Power is a rate in watts. Energy adds that rate over time: 1 kilowatt-hour (kWh) is the energy transferred by a steady 1,000W load in one hour.

For a constant 2kW load running for half an hour, energy is 2 × 0.5 = 1kWh. A thermostatically controlled appliance may switch on and off, so its nameplate power multiplied by elapsed time need not equal actual use.

A bill also depends on tariff rates, standing charges, dates and any other applicable adjustments. A meter reading is evidence about energy, not the total amount owed.