Calculation details
Equipment estimate only. Verify voltage, load profile, losses, duty cycle, manufacturer specifications, and electrical safety requirements before relying on the result.
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What does this calculator estimate?
A watts to amps calculator converts between electrical units. Enter any two of watts, amps, and volts to find the third.
- Amps = watts ÷ volts
- 1,200 W ÷ 120 V = 10 A
- The core Ohm's law conversion
Why the conversion matters
Circuit breakers are rated in amps; appliances are rated in watts — so choosing a circuit, extension cord, or generator means converting between them. Amps = watts ÷ volts is the formula that keeps loads within their circuit rating.
Limitations to watch for
The tool uses the simple DC/resistive formula; motors and inductive loads draw higher startup current. US household voltage is ~120 V (240 V for big appliances); other countries use 220–240 V, which halves the amps for the same watts.
How to use it in practice
Add the watts of everything on a circuit and divide by the voltage to check it against the breaker (15 A or 20 A typical). Allow 80% headroom on continuous loads.
['Enter the watts and the voltage.', 'The tool computes the amps.', 'Check against the circuit rating.']
The power formula behind the conversion
The core relationship is watts = volts × amps. At a fixed voltage, dividing watts by volts gives amps: a 1,200-watt heater on a 120-volt circuit draws 10 amps. The calculator applies this directly, with voltage selectable because the same appliance behaves differently on 120 V, 240 V, or 12 V systems.
Why amps matter for circuits
Circuit breakers are rated in amps because the wiring heats up with current. A 15-amp circuit is designed for 80 percent continuous load — about 1,440 watts at 120 volts — which is why a 1,500-watt space heater on a 15-amp circuit with anything else on it can trip the breaker. The conversion tells you whether your equipment fits the circuit before you plug it in.
AC vs. DC and power factor
For DC, watts = volts × amps exactly. For AC, the formula includes power factor: watts = volts × amps × power factor, which is below 1 for motors and other inductive loads. The calculator covers the common DC and resistive AC cases and notes when power factor matters — most household resistive loads (heaters, incandescent bulbs) run near 1.
Battery and solar applications
On a 12-volt battery system, the same 100-watt load draws about 8.3 amps, which is why deep-cycle batteries and inverters are rated in amp-hours. The conversion is the first step in sizing wire, fuses, and inverters for off-grid and vehicle systems.
Common appliance draws for reference
A phone charger draws about 0.5 amp, a laptop 1–2 amps, a refrigerator 3–6 amps (with startup spikes), and an electric range 30–50 amps on a dedicated circuit. The calculator gives exact numbers for your equipment's wattage and your voltage, which is more reliable than memory.
How this calculator works
Formula
Amps = watts ÷ volts. The tool converts between watts, amps, and volts (and optionally kVA) using Ohm's law relationships.
Worked example
A 1,200 W appliance on 120 V draws 10 amps.
Assumptions to verify
- Simple resistive loads at the stated voltage.
- US or specified supply voltage.
- No power-factor correction is needed.
Frequently asked questions
How do I convert watts to amps?
Divide by volts: 1,200 W ÷ 120 V = 10 A.
What's the formula?
Amps = watts ÷ volts. Rearranged: watts = amps × volts.
Why does voltage matter?
The same watts draw fewer amps at higher voltage — that's why 240 V appliances use smaller wiring.
How many amps can a 15 A breaker handle?
12 A continuously (80% rule) or 15 A for short periods.
What about motors?
They draw 3–5× their running amps at startup — size circuits for the surge.
How do I size a generator?
Total the running watts of everything you'll power and divide by the generator's voltage for amps.
Is this the same as the battery runtime calculator?
Related — that tool converts amp-hours and watt-hours for runtime; this converts live watts and amps.
Why do I need to know the voltage?
The same wattage draws more amps at lower voltage, so the voltage changes the answer completely.
Is watts-to-amps the same for AC and DC?
DC is exact; AC with motors needs the power factor. Resistive AC loads are close to 1.
How many watts can a 15-amp circuit handle?
At 120 V, about 1,800 watts peak, and about 1,440 watts for continuous use (the 80 percent rule).
Cite this tool
BoringToolsKit. “Watts to Amps Calculator.” boringtoolskit.com/watts-to-amps-calculator/ (reviewed 2026-08-25). Free to reference in articles, syllabi, and answer posts with a link.
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