Power is volts times amps, so amps are watts divided by volts — and that means there is no fixed watts-to-amps conversion. A 1,500 W appliance draws 12.5 A at 120 V and 6.5 A at 230 V: the same power, half the current. That relationship is the entire argument for higher distribution voltages, because losses in a cable go with the square of the current.
Every question in this family is the same equation rearranged. Knowing which two quantities you have tells you which form to use.
What are the four forms?
Ohm’s law and the power relationship between them cover four quantities, and knowing any two gives the other two.
| You know | Formula | Example |
|---|---|---|
| Volts and amps | P = V × I | 230 V × 13 A = 2,990 W |
| Watts and volts | I = P ÷ V | 3,000 W ÷ 230 V = 13.04 A |
| Watts and amps | V = P ÷ I | 60 W ÷ 5 A = 12 V |
| Amps and ohms | V = I × R | 0.5 A × 24 Ω = 12 V |
| Watts and ohms | V = √(P × R) | √(60 × 10) = 24.5 V |
The last two are why a plain watts-to-volts converter cannot exist. Power is a product of two independent quantities, so a 60 W load could be 12 V at 5 A, 230 V at 0.26 A, or anything else on the same curve — you need a second measurement.
Where does the power factor come in?
On AC, with anything that is not a simple heating element. Real power is V × I × pf, and the power factor is the fraction of the delivered volt-amps that does useful work.
For a resistive load — a kettle, a heater, an incandescent lamp — the power factor is 1 and watts equal volt-amps exactly. For a motor, a fluorescent ballast or an older switch-mode supply, current lags or distorts and the two diverge. Eight amps at 230 V is 1,840 VA of apparent power and only 1,472 W of real power at a 0.8 power factor.
This matters when reading a clamp meter. A clamp measures current, not power, and multiplying it by the voltage gives volt-amps. For a heater that is the answer; for a motor it is an overestimate of the power and the correct figure for sizing the supply.
Why do the low-voltage cases need such thick cable?
Because current is inversely proportional to voltage, and cable is sized on current. At 12 V a 60 W load pulls 5 A and a 500 W inverter load pulls 41.7 A — nearly twenty times what the same 500 W would draw at 230 V.
Voltage drop compounds the problem. A 0.5 V loss in the cable is 0.2 per cent of a 230 V supply and 4 per cent of a 12 V one, so the same absolute loss is twenty times more damaging. That is why vehicle and van wiring is so much heavier than mains wiring for the same power, and why anything larger than a small off-grid system moves to 24 or 48 V.
Doubling the system voltage halves the current, which quarters the resistive loss, because loss goes with the square of current. Going from 12 V to 24 V is the single largest efficiency decision in a small DC installation.
What is the 125 per cent rule?
A sizing margin for continuous loads. Breakers and cables are rated for a steady thermal limit, and a load running for three hours or more sits right at that limit with nothing in reserve — so the protective device is sized at 125 per cent of the calculated current.
A 3,000 W load at 230 V draws 13.04 A, which becomes 16.3 A after the margin and a 20 A device in practice. Motors need more thought again: starting current can be six or seven times the running figure for a second or two, which is a question about the breaker curve rather than its rating.
What are the practical ceilings?
Lower than people expect, and they are why some appliances need their own circuit.
| Supply | Practical ceiling |
|---|---|
| 120 V, 15 A | 1,800 W |
| 230 V, 13 A plug fuse | 2,990 W |
| 230 V, 16 A circuit | 3,680 W |
| 230 V, 32 A circuit | 7,360 W |
| 400 V three phase, 32 A | 22,170 VA |
A 7.4 kW car charger is 32 A at 230 V single phase and only 10.7 A per line at 400 V three phase. Adding that charger, an 11 kW shower and an induction hob to one single-phase supply is arithmetic that stops working quickly, which is why load management exists.
Questions people ask
How many amps is 1,500 watts? It depends entirely on the voltage: 12.5 A at 120 V, 6.5 A at 230 V, and 125 A at 12 V. There is no single answer, which is why a chart cannot replace the division.
Is VA the same as W? Only for a resistive load. Volt-amps is what the supply must deliver; watts is what does work. Supplies, cables and generators are sized in VA.
Why is 230 V not exactly 230? European harmonisation set 230 V with a tolerance of +10%/−6%, which covers historic 220 V and 240 V systems without anyone rewinding a transformer. A UK socket often measures nearer 240 V.
Does DC use the same formula? Yes, and more simply — there is no power factor, so watts are exactly volts times amps.
One equation, rearranged five ways, and the voltage decides everything. The power calculator, current calculator and voltage calculator cover the general cases; watt to amp, amp to watt, watt to volt and kW to amp fix one direction each; and 12 V, 24 V and 230 V start from the supply you are actually on.