Amps to Watts Calculator
Enter current in amps and voltage in volts — get power in watts and resistance in ohms in one step using Watt's law.
230 V European default
The calculator pre-fills 230 V, the standard European mains voltage. If you are working with North American 120 V circuits, just change the voltage field.
Resistive loads only
Watt's law P = V × I gives real power only for resistive loads (heaters, incandescent bulbs). AC motors, fluorescent lights, and switch-mode supplies have a power factor below 1 — they draw more apparent current than this simple formula accounts for.
What is Watt's law?
P = V × I — the foundation of electrical power
Watt's law states that electrical power equals voltage multiplied by current: P = V × I. A circuit running at 230 volts and drawing 10 amperes consumes 2 300 watts. The law applies to DC circuits and to AC circuits with a purely resistive load (power factor = 1). It is named after James Watt, whose work on the steam engine led to the standardisation of power as a physical quantity.
Enter current (A) and voltage (V) to get power (W) and resistance (Ω) instantly.
The calculator applies two complementary formulas simultaneously. Watt's law gives power; Ohm's law gives resistance.
P (W) = V (V) × I (A) R (Ω) = V (V) ÷ I (A)At 5 A on a 12 V DC supply the power is 60 W and the resistance is 2.4 Ω — the kind of calculation used for LED drivers, car accessories, and battery management. At 16 A on 230 V — a fully loaded European socket — the power reaches 3 680 W, which is why a typical circuit breaker trips at that current.
The watts figure tells you how much energy the load converts every second. A result under 100 W is typical for lighting and small electronics; 500–2 500 W covers kettles, irons, and hair dryers; 3 000–3 680 W is the range of high-draw appliances such as electric showers and fast EV chargers on a standard socket. The resistance figure is useful for checking whether a load matches your power supply: according to Ohm's law, lower resistance means higher current for the same voltage — a very low resistance on a mains circuit can indicate a fault or an undersized fuse. If the calculated watts are significantly higher than the appliance's rated wattage, re-check your measured current; a clamp meter reading can be misleading if the waveform is non-sinusoidal. For resistive loads the result is exact; for reactive loads (motors, capacitors, inductors) multiply P = V × I by the power factor to obtain true power.
This calculator performs a mathematical conversion only.
AC loads with power factor below 1
For AC circuits driving inductive or capacitive loads — motors, fluorescent ballasts, switch-mode power supplies — the real (active) power is P = V × I × power factor. Without accounting for the power factor, P = V × I overstates actual power consumption. Always check the appliance's rated power factor or use a power meter for accurate measurements. This calculator is for informational purposes only; consult a qualified electrician before making wiring or circuit-protection decisions.