Watts, Amps and Volts, Explained Once and For All
By the VoltConvert team · June 12, 2026
Every electrical question — can this circuit handle that heater, why did the breaker trip, what size wire do I need — eventually comes down to three units: volts, amps and watts. Most explanations either drown you in physics or hand-wave with bad water analogies. Here's the working version that electricians actually use.
The three players
Volts (V) are electrical pressure — how hard the supply pushes. In North America, wall outlets push at 120 V with 240 V available for big appliances; the UK, Europe, and most of the world push at 230 V. Amps (A) are flow — how much current actually moves through the wire. Amps are what heat conductors, what breakers count, and what hurts you. Watts (W) are work — the rate at which energy is actually being used to make heat, light or motion.
The relationship is one multiplication: watts = volts × amps. Pressure times flow equals work. Everything else in basic electrical math is this formula rearranged.
Why the same appliance draws different amps in different countries
A 1,500 W space heater is a 1,500 W heater anywhere on Earth — but in a US bedroom at 120 V it draws 12.5 A, while the same heat in a UK bedroom at 230 V draws only 6.5 A. Higher voltage means less current for the same work, which is why high-power appliances everywhere (dryers, ranges, EV chargers) get higher-voltage circuits: half the amps means thinner, cheaper copper and cooler wiring. Try it yourself with the watts to amps calculator — flip between 120 V and 230 V and watch the current halve.
What breakers actually protect
A circuit breaker doesn't know or care about watts — it counts amps, because current is what overheats wire. A US 15 A circuit at 120 V can theoretically carry 1,800 W, but the NEC's 80% rule for continuous loads brings the practical ceiling to 1,440 W. That's why a single space heater plus almost anything else trips a bedroom circuit, and why "how many watts can this circuit take" is always really an amps question. Going the other way — you know the current, want the power — is the amps to watts calculator.
The AC complication: power factor
For heaters and old-fashioned bulbs, volts × amps = watts, full stop. But motors, compressors and electronics draw some current that does no useful work — it just sloshes back and forth sustaining magnetic fields. The fraction of current doing real work is the power factor (PF), and for AC the honest formula is watts = volts × amps × PF. A typical motor at PF 0.85 draws about 18% more current than its wattage suggests. Multiply volts by amps without PF and you get volt-amps (VA) — apparent power — which is its own useful unit for sizing supplies (see the VA to amps calculator).
Ohm's law, the other rearrangement
When the load is a simple resistance — a heating element, say — current also follows I = V ÷ R. A 240 V dryer element of 12 Ω draws 20 A, no power factor involved. The volts to amps calculator handles both the resistance route and the wattage route.
The five-second summary
Volts push. Amps flow. Watts work. W = V × A (× PF for AC motors and electronics). Breakers count amps, bills count watt-hours, and every electrical estimate you'll ever make starts from one of those rearrangements. Bookmark the calculators, and the formula sheet lives in your pocket.