Watts, volts, and amps
Watts, volts and amperes measure different electrical quantities. A watt measures power: the rate of energy transfer. A volt measures electrical potential difference. An ampere, usually shortened to amp, measures current. A generator and its connected equipment must be compatible in all three quantities. [1]
Three quantities, three questions
Voltage describes the electrical potential across a circuit; current describes the charge flowing through it. Power describes how quickly energy is delivered or used. These are related, but a voltage rating alone does not describe capacity, and an amperage value needs a voltage and circuit context to be interpreted. [1]
For example, two devices can both use 1,000 watts while operating at different voltages and drawing different currents. This does not make them electrically interchangeable. The generator must supply the voltage and frequency specified for each device.
Power formulas
For a steady DC circuit, P = V × I, where P is watts, V is volts and I is amps. In single-phase AC, S = Vᵣₘₛ × Iᵣₘₛ gives apparent power in volt-amperes. Real power is P = Vᵣₘₛ × Iᵣₘₛ × PF, where PF is the load’s power factor. [2]
The shortcut W = V × A applies to AC real power when power factor is 1, or approximately so for a suitable resistive load. It should not be assumed for every motor or electronic device. True power factor also accounts for waveform effects, rather than only the phase angle of ideal sine waves. [2]
| Quantity sought | Single-phase AC relationship |
|---|---|
| Apparent power (VA) | RMS volts × RMS amps |
| Real power (W) | RMS volts × RMS amps × power factor |
| Current (A), when real power and PF are known | Watts ÷ (RMS volts × power factor) |
An illustrative calculation
A single-phase load drawing 8 A at 120 V has an apparent power of 960 VA. At a power factor of 0.75, its real power is 720 W. Those numbers describe the same load: the conductors and supply still carry 8 A even though real power is less than 960 W. This is an illustration, not an appliance rating.
Power, energy and circuit limits
Kilowatts describe power. Kilowatt-hours describe energy accumulated over time. A steady 1 kW load operating for two hours uses 2 kWh. An annual energy figure cannot by itself establish instantaneous running demand or starting demand. [3]
Generator total capacity and individual connection limits must both be checked. A suitable total wattage does not establish that one receptacle, cable or circuit can carry the intended load. Protective-device and circuit ratings remain separate constraints. [4]
Sources and further reading
- Department of Energy — Building America electrical terminology ↗www.energy.gov
- Keysight — Real power, apparent power and power factor ↗www.keysight.com
- Department of Energy — Energy Savers: kilowatt-hours ↗www.energy.gov
- Generac — Portable generator power and outlet limits ↗support.generac.com
Use the manual and nameplate for the exact model. Local requirements and site conditions take precedence over general examples.