Volts, Amps, Watts and Ohms: What Is the Difference?

What does each measurement tell you?

Volts (V) measure potential difference between two points: energy per unit charge. Amperes, usually called amps (A), measure current: how much charge passes a point per second. Ohms (Ω) measure resistance. Watts (W) measure power: the rate at which energy is transferred.

The units are connected, but they are not interchangeable. Saying equipment is “high power” does not tell you its supply voltage, phase arrangement or protection requirements. An amp rating alone cannot tell you how much energy it will use during the day.

Further reading: 9.1 Electrical Current · 7.2 Electric Potential and Potential Difference · 9.4 Ohm's Law · 9.5 Electrical Energy and Power

Read a label as a set of conditions.

Imagine a fictional resistive heater labeled 120 V, 1,200 W. Those values describe its intended supply and rated power under specified operating conditions. They do not mean every appliance connected to a 120 V circuit draws the same current.

The supply rating also does not mean a device draws its full rated power continuously. A thermostat can switch a heater off. A variable-speed drive can change motor operation. An electronic device may draw power differently in standby and active use.

Real equipment can list frequency, phase, a range of acceptable voltages and other electrical requirements. Read those conditions together with the manufacturer’s installation instructions. A matching voltage is one part of compatibility.

Worked example: 1,200 watts at 120 volts.

For the fictional heater at its rated condition, treat the load as resistive with power factor 1. Real power is then voltage multiplied by current, so current is power divided by voltage.

1,200 W ÷ 120 V = 10 A. This is the current in the example while the heater is operating at that condition. It is an explanation of the relationship, not a selection of a breaker, receptacle or wire.

If the heater runs steadily for two hours, it transfers 2,400 watt-hours, or 2.4 kilowatt-hours, of energy. That adds time to the calculation. The watts describe the rate; the kilowatt-hours describe the amount.

Further reading: 9.5 Electrical Energy and Power

Circuit capacity and actual draw are different.

A circuit’s rating is not a statement that the circuit constantly uses that many amps. Connected loads draw current according to their behavior and operating conditions. Multiple loads can contribute to the current supplied by a shared circuit.

The circuit also has installation limits and protective requirements. You cannot determine remaining capacity simply by subtracting a single nameplate number, or determine service capacity by adding all the breaker handle numbers. The applicable evaluation considers the complete installation and intended use.

Where a simple watts-to-amps shortcut stops.

For a steady DC load, electrical input power is voltage multiplied by current. For an AC load, multiplying RMS voltage and RMS current gives apparent power in volt-amperes. Real power in watts also depends on power factor.

Motors and electronic power supplies may have starting behavior or non-sinusoidal current that a fixed-resistor example does not represent. Three-phase equipment introduces additional relationships. Use the equipment information and appropriate analysis rather than extending the heater example to every device.

Further reading: 15.4 Power in an AC Circuit · What is a nonlinear load?

Questions you may be asking.