Unit Errors in Energy Policy

Picture a fact sheet describing a proposed wind farm as capable of "400 megawatts of clean power every year." Megawatts already measure a rate, energy per unit time. Adding "every year" turns the number into energy per time squared, a quantity that describes nothing physical. Versions of this error clear editors and enter hearing records every week.

Unit errors in energy policy are not rare. They are endemic. Three types account for most of them.

Confusing power and energy. A press release says a battery can "store 100 megawatts." Megawatts measure power (the rate of energy flow), not energy. The battery stores megawatt-hours. This error understates or overstates the battery's usefulness depending on which number the reader infers.

Confusing capacity and generation. A state announces "5 GW of new solar capacity." Journalists report it as "5 GW of new solar generation." But 5 GW of solar at 25% capacity factor produces about 1.25 GW of average generation. The error overstates actual output by 4x.

Omitting the denominator. "The U.S. added 30 GW of renewables last year" sounds impressive. Total U.S. generation capacity is about 1,200 GW. The addition is 2.5% of the installed base. Without the denominator, the reader cannot evaluate significance.

Worked Example

A policy brief states: "The proposed solar farm will generate 200 MW, enough to power 150,000 homes."

• Check the power claim. 200 MW is the rated capacity, not average generation. At ~25% capacity factor, average output is 50 MW.
• Check the homes claim. One home averages ~1.2 kW. 50,000 kW / 1.2 kW = about 42,000 homes.

The brief overstates the number of homes served by what factor?

150,000 / 42,000 = about 3.6x. The error came from using rated capacity (200 MW) instead of average output (50 MW). This is the most common unit error in renewable energy coverage.

Every one of these errors is arithmetic, not opinion. A reader who checks units before checking sources catches most of what should never have been published.

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