[kragen]

$4000 is also pretty expensive; at my 7% yearly interest that's $23 a month. at my number of 0.9¢ per kilowatt hour that's 2600 kilowatt hours per month, an average of 3600 watts. i will be very surprised if you are using 3600 watts round-the-clock average unless you have a machine shop in your garage

so i think you are dramatically overestimating the cost of rooftop solar