Electrical capacity is the maximum firm (sustainable) amount of electrical power that can be generated or delivered at a point in time.

For example, the capacity of BC Hydro’s Site C dam is 1,100 MW, because it generates that amount of electricity at a point in time on a firm basis.

Electrical capacity is measured in watts, the same units as electrical power. The distinction between capacity and power is that electrical capacity must be firm, whereas electrical power may only be intermittent.

This is an important distinction. Solar panels are an intermittent source of power because the sun does not shine constantly, whereas hydroelectric dams or nuclear generators produce firm energy, as they operate throughout the day and night all year round.

Utilities plan to meet their forecast peak demand using only firm sources of power. A utility’s electrical system capacity does not include any non-firm sources of power, even though they might be used to generate electricity intermittently during the year.

This illustration explains the key concepts of electricity.