Watt Calculator. Enter any two of the four electrical values — Power (W), Voltage (V), Current (A), or Resistance (Ω) — and the Watt Calculator solves for the remaining two. Based on Watt's Law and Ohm's Law, it covers all valid combinations of P, V, I, and R. Also try the Kinetic Energy Calculator.
Watt's Law states that Power (P) equals Voltage (V) multiplied by Current (I): P = V × I. So if you have a 12V circuit drawing 5 amps, the power is 60 watts. You can also calculate watts from resistance using P = V² / R or P = I² × R. See also our use the Power-to-Weight Ratio Calculator.
A Watt is the SI unit of electrical power. It measures the rate at which electrical energy is transferred or consumed. One watt equals one joule of energy per second (1W = 1J/s), or equivalently, the power when one ampere flows through a potential difference of one volt.
Volts and watts are not directly interchangeable — they measure different things. However, using Watt's Law (P = V × I), if you know the current, you can find the voltage: V = P / I. For example, 60 watts at 5 amps equals 12 volts.
There are 1,000 watts in one kilowatt (kW). To convert watts to kilowatts, divide by 1,000. For example, a 1,500-watt heater consumes 1.5 kW of power. You might also find our calculate Power (P), Voltage (V) & Current (I) — Electrical Power useful.
Ohm's Law states that Voltage (V) equals Current (I) multiplied by Resistance (R): V = I × R. Combined with Watt's Law (P = V × I), you can derive all four electrical quantities — power, voltage, current, and resistance — from any two known values.
Watt hours (Wh) = Watts × Hours. Multiply the power draw in watts by the number of hours the device runs. For example, a 10W device running for 5 hours uses 50 Wh. For batteries rated in milliamp hours (mAh), use: Wh = (mAh × V) / 1000.
One watt is approximately equal to 3.41214 BTU per hour (BTU/hr). So a 1,500-watt heater produces roughly 5,118 BTU/hr of heat output. This conversion is commonly used when comparing electric heaters and HVAC equipment.
You need to enter exactly two of the four values: Power (W), Voltage (V), Current (A), or Resistance (Ω). The calculator will solve for the remaining two using Watt's Law and Ohm's Law. Valid combinations include P+V, P+I, P+R, V+I, V+R, and I+R. Check out our Work Calculator as well.