Amps to watts, and the voltage people assume

Watts equal amps times volts. That is the whole formula, and it is not where conversions go wrong.

They go wrong one step earlier, at the point where somebody decides which voltage to put in.

The voltage is an input, not a constant

A 15 A load at 120 V is 1,800 W. The same 15 A at 240 V is 3,600 W. Nothing about the appliance changed and the answer doubled.

In a US house this matters constantly, because the building carries both. Lighting and general outlets are 120 V. The range, the dryer, the water heater, the well pump and most central air conditioning are 240 V on a double pole breaker. Running the whole load list at 120 V, which is the default assumption, understates the big appliances by half.

In most of Europe the single voltage removes that trap and introduces a different one, which is three phase.

Three phase needs the extra factor

For three phase, watts equal amps times volts times the square root of three times power factor. The 1.732 is easy to forget and large enough to matter.

Miss it and a three phase load calculates 42% low. That is the direction that undersizes a cable or a generator, so it is worth a deliberate check any time the equipment has three live conductors rather than one.

The watts calculator handles DC, single phase and three phase separately, which removes the decision from memory, and the kVA calculator covers the apparent power side when a spec sheet gives kVA instead.

Power factor separates two kinds of watts

Multiply amps by volts and you get volt-amperes, which is apparent power. For a kettle or a heater that is the same as watts, because resistive loads use everything they draw.

Anything with a motor or a switch mode power supply does not. Part of its current builds magnetic fields or charges capacitors without doing work, so real power is lower than the product suggests. The ratio is power factor, often around 0.8 for a motor, and it is why a generator rated in watts can struggle with a motor load that looks like it fits.

Reading a nameplate without guessing

Nameplates are written for inspectors rather than for owners, so a little translation helps.

A plate giving volts and amps describes the running draw at that voltage. A plate giving watts has done the arithmetic for you. Where both appear and disagree slightly, the watts figure usually accounts for power factor and the volts times amps does not.

Look out for a locked rotor amps figure on anything with a compressor. That is the startup draw, frequently three to six times the running current, and it is the number that decides whether a generator stalls rather than the one on the front of the plate.

Breakers describe the circuit, not the load

A 20 A breaker does not mean the thing plugged into it draws 20 A. It means the wiring behind it is protected at 20 A.

For load planning the breaker is a ceiling, not a measurement, and for continuous loads the working limit is 80% of it. On a 20 A circuit that is 16 A, which at 120 V is 1,920 W, and that is the honest figure to plan a long-running load against.

Measuring beats calculating

For anything that matters, a plug-in energy meter settles the question in a way a nameplate cannot. It shows the actual draw under real conditions, which is usually below the nameplate figure and sometimes well below.

The exception is anything hardwired or on 240 V, where a plug meter will not fit. There the nameplate, with the locked rotor figure noted separately, is what you have, and treating it as an upper bound rather than a measurement is the right posture.