Capacity Factor

Capacity factor is energy actually generated divided by the energy the plant would have made running at its nameplate rating for the whole period: $CF = E/(P_r t)$. It is the number that turns a rating into a power station, and the one whose absence lets a press release quote megawatts as if they were megawatt-hours.

Quoting a nameplate as though it were continuous output is the classic error. A "100 MW wind farm" at a 35% capacity factor delivers the annual energy of a 35 MW plant that never stopped. For scale: modern onshore wind runs 30–45%, offshore wind 45–60%, fixed-tilt solar PV 12–20% in Canada and northern Europe and 20–28% in the American southwest, and a nuclear station above 90%.

What a low capacity factor does not mean is that the machine works poorly, and this is the subtler misreading. A wind turbine is deliberately rated for a wind speed it seldom sees, because power goes as the cube of speed and a generator sized for the rare gale would cost far more while sitting idle nearly all year. Rating below the peak is an economic optimisation, not a compromise. The same logic explains why an oversized solar array behind a smaller inverter — deliberate DC/AC "clipping" — raises capacity factor and lowers the cost of the energy, while throwing away a little peak production on the brightest days.

Three things quietly wreck the arithmetic. The period must match the energy: 8,760 hours for a calendar year, not 12 and not 365. The rating must belong to the same plant the energy came from, not to one turbine in a farm. And the units must match — mistaking MWh for kWh moves the answer by a factor of a thousand and is easy to do when reading a utility report. Finally, capacity factor is not availability: a plant can be available 98% of the time and still run at 35%, because the fuel simply is not always blowing.