/2.7, not 1/3. (37% efficient source to site.) So… since we have used approximately 11000KWh/year, that is 29700 KWh/year in source energy that we REALLY used.

(Worry about PVs later… in step 3.)

2. We are 4 people, so look up our “fair share” according to his “people = bedrooms+1″ equation and we should be trying to meet 13,600 KWh/year in source energy. For everything… heating, cooling, hot water, appliances, cooking, lighting.

3. One should be able to offset some percentage of hot water and electricity usage from onsite generation. Marc explains (I believe) that the Passivhaus PHPP allows up to 20% offset for a traditional solar hot water system, so in his mind, why not allow up to 20% of electricity use as well. And indeed… we heat our hot water with an air-source heat pump too, so I am lumping it all together. And actually, since I am dealing with real data, not estimates, I see from my record keeping that in the last year we have exported 5600 KWh to the grid. So that is the equivalent of 2.7 that much in “source energy” that we have offset = 15,120 KWh

So if we reduce our 29,700 by 15,120 => 14,580 KWh/year

4. 14,580 > 13,600 KWh (Marc’s limit for 4 people)

So we didn’t meet Marc’s proposed criteria for PEC for a Passivhaus in New England, even assuming my generous PV offset based on our grid export numbers.

Pretty close though. Why didn’t we meet it? I assume:
– Our house is too big (1741 TFA via PHPP)
– 2 of us work at home and waste energy
– Windows and HRV not efficient enough
– We should use more solar thermal heating (I have plans on this front)

‎”Your house is a leaky bucket and the sun is a hose. To raise the water level, you need fewer leaks or a bigger hose.” — Nick Pine in a discussion of his “box on the lawn” solar collector design in http://tech.groups.yahoo.com/group/SolarHeat/

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