Three of the bids specified BP Solar panels, which are ok, but the fourth specified the more expensive Mitsubishi Solar panels. Looking at the specs, I found two things that made Mitsubishi stand out: first, their power tolerances are tighter (+-3% instead of +- 9%), which is a mark of quality in my book. Second, they're lead free, which is attractive for someone trying to go green. (One of these days we've got to stop using poisonous heavy metals in everything we build, may as well be now.) So that's what we went with, and how we picked our contractor.
We are installing 46 panels, each with a peak power rating of 175 watt, for a total of 8050 watts, made with polycrystalline cells, model Mitsubishi PV-UD175. The power is converted to AC by two Xantrex GT4.0 inverters (one for each unit of our duplex). The inverters are supposedly 95.5% efficient.
So, if those assumptions hold, we'll only be paying the power company about $0.50/day, and our total daily cost for power will go down from $5 to $3.50. (It can't be true, can it? Tell me where I pencilled that out wrong...)
Incidentally, if you're a do-it-yourselfer, you might be able to get a better deal by doing it yourself. We were too lazy to even think about this, but if you've got the energy, consider it.
One set of panels produces about 14% less power than the other. This is probably a result of shade (the east side of our roof has a 2-3 foot high false pitched roof, and maybe only one system is in its shade) and fewer solar panels (we don't know which one has 24 and which one has 22). The difference is mostly during the first hour of the day, which makes me suspect shade is the bigger factor.
If the systems were reversed, we'd be at 371/480 and 433/424, or 77% and 102% respectively. Anything over 100% would be a waste because our power company doesn't pay for extra energy. I don't know if our installer was brilliant, or just lucky.
The PVWATTS calculator at nrel.gov lets you calculate estimated output over the year. It predicts a 4KW system will produce 441 KWH in February in Los Angeles (if tilted at 34 degrees). We almost hit that with one of our two systems.
Looking at the power indicator made me want to get to 100% solar power, so now I'm more careful about turning things off when we're not using them. We've set our computer to suspend itself when idle, which saves a lot of power.
According to a 2005 UK report, the CO2 footprint of PV panels averaged over their lifetime is one-tenth that of LADWP power, ~0.06 grams per watt-hour. So our panels "generated" 0.4 tons of CO2, for a net savings of 3.6 tons.
Burning gasoline produces about 9KG of CO2 per gallon, so this saved the CO2 equivalent of 400 gallons of gas.
Here's a much larger installation a few miles north of us, using the same solar panels (well, they use the 185 watt panels; I suspect Mitsubishi sorts cells by efficiency and sells the less-efficient ones a bit cheaper).
|What||Model||Power each||#||Total power|
|Refrigerator||GE PSF26MGW||615 KWH/year = 70W average||1||70|
|Cable box||Mot. DCT2224||15 watts||2||30|
|PVR||Tivo series 2||40||2||80|
|Cordless phone||ATT SL82408||5?||5||25|
To check this, I used the Itron Centrol Watthour meters's one dot per watt-hour display and a stopwatch.
(To do this, turn off all lights in the house, turn off and unplug all computers in the house, throw all the circuit breakers to OFF (including the one for the solar feed), and then for each breaker, turn it on by itself and measure how many seconds between dots. (If you don't see any dots change in a minute, give up and move on to the next. You might want to group all of those together.) To convert those times into watts, divide them into 3600. So, if you counted ten seconds between dots, that's 360 watts.)
I had some trouble getting repeatable measurements, but here's my data:
|18 dining room||50||72|
|21 living room||50||72|
|actual measurement of all||n/a||309|
|all plus mac||8.4||430|
|all plus mac and PC||7.0||514|
|7,8 counter, bedroom||143||25|
|28 living room plugs (including r3000 laptop)||52||69|
|actual measurement of all||18||200|
|all plus kitchen lights||8||450|
I omitted individual measurements of the fridges because they suck lots of watts briefly at startup, then nothing during idle; we probably have to trust the mfr who says they take 70 watts average. To avoid counting them in the actual whole-house measurement, you'd have to unplug them; I just waited a while until they probably finished running.
Note that the sum of the measurements for downstairs exactly matches the estimate I made earlier. This is just a lucky coincidence, though, since my total measurement is lower.
Doing accurate measurement by flipping breakers and counting dots is hard, plan to iterate a few times until you really understand the results. I have not quite finished iterating. A Kill-a-watt power meter would be a heck of a lot easier way to measure things that can be plugged and unplugged.
total pounds of CO2 emitted = 1.4 * KWH electricity used + 12 * therms natural gas burned in home + 18 * number of thousand gallons of water usedSo not only do you want to switch to solar, you also want to minimize your use of natural gas and water.
(sources: [1, 2.1 lbs CO2/KWH for coal, 1.3 lbs CO2/KWH for natural gas], [2, LADWP 45% coal, 33% natural gas], [3, 13 KHW/thousand gallons of water]
Another measure is how long it takes before the panels have generated as much energy as it took to make and install them. Wikipedia says that recent studies estimate this at 1.5 to 3.5 years.
Mitsubishi has lots of environmental information online at global.mitsubishielectric.com/company/csr. They even mention at global.mitsubishielectric.com/company/csr/ecotopics/pv/manufacture that at their new solar panel factory, "PV systems will be installed on the roof of the building, reducing carbon dioxide emissions from PV cell production." However, I have not yet seen actual figures on the carbon footprint of the panels we are installing. It would be nice if Mitsubishi provided carbon emission labels on their solar panels so consumers could make informed choices more easily.
16 kWh is a metric f-ton of power use. Where does it all go, again?
Hmm, why didn't we get a gas-heated hot tub, again?
Copyright 2008-2009, Dan Kegel