Perhaps my number seem pessimistic, but they were taken right off of the manufacturers web sites, and I only looked at higher efficiency panels. A LOT of them were listed at less than 10% efficiency.
Those just have to be thin film panels. Inefficient but cheap. Cost/watt still cheaper than silicon. Mostly used by utility companies who put up acres. Solar City has begun selling these to home owners. I wonder if anyone is buying them. I've never seen them installed.
Inverters have a quiescent current draw, so a lightly loaded inverter is very inefficient while a heavily loaded one would be highly efficient. Since most home owners don't bother to regulate loads, assume an average efficiency.
I just checked the spec on a typical Xantrex inverter. The lowest efficiency with 240V mains is 88%. That's at 10% load. At 30% load efficiency is above 90%. At 50% it's 95%. The "night time tare loss" is 1W.
Most power companies will 'buy back' excess power produced by a home system, although it will be at the 'wholesale rate' which is substantially below what you see on your bill. Still getting it's money in the right direction.
Grid connected systems are usually sized no larger than net zero dollars over a year. There's a monthly bill but you don't pay it. The bills accumulate for a year until a "true up" date. That goes for you owing them money and vice versa. Power going both ways is priced at retail rates, seasonal, time of use retail rates.
On summer afternoons the panels produce a lot of extra power which the utility buys from you at summer afternoon rates like $.30/kwh. In the evening and winter you buy power from them at off peak rates like $.11/kwh. If at the end of a year they owe you money, you either just plain lose it or you get a discounted check. Those are the bargain basement rates which you try to avoid. If you've done your homework and sized the system correctly, though, they don't owe you money and you've effectively sold them power at $.30/kwh.
But wait. There's more. If you are a big user of power and have an escalating rate structure like where I live, the highest tier rates can be something like $.50/kwh. You're most certainly in that tier on summer afternoons when the AC is cranking. That's right when a solar panel puts out near peak efficiency so it's offsetting that very expensive power. You're effectively getting $.50/kwh power from your panels.
That's why the most economically sensible PV systems are sized to just knock you out of the top tiers. Buy the rest of your power. Just don't buy any $.50 power.
If I had to build a reliable cheap power source, I'd go 4with a Gas Generator system and use a big generator. Salvage the waste heat for living space and hot water, and sell back the unused system. Capturing the waste heat will increase the efficiency to somewhere on the order of 60-70% or better. Fuel cost (around here) would be free, the town dump charges the tree guys a flat rate per ton for chipped waste, a perfect fuel for gasifiers, I'm sure they'd love to get rid of it for no cost to them.
All I have to do is get off my lazy a$$ and build it...
Sounds right to me. Big guys out here are beginning to buy the new whiz, bang fuel cells. Apparently these save money right off the bat. They are too big for home use though. These things:
http://www.bloomenergy.com/
