Electrical Power Calculator
Compute power dissipated by a resistance from an applied voltage.
Formula
P = V² / R
Inputs
- Voltage (V)
- Resistance (Ω)
What it does
Finds the power a resistance dissipates from the voltage across it: P = V² / R, plus the current it draws.
Where it's used
Heaters, incandescent loads and checking resistor heating.
How to use it
- Enter the voltage across the load.
- Enter its resistance.
- Read the power and current.
The formula explained
- P
- power, W
- V
- voltage across the load, V
- R
- resistance, Ω
Worked example
A 230 V heater element of 52.9 Ω.
Inputs:
- Voltage: 230 V
- Resistance: 52.9 Ω
The calculator gives:
- Result: 1000 W
- Current: 4.3478 A
Tips and common mistakes
- Power rises with the square of voltage: doubling V gives 4× the power.
- Heating elements change resistance as they warm up.
- For AC, use RMS voltage.
Questions people ask
How do I find a heating element’s resistance from its rating?
R = V² / P. A 2 kW element for 230 V has 230² / 2000 ≈ 26.5 Ω when hot. Measured cold with a multimeter it often reads a little lower.
What happens if I run a 230 V heater on 115 V?
Power goes with the square of voltage, so half the voltage gives a quarter of the power: a 1000 W heater gives only about 250 W.
What current does it draw?
I = P / V, or V / R. The 1000 W example draws about 4.35 A at 230 V. Choose the fuse and cable for that current with some margin.
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Enter your values to get the result with the formula, substituted values and an explanation of each step.