Thursday, July 17, 2008

Kilowatts are not Kilowatt-hours.

Heard two pieces about alternate energy this morning. Both of them described the size of the device as so many kilowatts. In both cases they should have said kilowatt hours. Most reporters are too dumb to read their own electric bills.
Kilowatts measures the rate of using electricity. A 100 watt (0.1 KW) light bulb uses electricity faster than a 60 watt (0.06 KW) light bulb. But you pay for electricity by the kilowatt hour. An ordinary two slice toaster draws a kilowatt. but it has the toast nice and brown in a minute so it doesn't draw all that much electricity overall. You'd have to toast 60 batches of toast in order to consume a kilowatt hour.
If you are thinking of buying a solar electric rig, you want to know both numbers. Kilowatt hours per day tells you how much money you save using your solar power as opposed to buying juice from the electric company. Kilowatts tells you the heaviest load the rig can power. For example if your air conditioner needs 3 kilowatts to work, it would be nice if your solar electric rig could produce 3 KW to power the AC.
The kilowatt-hour rating of a rig can be estimated from the kilowatt rating. The sun stays up 12 hours (on average) so each day it will produce 12 times the kilowatt rating. So a 1 kilowatt solar collector will furnish 12 kilowatt-hours in the course of a day. Up here the electric company will furnish 12 kilowatt-hours for $2.40. If the solar electric rig costs $7000 (as quoted in the NPR piece) it will take 8 years for the electricity produced to pay for the rig.

1 comment:

ragreiner said...

The sun may be up 12 hours, but it does not produce solar power for that time. In Wisconsin the average sun light is only about 4.5 hours per day.

Your figures are wildly optimistic.