Electrical power and energy calculator

Formula

How it works

Electrical power can be found from any two of voltage, current and resistance, since Ohm's law links all three. Multiplying power by the time it is used, then dividing by 1000, gives the energy in kilowatt-hours — the unit electricity is usually billed in.

Worked example

The same calculation with the default values:

• Known values Voltage and current
• Voltage, or current (see choice above) 230
• Current, or resistance (see choice above) 2
• Hours used 5 h

1. P = 230 × 2 = 460 W
2. E = 460 W × 5 h ÷ 1000 = 2.3 kWh

Result: Power (W): 460 W · Power (kW): 0.46 kW · Energy used (kWh): 2.3 kWh

Common mistakes

• Using the wrong pair of known quantities for the chosen form of the equation - P=I²R needs current and resistance, not voltage; mixing in a voltage value here gives a meaningless number.
• Forgetting to convert energy (E = P × t, in watt-hours or joules depending on the time unit) into the units an electricity bill actually uses (kWh) - this is the same quantity the electricity-cost calculator handles with explicit kWh conversion.
• Assuming power stays constant over the whole time period t - if a device's draw changes (e.g. a motor under varying load), E = P × t only holds for each interval of truly constant power, not the whole varying period at once.