[Boot Hill]

r9etb suggests:   "For example, it's possible to run a home air conditioning unit using power generated by roof-top cells.."

You just couldn't do it and make any kind of economic sense (without government subsidies, that is). Let's look at an example.

1. We have a 3,000 watt window air conditioner unit running 8 hours per day for 180 days per year at a utility rate of 15¢ per kW-hr. Cost per year = $648.

2. When calculating PV output we will use no loss for night time, sun angle or dirt. The starting value of 790 W/m^2 is reduced by PV loss (85%), packaging loss (20%), heat loss (15%), inverter loss (20%), leaving 64.5 W/m^2. For 3,000 watts, that's 46.5 m^2 of PV panel array needed. The area of the SP-150 (for instance) is 1.32m^2, meaning that you need at least 35 of them at $700/unit or $24,500. Plus another $3,000 for the 3kW inverter and another $5,000 for someone to install and wire this system. Now we're up to $32,500. I'll give you the building permits for free.

3. But for that same $32,500, I could have run the air conditioner off the power grid for 50 years! And this does not even begin to account for the time-value of the $32,500 I had to plunk down on day one to start this venture!

So maybe you could explain to me again just why it would be such a good idea to buy the solar cells to run the air conditioner?

--Boot Hill

[mvpel]

You make the assumption that the price of electricity from the grid will always be 15c for the next 50 years. I don't think that's likely. In California, you'd be paying upwards of 25c per kWh for that A/C unit after you burned through your monthly peak usage limits.

[biblewonk]

Even Homepower magazing, one of the biggest PV advocates that exists showed similar numbers. Lately they have been showing how much cheaper it is to simply conserve rather than try and produce power with solar panels. They showed how they could save tons of money in a library by simply going to compact florescent bulbs.

Windpower, on the other hand, is perfectly economical and will produce 2-4 percent of the worlds power in 15 years.

[DB]

The inverter the other person said he used in his system has a specified nominal efficiency of better than 90%.

http://www.advancedenergy.com/gc1000.htm

Significantly better than the 75% you've been posting.