Calculate electrical power in watts from voltage and current. See the power in kilowatts and how much energy it corresponds to over one hour.
Electrical power (P) equals voltage (U) multiplied by current (I), measured in watts (W). Power is also related to Ohm's law: since U = I × R, power can also be written as P = I² × R or P = U² ÷ R, depending on which quantities you know.
Power tells you how much energy something uses per second. Multiply the power by the time the appliance is in use, and you get the energy consumption — for example in kilowatt-hours (kWh), which is what your electricity bill uses.
Watts (W) measure power — how much energy something uses right now. Kilowatt-hours (kWh) measure energy over time — the power in kilowatts multiplied by the number of hours in use. Your electricity bill is calculated from kWh, not watts.
Find the appliance's power in watts (often listed on a label or in the manual), convert to kilowatts, and multiply by the number of hours it's used. A 1000 W heater running for 3 hours uses 3 kWh.
Ohm's law (V = I × R) and the power formula (P = U × I) are closely related. If you also know the resistance in addition to voltage or current, you can substitute Ohm's law into the power formula to get P = I² × R or P = U² ÷ R. See the Ohm's law calculator to find voltage, current or resistance.