Power vs. Energy

A 100 MW solar farm and a 100 MW gas plant have identical power ratings. But the solar farm produces about 250,000 MWh per year while the gas plant produces about 700,000 MWh. Same power, very different energy. Confusing these two quantities is the single most common error in energy policy debates.

Energy is the total amount of work done or heat transferred, measured in joules or kilowatt-hours. Power is the rate at which energy flows, measured in watts (one watt = one joule per second). Energy = Power x Time. A 1,000 MW plant running for one hour delivers 1,000 MWh of energy. The same plant running for 8,760 hours (one year) delivers 8,760,000 MWh, if it never stops. Real plants stop.

Worked Example

A headline reads: "New wind farm will power 500,000 homes with 1,500 MW of clean energy."

• Check units. MW is power, not energy. The headline conflates them.
• Calculate actual energy. Wind farms typically operate at 30-40% capacity factor. At 35%: 1,500 MW x 0.35 x 8,760 hours = 4,599,000 MWh/year.
• Average U.S. household uses about 10.8 MWh/year. 4,599,000 / 10.8 = ~426,000 homes.

Does the headline's claim of 500,000 homes hold up?

It is close but slightly overstated. More importantly, the correction required knowing the difference between power and energy, then applying a capacity factor. Without that distinction, the claim is uncheckable.

Every energy claim you encounter uses power, energy, or both. Distinguishing them is the first filter for evaluating any number in this field.

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