Amps to Watts Converter
| Circuit | Breaker | Volts | Max watts | 80% continuous |
|---|---|---|---|---|
| Bedroom / general outlets | 15 A | 120 | 1,800 | 1,440 |
| Kitchen outlets | 20 A | 120 | 2,400 | 1,920 |
| Water heater / dryer | 30 A | 240 | 7,200 | 5,760 |
| Range | 40 A | 240 | 9,600 | 7,680 |
| Hardwired EV charger | 48 A | 240 | 11,520 | 9,216 |
One multiplication runs the show: watts = amps x volts. The numbers worth memorizing: a 15-amp circuit at 120 V delivers 1,800 watts, a 20-amp one 2,400 - and at 240 V the same breakers carry double, 3,600 and 4,800 watts, because voltage multiplies what each amp can move. Type amps and voltage and the converter gives watts live, or run it backwards from a device's wattage to see which circuit it belongs on.
The second number on every breaker matters more than the first: the 80% continuous rule. A 20-amp breaker is built to carry 2,400 watts all day, but only 1,920 watts forever - loads that run three hours or more (heaters, EV charging, server racks) must stay under it. That single line explains most tripped-breaker mysteries: the math said 2,200 watts fits, the breaker disagreed an hour later. The table below lists the common circuits with both figures.
How to use
- Type the circuit amps (the number on the breaker handle) and the voltage, and watts appear instantly.
- Pick 120 V for standard outlets, 240 V for dryers, ranges, water heaters and EV chargers; three-phase for shop or commercial panels.
- Compare the result to the 80% column in the table before plugging in anything that runs for hours.
Frequently asked questions
How many watts is 15 amps?
1,800 watts at 120 V (15 x 120) or 3,600 watts at 240 V. For anything that runs more than three hours, the safe continuous figure is 80% of that: 1,440 watts at 120 V - one space heater, and nothing else on the circuit.
How many watts is 20 amps at 240 volts?
4,800 watts (20 x 240). This is the window air-conditioner and small-EV-charger territory: 4,800 watts at 100% and 3,840 watts continuous. The same 20-amp breaker at 120 V carries just 2,400 watts - voltage is half the product, which is why big appliances get their own 240-volt circuits.
What is the 80 percent rule for breakers?
Breakers are thermal devices: a 20-amp unit tolerates brief spikes above 20 amps but heats up and trips when a sustained load sits near its rating. Code therefore caps continuous loads (three hours or more) at 80%: 12 A on a 15-amp circuit, 16 A on 20. It is why a 1,500-watt heater plus a 700-watt microwave - 2,200 watts on a 2,400-watt circuit - holds at breakfast and trips mid-afternoon: the load never spikes past the rating, it just never cools down either.
Can a space heater and a microwave share a circuit?
Do the watts first: 1,500 W heater + 1,000 W microwave = 2,500 watts, well over the 1,920 continuous / 2,400 peak limit of a 20-amp 120-volt circuit. The breaker will hold for a while (it tolerates 120% briefly) and trip mid-afternoon - which feels random but is the thermal trip doing its job. Rule of thumb: heater gets a circuit, everything else negotiates around it.
Why does 240 volts double the watts at the same amps?
Watts = volts x amps: hold amps constant and every extra volt carries more energy per unit charge. A 30-amp dryer circuit is 3,600 watts at 120 V but 7,200 watts at 240 V - the same copper, twice the work. That is the whole reason heavy appliances use 240 V: heat lost in the wiring scales with amps alone (P = I2R), so pushing power with volts instead of amps keeps the walls cool.