Power vs energy (kW vs kWh)
Power (kW) is the rate of energy flow at an instant; energy (kWh) is power accumulated over time. Capacity is measured in power, output in energy.
Power is how fast energy is delivered; energy is the total amount delivered. The analogy is a water pipe: power is the flow rate (litres per second), energy the volume in the bucket (litres). A 2 kW heater running for 3 hours uses 6 kWh; a 6 kW heater running for 1 hour uses the same energy at three times the power.
E (kWh) = P (kW) x t (h); P (kW) = E (kWh) / t (h)
Annual output (MWh) = capacity (MW) x 8,760 h x capacity factor
Battery duration (h) = energy (kWh) / power (kW)
Common errors include writing "kW per hour" (meaningless for consumption; "kWh" is correct), quoting battery size in kW, or comparing power stations by capacity alone. A 1 GW solar park at 15–20% capacity factor produces roughly 1.3–1.75 TWh a year, whereas a 1 GW nuclear unit at 90% produces about 7.9 TWh; equal power, very different energy.
Both dimensions matter for design:
- Batteries: power (kW) sets how many appliances can run at once; energy (kWh) sets how long. See C-rate.
- Grid connections and tariffs: capacity or demand charges bill peak kW; energy charges bill kWh.
- EV charging: charger power in kW, battery capacity in kWh.
Size home storage with the solar battery calculator.
Related terms
All terms →- Watt (W)
The SI unit of power, equal to one joule per second; it measures the rate at which energy is produced, transferred or used.
- Kilowatt-hour (kWh)
A unit of energy equal to one kilowatt of power sustained for one hour, or 3.6 megajoules; the standard billing unit for electricity.
- Capacity factor
The ratio of the energy a power plant actually produces over a period to the energy it would produce running at full nameplate capacity for the whole period.
- C-rate
The charge or discharge current of a battery relative to its capacity; 1C empties a full battery in one hour, 0.5C in two hours.
- Full-load hours
The number of hours per year a plant would need to run at rated power to deliver its actual annual output; equal to capacity factor multiplied by 8,760.
- Megawatt-hour (MWh)
A unit of energy equal to 1,000 kWh or 3.6 GJ; the standard unit for wholesale electricity prices and utility-scale generation and storage.