Capacity vs. Energy: kW and kWh

A headline announces "10 GW of new solar installed." A critic responds "but that only generated 22 TWh." Both are correct. One describes the pipe (capacity: how fast energy can flow) and the other the water that passed through it (energy: how much total). Conflating these two is the most common analytical error in energy journalism.

Capacity (measured in watts) is the maximum instantaneous rate of generation or consumption. Energy (measured in watt-hours) is the total generated or consumed over time. The relationship: Energy = Capacity x Time x Capacity Factor. A "100 MW / 400 MWh" battery can deliver 100 MW for 4 hours. Without both numbers, you cannot evaluate what a power source or storage system actually does.

When someone says "the U.S. added 30 GW of solar and 5 GW of nuclear capacity," the implied equivalence is misleading. 30 GW solar at 25% capacity factor produces about 66 TWh/year. 5 GW nuclear at 92% capacity factor produces about 40 TWh/year. Solar added 6x more capacity but only 1.6x more energy.

Worked Example

A battery is marketed as "500 MW of storage."

• Identify the missing number. MW is power (rate of discharge), not stored energy. How long can it sustain 500 MW?
• The spec sheet says 2,000 MWh. Duration = 2,000 MWh / 500 MW = 4 hours at full power.

A second battery is also rated at 500 MW but stores only 500 MWh. Are these batteries equivalent?

No. The first provides 4 hours of backup; the second provides 1 hour. Same power, very different energy. For grid planning, duration determines whether a battery can cover an evening peak or just smooth a momentary fluctuation.

Capacity tells you what the grid can deliver at peak. Energy tells you how much electricity is actually produced or stored. Both numbers are required.

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