Watt 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 Bullet Energy 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 Power-to-Weight Ratio Calculator 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 Efficiency Calculator as well.