A full explanation is here but to boil it down, they measure different things. The kilowatt (kW) is a unit of power – 1,000 watts. It measures the rate at which electrical energy is generated or consumed. Solar panel manufacturers rate the capacity of panels in watts to indicate the output of a panel in full sun. Commonly available panels range from 250 to 300 watts rated capacity. Solar contractors rate a full PV array in watts or kilowatts (number of panels multiplied by number of watts per panel).
On the other hand, the kilowatt hour (kWh) is a unit of energy – 1,000 watts of electricity running for one hour. It is also the billing unit on residential electric bills. You can use it to determine how much you can reduce that bill. On average, an unshaded 250 watt panel produces 300 kWh a year in Indiana.
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