Electrical power is the rate energy is delivered: P = V × I. If you know current and resistance, use P = I² × R; with voltage and resistance, P = V² ÷ R. A kettle on 230 V drawing 10 A uses 2300 W, or 2.3 kW, and its element has a resistance of 23 Ω.
Sizing a fuse, picking a resistor wattage or checking what an appliance pulls all come down to one number: power in watts. This calculator takes any pair of voltage, current and resistance, works out the power, and fills in the third quantity through Ohm's law so you can see the whole circuit at once. It also shows the result in kilowatts and writes out each line of the working. It treats the load as a pure resistance with DC or RMS AC values, so it does not account for power factor in motors, transformers or switch-mode supplies; for those, real power is lower than volts times amps.
Choose voltage and current, current and resistance, or voltage and resistance. Only the two matching fields are shown.
Type volts, amps or ohms. For mains AC use RMS values, such as 230 V or 120 V.
Power appears in watts and kilowatts, along with the third quantity and the formula lines used.
Result Power: 2,300
Power in kW: 2.3
Voltage (V): 230
Current (A): 10
Resistance (Ω): 23
A typical European kettle element: 2.3 kW of heat from a 23 Ω element.
Result Power: 0.088
Power in kW: 0.000088
Voltage (V): 4.4
Current (A): 0.02
Resistance (Ω): 220
0.088 W fits comfortably inside a 0.25 W resistor rating.
Result Power: 1,200
Power in kW: 1.2
Voltage (V): 120
Current (A): 10
Resistance (Ω): 12
V² ÷ R gives 1200 W, and the element draws 10 A.
P = V × I = I² × R = V² ÷ R
PVIRJoule's law gives P = V × I. Substituting Ohm's law, V = I × R, yields the two other forms, so any two known quantities fix the power. The tool picks the form that matches your inputs, then uses Ohm's law again to report the quantity you did not enter. Kilowatts are watts divided by 1000.
They are one law written three ways. P = V × I is the definition, and replacing V with I × R or I with V ÷ R from Ohm's law gives I²R and V²/R. Use whichever matches the two values you have measured.
At 230 V, 13 A allows about 2990 W and 16 A about 3680 W. At 120 V, a 15 A circuit gives 1800 W, though continuous loads are usually kept to 80 percent of that.
Work out I²R or V²/R for the resistor itself, then choose a part rated at roughly twice that value. A 220 Ω resistor carrying 20 mA dissipates 0.088 W, so a 0.25 W part leaves ample margin.
Yes, for resistive loads such as heaters, kettles and incandescent lamps, provided you enter RMS voltage and current. For inductive or electronic loads, multiply V × I by the power factor printed on the nameplate to get real watts.
Multiply kilowatts by hours of use. A 2.3 kW kettle running for 6 minutes, 0.1 hour, uses 0.23 kWh. Your electricity bill is charged per kWh.
With fixed voltage, P = V² ÷ R, so halving resistance doubles power. That is why a short circuit, with resistance near zero, draws enough current to trip a breaker.
Using this tool in an assignment, article or report? Copy a citation, or a link for your website.
Tools Directory Editorial Team. (2026, October 12). Power Calculator (Electrical). Tools Directory. Retrieved October 26, 2026, from https://tools.milunir.com/electronics-physics/power-and-energy/power-calculator-electrical/
Tools Directory Editorial Team. “Power Calculator (Electrical).” Tools Directory, 12 Oct. 2026, https://tools.milunir.com/electronics-physics/power-and-energy/power-calculator-electrical/. Accessed 26 Oct. 2026.
Tools Directory Editorial Team. “Power Calculator (Electrical).” Tools Directory. Last modified October 12, 2026. Accessed October 26, 2026. https://tools.milunir.com/electronics-physics/power-and-energy/power-calculator-electrical/.
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