Amps to Watts Calculator

What an amp is, and what a watt is

An ampere measures flow: how much electric charge passes a point each second. A watt measures power: how fast energy is being converted. The two are linked by the voltage, which is why the same 10 A means something different at 12 V than at 240 V.

The formula for a resistive load, DC or AC, is one multiplication: watts = volts x amps. The US Environmental Protection Agency explains the same relation for EV charging: a charger listing amps and one listing kilowatts are describing the same thing once voltage is known, and amps = watts / volts is the rearrangement that converts between them.

Because the voltage does the scaling, the answer is different on every system. The same 12 A of current is 144 W at 12 V, 1,440 W at 120 V and 2,880 W at 240 V. Converting amps to watts without stating the voltage is the single most common error on this calculation, and the table below shows how wide the spread is.

The US Energy Information Administration lesson on measuring electricity defines volts, amps and watts the same way, and it is the reference to keep at hand.

The three formulas, one per system

AC systems add two complications: a power factor, and in three phase work a square root of three. The three formulas cover every case a home or shop calculation meets.

The power factor, written PF, accounts for the phase shift between voltage and current that inductive loads create. A value of 1.0 means voltage and current peak together, which is heaters and incandescent lamps. Motors without correction run 0.80 to 0.90, which is why the nameplate current of a motor cannot be turned into watts without knowing the PF. When the nameplate only states amps and volts, a PF of 0.85 is the standard estimate for a small motor, and the calculator lets you change it.

The 1.732 in the three phase formula is the square root of 3. A three phase system delivers power over three conductors whose voltages are offset by 120 degrees, and the geometry of that offset produces the constant. Line to line voltage, such as 208 V or 480 V, is what the formula expects, because that is what a meter reads between any two conductors.

Worked example: a 120 V space heater

A 1,500 W portable heater is the standard plug-in space heater sold in North America, and its nameplate says 120 V, 12.5 A. The conversion checks both ways: 120 x 12.5 = 1,500 W, and 1,500 / 120 = 12.5 A, the same arithmetic in both directions.

Heating elements are resistive, so the power factor is 1.0 and no correction applies. The resistance implied by those numbers is R = 120 / 12.5 = 9.6 ohms, a number that only means anything while the element is hot, because a cold element reads lower.

The circuit side matters more than the heater side. A 15 A, 120 V outlet serves 15 x 120 = 1,800 W before the breaker trips, so a 1,500 W heater fits. The 80 percent rule for continuous loads in the National Electrical Code limits a load running three hours or more to 80 percent of the breaker rating, which on that outlet is 12 x 120 = 1,440 W, which is why the instructions on most 1,500 W heaters ask that they be the only thing plugged into the circuit.

Applying the same conversion to common nameplates:

Worked example: a 12 V system, RV and marine

Low voltage systems flip the numbers around. An RV refrigerator drawing 5 A on a 12 V battery consumes 12 x 5 = 60 W, and over a full day that is 60 x 24 = 1,440 Wh, or 1.44 kWh, from the battery bank.

Inverter loads are where amps mislead most. A 1,500 W microwave on a 12 V inverter draws 1,500 / 12 = 125 A from the battery side even though the outlet side is only 12.5 A at 120 V, a factor of 10 difference. That battery-side figure decides the wire gauge and the fuse size, not the outlet-side one, and it is the reason high draw appliances list both numbers in their manuals.

The same arithmetic at 24 V halves the current: a 1,500 W load on a 24 V bank draws 1,500 / 24 = 62.5 A. Marine and off-grid systems often choose 24 V or 48 V on purpose for exactly this reason, because halving the current quarters the heat loss in the wiring, since P = I^2 x R.

Worked example: a three phase shop tool

A machine shop motor nameplate reads 208 V three phase, 24 A, PF 0.90. The three phase formula gives 208 x 24 x 1.732 x 0.90 = 7,782 W, which rounds to 7.8 kW.

The same 7,782 W of load on a 480 V line to line supply draws 7,782 / (1.732 x 480 x 0.90) = 10.39 A instead of 24 A. Higher voltage lowers the current for the same power, which is why long feeds and fixed machinery run on the higher voltage in the first place.

Checking a car charger in Europe shows the formula travels. A 7,360 W charger on 400 V three phase at PF 1.0 draws 7,360 / (1.732 x 400) = 10.62 A per line, which fits a 16 A three phase outlet with room to spare, and the same charger pulls 7,360 / 230 = 32 A on a single phase 230 V socket.

Starting watts versus running watts

Generators are sized on starting watts, not running watts, and the conversion is what exposes the difference. A refrigerator that draws 6 A at 120 V while running converts to 720 W of running power. Its compressor may ask two to three times that for the fraction of a second it spins up, which is 1,440 to 2,160 W of starting demand.

This is why generator sizing guides tell you to add the largest starting surge on top of the sum of running watts, not to multiply the whole sum. Our generator size calculator does that arithmetic, and the amps to watts conversion in this page is the step before it: turn every nameplate into watts first, then let the sizing logic run.

Where the power factor comes from

Volts times amps gives apparent power, measured in volt-amperes (VA). Real power in watts is what the meter bills and what turns into heat or work. The power factor is the ratio between them: PF = W / VA. A load drawing 500 VA that delivers 450 W runs at 450 / 500 = 0.90.

Resistive loads have PF 1.0, so VA and watts coincide, and the conversion needs no factor. Inductive loads, which is any motor, transformer or magnetic ballast, lag their current behind the voltage, and the gap between VA and W opens up. Old fluorescent ballast setups ran as low as 0.50, and modern corrected electronic gear runs 0.95 or better.

The practical consequence for this conversion is simple: if the nameplate states watts, no conversion is needed at all. If it states only volts and amps, multiply by a power factor, or you will overestimate the real draw. An 1875 W hair dryer advertisement works out to 15.6 A at 120 V, which is 1,875 VA before any factor, and heating appliances are the honest ones: their VA is their W.

Mistakes that throw the conversion off

Forgetting the voltage. Amps without a stated voltage cannot be converted. The same 12 A is 144 W at 12 V and 2,880 W at 240 V, a factor of 20.

Using peak instead of RMS. North American 120 V is an RMS value. Multiplying 12 A RMS by 120 V RMS gives real watts directly. Multiplying by the peak value, which is 170 V for a 120 V line, inflates the answer by 41 percent.

Skipping the power factor on motors. A 10 A motor nameplate at 120 V is 1,200 VA, not 1,200 W. At PF 0.85 the real power is 1,020 W. Resistive loads are the exception where the factor can be left at 1.0 safely.

Mixing line to line and line to neutral in three phase. The 1.732 formula expects line to line voltage. A 120/208 V three phase panel gives 208 V line to line and 120 V line to neutral, and using the wrong one scales the answer by 1.732. Divide the line to neutral formula instead: W = 3 x V_line_neutral x A x PF, which on the same motor gives 3 x 120 x 24 x 0.90 = 7,776 W, the same answer from the other direction.