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Watts vs Volts vs Amps: A Practical Explanation

Volts are the electrical pressure, amps the rate of flow, watts the power delivered: volts times amps. That one relationship sizes cables, fuses, circuits, inverters and electricity bills.

By the Akermin Editorial Team · 3 min read

Volts (V) measure electrical pressure, the push that drives current. Amps (A) measure current, the rate at which charge flows. Watts (W) measure power, the rate at which energy is delivered, and for DC and simple AC loads watts equal volts times amps. A 120 V circuit carrying 10 A delivers 1,200 W. Volts are set by the supply, amps by what the load demands, and watts is what you pay for over time as kilowatt-hours.

The water analogy

Think of a pipe. Voltage is the water pressure; current is the flow rate in liters per second; power is pressure times flow, the rate at which the water can do work. A narrow pipe (high resistance) passes less flow for the same pressure; a wide one passes more. Double the pressure through the same pipe and the flow doubles. The analogy breaks down eventually, but it carries the essential relationships.

The formulas

  • Power: P = V × I (watts = volts × amps)
  • Ohm’s law: V = I × R (volts = amps × ohms)
  • Combined: P = I² × R and P = V² ÷ R
  • AC with power factor: P = V × I × PF

The watts / volts / amps calculator solves any of these from two known values.

Worked examples

Situation Volts Amps Watts
Kettle on 230 V mains 230 13 3,000
Space heater on 120 V mains 120 12.5 1,500
Phone charger output 5 2 10
Car headlight bulb 12 4.6 55
Solar panel at maximum power 36 11 400
LED bulb 230 0.04 9

Why each number matters

  • Volts decide compatibility. A 120 V appliance on 230 V draws far too much current and fails; a 230 V appliance on 120 V underperforms. Battery systems are 12, 24 or 48 V and everything must match.
  • Amps decide cable and fuse size. Cables heat in proportion to current squared, so the current rating, not the wattage, sizes the wire and the breaker. A 15 A circuit is a 15 A circuit whatever the voltage.
  • Watts decide capacity and cost. An inverter is rated in watts; a generator in watts; the electricity bill in watt-hours. See what a kilowatt-hour is.

Power factor

For AC loads that are not purely resistive (motors, transformers, most electronics), current and voltage are out of step and volts times amps overstates the real power. The ratio is the power factor, typically 0.6 to 0.95. A motor drawing 5 A at 230 V with a power factor of 0.8 uses 920 W of real power but the circuit must carry the full 5 A. Cables and breakers are sized on amps; bills are charged on watts.

Measuring each

For how much a device costs to run, see how much electricity a 100-watt device uses.

P3 P4400 Kill A Watt Electricity Usage Monitor

P3 P4400 Kill A Watt Electricity Usage Monitor

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Klein Tools CL390 AC/DC Digital Clamp Meter, Auto-Ranging, 400 Amp

Klein Tools CL390 AC/DC Digital Clamp Meter, Auto-Ranging, 400 Amp

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Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.

Frequently Asked Questions

How do I convert amps to watts?

Multiply by the voltage: 10 A at 120 V is 1,200 W; 10 A at 230 V is 2,300 W. For AC loads with motors or electronics, multiply by the power factor as well.

How many amps does a 1,500 W heater draw?

About 12.5 A at 120 V or 6.5 A at 230 V. On a 15 A circuit at 120 V that leaves little margin for anything else.

What is a kilowatt-hour then?

Energy: power multiplied by time. A 1,000 W device running for one hour uses 1 kWh, which is what the electricity meter counts.

Affiliate disclosure: Akermin earns a commission from qualifying Amazon purchases made through links on this page. Our editorial picks are based on documented specifications and owner feedback, not commissions.