Electrical power (watts) calculator
Electrical power — the rate at which energy is used or dissipated, measured in watts — can be found three equivalent ways depending on which quantities you know. This calculator lets you pick the pair you have (voltage, current or resistance) and returns the power instantly, so you never have to rearrange the formula yourself. Handy for circuit design, sizing components and electronics homework.
How it works
All three formulas come from the power law combined with Ohm’s law (V = I × R):
P = V × I (voltage × current)
P = I² × R (current squared × resistance)
P = V² ÷ R (voltage squared ÷ resistance)Choose the mode matching your known values, enter the two numbers, and the watts appear below.
Worked examples
A circuit with 12 V and 2 A:
- P = V × I = 12 × 2 = 24 W
The same power from current and resistance, or voltage and resistance:
| Known values | Formula | Power |
|---|---|---|
| 12 V, 2 A | V × I | 24 W |
| 2 A, 3 Ω | I² × R | 12 W |
| 12 V, 4 Ω | V² ÷ R | 36 W |
Real-world uses
Resistor sizing — a resistor in a circuit dissipates power as heat. Use P = I² × R to find the wattage the component must handle; then choose a resistor rated safely above that value (common ratings: 0.125 W, 0.25 W, 0.5 W, 1 W, 2 W). Undersized resistors overheat and fail.
Cable selection — the power dissipated in a cable run is P = I² × R_cable. A higher-resistance (thin or long) cable wastes more energy as heat and also drops voltage at the load. Use this formula to confirm cable losses are acceptable before finalising wire gauge.
Power supply sizing — knowing the total watts drawn by a circuit (sum of each component’s P) tells you the minimum supply rating. Always add a margin for efficiency losses and startup surges.
Battery runtime — dividing battery capacity (in watt-hours) by the load’s power in watts gives a rough runtime estimate. For example, a 100 Wh battery powering a 25 W load runs for about 4 hours under ideal conditions.
Understanding the three formulas
The three formulas are all the same law under different substitutions of Ohm’s law (V = I × R):
- Start with P = V × I (the fundamental definition of electrical power)
- Replace V with I × R → P = I × (I × R) = I² × R
- Replace I with V / R → P = V × (V / R) = V² ÷ R
They always give the same result for a consistent set of values. Pick whichever formula uses the quantities you actually measured or know from a component’s datasheet.
Every calculation runs locally in your browser, with no network requests.