Watts to Amps at 120V vs 240V: Why Voltage Changes Everything
By the VoltConvert team · June 20, 2026
The watts to amps conversion you get depends entirely on voltage — and getting it wrong can mean undersized wiring, a tripping breaker, or a dangerously overloaded circuit. A 1,500 W space heater draws 12.5 A on a 120V US outlet but only 6.25 A on a 240V UK socket. Same heater, same heat, half the current. This guide explains exactly why, shows the numbers for both voltages, and links you to the free calculator so you can run your own figures in seconds.
The watts to amps formula (and why voltage is in it)
The fundamental relationship is: Amps = Watts ÷ Volts. This comes straight from the power formula W = V × A, rearranged. Voltage is in the denominator, which means as voltage goes up, amps go down — for the same number of watts. Double the voltage and you halve the current. This is not a trick of physics; it is the physics.
For resistive loads (heaters, incandescent bulbs, kettle elements) this formula is exact. For motor loads and electronics there is a power factor correction, but for most household appliance questions the simple division is close enough. Use the watts to amps calculator to handle both cases — it lets you enter a power factor when you need precision.
120V vs 240V: the numbers side by side
North American homes run on 120V for standard outlets and 240V for large appliances (dryers, ranges, EV chargers, central AC). The UK, Europe, Australia and most of the rest of the world use 230–240V for everything. Here is what common wattages look like at both voltages:
| Appliance (watts) | Amps at 120V | Amps at 240V |
|---|---|---|
| Phone charger — 20 W | 0.17 A | 0.08 A |
| LED bulb — 10 W | 0.08 A | 0.04 A |
| Laptop — 65 W | 0.54 A | 0.27 A |
| Refrigerator — 150 W | 1.25 A | 0.63 A |
| Microwave — 1,000 W | 8.33 A | 4.17 A |
| Space heater — 1,500 W | 12.5 A | 6.25 A |
| Electric kettle — 2,000 W | 16.7 A | 8.33 A |
| Clothes dryer — 5,000 W | 41.7 A | 20.8 A |
| EV charger (Level 2) — 7,200 W | 60 A | 30 A |
Notice the kettle: at 120V it pulls 16.7 A — more than a standard 15 A US circuit can safely handle. That is why the US kettle market sells lower-wattage models (around 1,000–1,200 W). In the UK at 240V, a 2,000 W kettle draws only 8.3 A, well within the 13 A plug fuse fitted to every UK appliance lead.
Why 240V appliances use thinner wire
Wire heating is determined by current, not voltage — this follows from P = I² × R (power lost as heat equals current squared times resistance). Half the amps means one quarter the heat generated in the wire for the same load. That is why a US 240V dryer circuit uses 10 AWG cable where an equivalent 120V circuit for the same appliance would need heavy 6 AWG. Thinner wire is cheaper, lighter, and easier to install. It is the main practical reason utilities around the world settled on higher distribution voltages over time.
For the reverse calculation — if you know the amps and want the watts — see the amps to watts calculator.
The 80% rule: why your circuit's real limit is lower than its breaker rating
A 15 A breaker does not mean you can run 15 A continuously. The NEC (National Electrical Code) in the US requires that continuous loads — anything running more than 3 hours — stay at or below 80% of the breaker rating. For a 15 A breaker that is 12 A, or 1,440 W at 120V. For a 20 A kitchen circuit it is 16 A, or 1,920 W at 120V. In the UK, BS 7671 (the IEE Wiring Regulations) applies similar diversity factors. The practical takeaway: when calculating whether a circuit can handle your load, use 80% of the breaker rating, not 100%.
When to use 120V and when 240V matters for your calculation
Always use the actual supply voltage for the outlet or circuit you are working with:
- US standard outlet: 120V (some older homes have 110V — close enough for most purposes)
- US large-appliance outlet (dryer, range, EV charger): 240V
- UK, Europe, Australia, most of Asia: 230V (use 230 or 240 — the difference is minimal)
- 12V DC (car, RV, boat, solar storage): use 12 — see our separate guide on watts to amps for 12V and 24V systems
If you are not sure what voltage a circuit runs on, check the breaker panel label or use a multimeter. Never assume — the difference between 120V and 240V doubles the current your wiring must carry.
Frequently asked questions
How many amps is 1,500 watts at 120V?
1,500 watts at 120V draws 12.5 amps (1,500 ÷ 120 = 12.5 A). This is close to the 15-amp limit of a standard US bedroom circuit, which is why a 1,500 W space heater on its own can trip a breaker if anything else is running on the same circuit.
How many amps is 1,500 watts at 240V?
1,500 watts at 240V draws only 6.25 amps (1,500 ÷ 240 = 6.25 A) — exactly half the current compared to 120V. This is why high-power appliances like dryers and ovens use 240V circuits: less current means thinner, cheaper wiring and less heat in the cable.
Why does higher voltage mean fewer amps for the same watts?
Because watts = volts × amps. If you increase voltage while keeping watts the same, amps must fall to keep the equation balanced. Think of it like water: more pressure (voltage) through a pipe means you need less flow rate (amps) to deliver the same power (watts).
Does a 240V appliance use less electricity than a 120V one?
Not necessarily. A 240V appliance doing the same job uses the same wattage and therefore the same energy in kWh. The advantage of 240V is that it delivers that power with half the current, which reduces wire heating, allows longer cable runs without excessive voltage drop, and permits thinner, cheaper conductors.