Electrical Power and Scale

A phone charger draws 5 watts. A house draws 5,000 watts. A city draws 5,000,000,000 watts. The same formula (P = VI) governs all three scales, spanning nine orders of magnitude. Being comfortable with power scales from watts to terawatts is essential for reading any energy statistic.

The power scale relevant to energy systems: a watt (W) is one joule per second. A kilowatt (kW, 10^3 W) is a household appliance. A megawatt (MW, 10^6 W) is a utility-scale unit, roughly 1,000 homes. A gigawatt (GW, 10^9 W) is a large power plant or a small country's total capacity. A terawatt (TW, 10^12 W) is the scale of global energy: humanity's average power consumption is about 19 TW.

Nameplate capacity (rated power) does not equal average power. A 1 GW solar farm with a 25% capacity factor delivers an average of 250 MW. Converting nameplate to average power requires the capacity factor: P_avg = P_nameplate x CF.

Worked Example

A utility announces "5 GW of new solar capacity."

• Convert to average power. At 25% capacity factor: 5 GW x 0.25 = 1.25 GW average.
• Convert to annual energy. 1.25 GW x 8,760 hours = 10,950 GWh = 10.95 TWh.

The U.S. consumes about 4,000 TWh per year. What fraction does this 5 GW of solar provide?

10.95 / 4,000 = 0.27%. Five gigawatts of nameplate solar, despite sounding large, provides about one-quarter of one percent of U.S. electricity. Scale matters.

A gigawatt sounds impressive until it gets multiplied by a capacity factor and divided by national demand. That arithmetic, not the headline number, is what separates a serious energy claim from a press release.

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