Power is the capacity to perform work or produce a specific change to a condition. Energy is the measure of the actual performance of that work, or the amount of change produced. The terms are often used interchangeably, even within the energy industry itself, but they have two distinct meanings.

In electricity, power is usually expressed in kilowatts; energy is usually expressed in kilowatt-hours. In other words, the value used to express energy has volume, whereas values that express power only imply the ability to provide that volume. A static electricity shock such as a carpet shock is an example of enormous power with very little energy. These shocks can deliver thousands of volts, but the total quantity of energy is so small that the shock is harmless. Power can also be available without providing energy. A household circuit connected to a lightbulb always has power, but it uses virtually no energy until the light is actually turned on.

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