[neurocline]

A roof is about 200 m^2 (more or less), so at 10 mw/m^2, you could generate a whopping 2 watts. While it's raining. California gets 563 millimeters of rain a year, so you might end up generating an entire 100 watts. For the year.

[NobodyNada]

A watt is a unit of power, not energy —- it represents the rate at which energy is produced (joules per second), not the total amount of energy produced. I’m not sure how you came up with the number 100, but the units for that number should be energy (such as joules or watt-hours).

[neurocline]

Meant to type watt-hours for that last sentence. Thanks for catching it.