Watts, volts and amps explained: what's the difference?

Every generator, power station and appliance label talks in watts, volts and amps. You only need one formula to make sense of all three, and it fits on a sticky note.

The water pipe picture

Think of electricity like water in a pipe.

• Volts are the pressure. It's how hard the water is pushed. US homes use 120 V for most outlets and 240 V for big loads.
• Amps are the flow. It's how much water moves through the pipe each second.
• Watts are the work done. It's what the moving water can do, like turning a water wheel.

More pressure or more flow gives you more work. That's the whole idea behind the formula.

The formula, three ways

• Watts = volts × amps
• Amps = watts ÷ volts
• Volts = watts ÷ amps (you'll rarely need this one)

Appliance labels often list amps, not watts. Multiply by 120 to get watts. A fridge label that says 3.5 A on 120 V is about 420 W.

Worked example: a 1,500 W space heater

A space heater on its high setting uses about 1,500 W. Your outlets are 120 V.

1,500 W ÷ 120 V = 12.5 A

That's a lot of flow for one plug. Now see what the outlet can take.

Why it matters for 15 A and 20 A outlets

Most household outlets are on 15 A or 20 A circuits. Multiply by 120 V to get the most each can carry:

Circuit or outlet	Volts	Amps	Most watts (V × A)
5-15 (standard household)	120	15	1,800 W
5-20 (common on generators)	120	20	2,400 W
TT-30 (RV, 30 A)	120	30	3,600 W
L14-30 (generator 240 V outlet)	120/240	30	7,200 W (30 × 240)

So one 1,500 W heater (12.5 A) fits on a 20 A outlet. Two of them would be 25 A, or 3,000 W. That's more than the 2,400 W a 20 A outlet can give. The breaker trips, on your house panel or on the generator.

Leave headroom for long loads. A heater or AC that runs for hours is a "continuous" load. Electricians plan those at about 80% of the rating. That's why a 30 A connector is listed for 24 A continuous. On a 20 A outlet, that's about 16 A, or roughly 1,900 W.

Quick watts-to-amps table at 120 V

Watts	Amps at 120 V
600 W	5 A
1,200 W	10 A
1,500 W	12.5 A
1,800 W	15 A
2,400 W	20 A
3,600 W	30 A

What changes at 240 V

At 240 V, the same watts need only half the amps. A 4,500 W electric water heater draws 4,500 ÷ 240 = 18.75 A. On 120 V it would need 37.5 A. That's why big loads like water heaters, well pumps and central AC run on 240 V. See 120 V vs 240 V for how to tell which yours is.

How this helps you pick a generator

Generators are rated in watts, but their outlets are rated in amps. You need both to line up:

1. Add up the watts of what you'll run at once. Our generator size guide shows how.
2. Check each outlet's amps. A generator might make 4,000 W in total, but a single 5-20 outlet still tops out at 20 A (2,400 W).
3. Remember motors. A fridge or pump needs extra watts for a moment at start-up. That's covered in running vs starting watts.

If your generator keeps tripping, the cause is often too many amps on one outlet. Our overload tripping guide walks through it.

Watts are not watt-hours

Watts are how fast you use power right now. Watt-hours are how much energy you use over time. A battery's size is in watt-hours. If you're shopping for a power station, read watt-hours vs watts next.

Safety: Run it outdoors only, 20+ feet from the house, away from windows and doors. Read our carbon monoxide guide.