[akira2501]

There are a few different forms and a few different rules surrounding it. I can probably assume what he means, but a more direct comparison would obviate any of these factors.

Right.. but that's not his total cost, that's his total savings. So, with those factors; which make sense for New England, he's got a 7kW system getting light for about 3 hours a day on average to net that 775kWh to earn the $233 savings in a month. Would that be right?

[Kirby64]

If the system was totally efficient, then yes, but the 7kW nameplate number is peak efficiency... so most likely it's some averaged number (say, 4-5kW?) that is across a longer period of time.