kW, kWh and amps: the practical difference

Power and energy are different quantities

Kilowatts (kW) describe the rate at which energy is transferred at a particular moment. Kilowatt-hours (kWh) describe an amount of energy accumulated over time. A 2 kW heater operating continuously for three hours uses 6 kWh. The same 6 kWh could be used by a 1 kW appliance over six hours. This distinction matters because an inverter may have enough battery energy for an appliance but still lack the instantaneous power needed to start it.

Where amps and voltage fit

Amps measure electrical current. For a simplified single-phase resistive load, power in watts is voltage multiplied by current. At 230 V, 16 A corresponds to about 3.68 kW. For a balanced European three-phase supply, the simplified relationship is √3 × line voltage × current × power factor. At 400 V and 16 A per phase, that is about 11.1 kW when the power factor is 1.

Why power factor changes the answer

The simple voltage-times-current result describes apparent power. Some motors and electronic loads draw current out of phase with voltage, so real power is lower. Power factor expresses that relationship. Resistive heaters are often close to 1, while other equipment can be lower. Do not assume a nameplate current always converts directly into the same real power.

Apply the units to EV charging

An 11 kW wallbox describes maximum charging power. In a hypothetical example, a battery has 72.6 kWh of usable energy. Adding 43.56 kWh to that battery cannot take exactly 3.96 hours in every case because the car may limit power and energy is lost in conversion. Charging power, battery energy and charging efficiency must be considered separately. This is not verification of a particular vehicle specification.

Apply the units to home backup

Battery capacity in kWh controls approximate runtime. Inverter output in kW controls how much equipment can run simultaneously. A 15 kWh battery with 90% usable capacity and 92% inverter efficiency can deliver roughly 12.42 kWh, but that does not prove it can start a pump, kettle or induction hob at the same time.

Safety boundary

These equations are useful for planning and checking orders of magnitude. They do not select a cable, breaker, residual-current device or earthing arrangement. Cable length, installation method, ambient temperature, voltage drop, fault current and national rules require a qualified electrical assessment.

Use a units check to catch a thousand-fold error

A 500 W load for two hours consumes 1,000 Wh, which is 1 kWh. At €0.25/kWh that costs €0.25. Multiplying 500 by two and treating the result as kWh would incorrectly produce €250. Keep the unit next to every intermediate value; do not rely on the number alone.

Power is a rate and energy is an accumulated quantity. A 10 kWh battery does not imply 10 kW output, and a 10 kW inverter does not imply ten hours of runtime. For a simple estimate, divide energy available to the load in kWh by average load in kW. Apply any reserve and conversion losses first, then check whether the equipment’s power ceiling can support the instantaneous load.

Sources and scope

EIA: measuring electricity · Schneider Electric: apparent power. References explain units and technology; hypothetical examples use the site’s own model. Owner observations are not laboratory tests.