How to Calculate Watts (With Worked Examples)

To calculate watts, multiply volts by amps: a 120-volt hair dryer pulling 10 amps draws 1,200 watts (120 × 10 = 1,200). That single formula, W = V × A, handles most household devices, but three situations trip people up: a nameplate that lists only amps, a rating given in volt-amps (VA) instead of watts, and a battery or solar setup that runs on DC voltage instead of 120V AC. This guide works through all three, then covers running watts versus surge watts and the watts-versus-watt-hours mix-up that causes most power station and generator sizing mistakes.

How to calculate watts from volts and amps

The formula is watts = volts × amps. Standard US wall outlets run at 120 volts, so a device drawing 5 amps there uses 600 watts (120 × 5). Large appliances wired on their own circuit — electric clothes dryers, ranges, EV chargers, and central air conditioners — typically run on 240-volt circuits, so the same 5 amps at 240 volts works out to 1,200 watts (240 × 5). Always note which voltage you’re working with before multiplying; an amps figure by itself tells you nothing about wattage until you pair it with a voltage. The watts-to-amps calculator and amps-to-watts calculator run this formula both directions if you’d rather skip the mental math.

When the label only lists amps

Most appliance nameplates print voltage and amps, not watts, because amps and voltage are what the wiring, plug, and breaker actually care about. A portable heater stamped 120V, 12.5A draws 1,500 watts (120 × 12.5) — worth knowing before you plug it into a circuit shared with anything else, since a standard 15-amp circuit is only rated for 1,800 watts total, and code treats continuous loads (running 3 hours or more) as limited to 80% of that, or 1,440 watts. A refrigerator or window air conditioner nameplate works the same way: read the amps, note the voltage, multiply. On a refrigerator the result is a maximum rating well above the 100-250W it draws while the compressor runs. See the appliance wattage chart for typical running-watt figures across dozens of household devices if a label is missing or worn off.

When the rating is in VA instead of watts

UPS units, some inverters, and a handful of electronics list volt-amps (VA) rather than watts. VA is calculated exactly like watts — volts × amps — but it measures apparent power rather than real power, and the two only match when the load’s power factor (PF) is 1.0. True watts = VA × PF. Resistive loads like space heaters and incandescent bulbs run at PF ≈ 1.0, so their VA and watt ratings are identical. Devices with switching power supplies — laptops, monitors, most modern electronics — pull power less efficiently, with PF commonly in the 0.6-0.8 range. A UPS labeled 1000 VA / 600 W is telling you its power factor: 600 ÷ 1000 = 0.6. If a device only lists VA, multiply by the PF on its label to get real watts. If no PF is given, treat the VA figure as watts when sizing a power station, generator or circuit, since a PF of 1.0 is the safe upper bound; use 0.6-0.7 only as a typical value when estimating energy use. The VA-to-watts calculator does the conversion once you know the PF.

DC devices: watts from a 12-volt battery

Anything that runs off a battery instead of a wall outlet — a car accessory, an RV appliance, a solar setup — uses the identical formula with DC voltage in place of 120V. A 12-volt cooler drawing 5 amps uses 60 watts (12 × 5); a 12-volt air compressor pulling 15 amps uses 180 watts (12 × 15). This matters for anyone sizing a battery bank or solar setup, since battery capacity is rated in amp-hours (Ah) at a specific voltage, and amp-hours × volts = watt-hours, the unit you actually need for runtime math. The volts-to-watts calculator handles both AC and DC cases.

Running watts vs. surge watts

The watts figure on a nameplate is a maximum rating, and for anything with a motor or compressor it is usually above the running draw; those appliances — refrigerators, air conditioners, well pumps, power tools — also pull a short surge of 3 to 7 times their running current for a fraction of a second at startup while the motor overcomes initial resistance. A mini fridge whose nameplate reads 120V, 1.3A (156 watts) typically runs at 50-100 watts and can surge to roughly 400-800 watts the instant its compressor kicks on, so plan for the top of that range. This is why a generator or power station rated for exactly your running-watts total can still stall or trip on startup: the surge, not the running load, is often what determines the minimum size you need. The air conditioner wattage guide breaks down running and surge figures for window and central units specifically.

Watts vs. watt-hours: the mix-up that causes the most confusion

Watts measure the rate of energy use right now; watt-hours (Wh) measure total energy consumed over time. Multiply watts by the hours a device runs to get watt-hours: a 65-watt laptop running for 3 hours uses 195 watt-hours (65 × 3). Divide watt-hours by 1,000 to get kilowatt-hours (kWh) — the unit on your electric bill — so 195 Wh equals 0.195 kWh. This distinction matters most when you’re estimating runtime from a power station’s capacity, because inverters lose roughly 15% of stored energy converting DC battery power to AC: usable runtime in hours is about equal to capacity in Wh × 0.85, divided by the load in watts. A power station rated at 1,000 Wh running that 65-watt laptop would last about 13 hours (1,000 × 0.85 ÷ 65 ≈ 13.1). The watts-to-kWh calculator handles the conversion for larger appliances and billing estimates.

From watt-hours to what it costs

To turn energy into a dollar figure, convert to kilowatt-hours and multiply by your rate: cost = kWh × price per kWh. A 1,500-watt space heater running 4 hours a day uses 6 kWh (1,500 × 4 ÷ 1,000). At 18.31 cents per kWh, the US residential average EIA reported for July 2026, those 6 kWh cost about $1.10 a day (6 × $0.1831), or roughly $33 over a 30-day month.

Use the price on your own bill rather than the national average, because the spread is wide: the same EIA table puts July 2026 residential prices at 12.72 cents in Louisiana but 33.61 cents in California and 48 cents in Hawaii, so that heater would cost about 76 cents a day in Louisiana, $2.02 in California and $2.88 in Hawaii. Many bills also use tiered rates, so the price per kWh can change with how much you use.

For appliances that cycle on and off, multiply average watts, not running watts, by the hours. A full-size fridge draws about 150 W while the compressor runs but averages about 60 W across a day, which is 1.44 kWh (60 × 24 ÷ 1,000), or roughly 26 cents a day at 18.31 cents. Multiplying the 150 W running figure by 24 hours would overstate that cost 2.5 times.

Adding up loads to size a generator or power station

List every device you want to run at the same time, calculate each one’s running watts with the formula above, and add them together for a total running-watts figure. Separately, note the single highest surge figure among those devices — surges from multiple motors rarely overlap, so you generally only need headroom for the largest one, not all of them stacked. A generator or power station needs a continuous AC rating above your total running watts and a surge rating above that single highest spike. Build in some margin rather than sizing to the exact number, since nameplate ratings are typically worst-case and real draw runs a little lower. The power station sizing guide walks through this process end to end, including how to handle mixed 120V and 240V loads. The table below shows the math for common devices.

Safety notes

Never connect a generator to your home’s wiring without a transfer switch. Plugging it into a wall or dryer outlet, often with a double-male “suicide” cord, is called backfeeding; CPSC warns it puts utility workers and neighbors served by the same transformer at risk of electrocution and bypasses some of the home’s built-in circuit protection. OSHA adds that a generator attached to a building’s wiring without a properly installed transfer switch can energize lines for great distances. The safe route is a transfer switch installed by a qualified electrician, which lets only one source, grid or generator, feed the panel at a time.

If you’re running a fuel-burning generator, keep it outdoors, at least 20 feet from the home with the exhaust pointed away from windows, doors and vents; carbon monoxide from a generator can build up to lethal levels in a garage, breezeway, or covered patio even with the door open. Put battery-operated or battery-backup CO alarms on every level of the home.

Frequently asked questions

How many watts is 15 amps at 120 volts?

15 amps × 120 volts = 1,800 watts. A standard 15-amp household circuit is rated for 1,800 watts total, but continuous loads (anything running 3 hours or more) should stay under 80% of that, or 1,440 watts, to leave headroom under electrical code.

How do I calculate watts from watt-hours?

Divide watt-hours by the number of hours the device ran: watts = Wh ÷ hours. A power station that delivered 1,000 Wh over 5 hours averaged 200 watts of draw (1,000 ÷ 5) across that period.

Does the watts formula change for 240-volt appliances?

No, the formula is the same — watts = volts × amps — you just substitute 240 for 120. A dryer drawing 20 amps at 240 volts uses 4,800 watts (240 × 20), twice the 2,400 watts the same 20 amps would carry on a 120-volt circuit.

What is the difference between watts and volt-amps?

Watts measure real power actually consumed; volt-amps (VA) measure apparent power, which includes some energy that flows back and forth without doing work. The two are equal only when a device’s power factor is 1.0; otherwise watts = VA × power factor, and VA is always the larger or equal number.

How do I calculate what size power station or generator I need?

Add the running watts of everything you’d run at once, then check the single highest surge watts among those devices separately. Choose a unit whose continuous AC rating clears your running-watts total and whose surge rating clears that top spike, with some margin on both numbers.