kW vs kWh: the difference between power and energy

A kilowatt (kW) is a rate: how fast a load uses energy at this moment. A kilowatt-hour (kWh) is a quantity: how much energy it used over a period. The two relate as speed relates to distance. A 2 kW heater draws 2 kW whenever it is on, and after 3 hours it has used 6 kWh, just as a car at 60 km/h covers 180 km in 3 hours.

kW vs kWh at a glance

A charger or a supply connection is rated in kW because that is how fast it can deliver energy. A battery's capacity is in kWh because that is how much it stores. A 10 kWh battery can supply 5 kW for 2 hours or 2 kW for 5 hours.

Everyday kW to kWh examples

The kW to kWh calculator converts any power and running time in either direction, with the running cost. Fridges, heaters and compressors switch on and off, so a rating multiplied by the hours connected overstates their use. The Appliance energy and running cost calculator allows for the time a cycling appliance actually runs.

kWh from kW and running time

For a constant load, energy (kWh) = power (kW) × time (hours). Convert minutes to hours first: 6 kW for 20 minutes is 6 × 20 ÷ 60 = 2 kWh.

A 2 kW load that runs for 3 hours at €0.25 per kWh uses 6 kWh and costs €1.50:

When the power changes, calculate the energy for each interval and add the results. A machine that averages 4 kW for half an hour and 8 kW for the next half hour uses 2 + 4 = 6 kWh. The peak of 8 kW multiplied by one hour would give 8 kWh, which is too high.

A nameplate rating is the maximum that the equipment is designed for. It is not what the equipment uses. Motors run at part load, compressors and heaters cycle, and most equipment has a standby state, so measure the real consumption.

Average kW from kWh readings

To go from energy to power, divide by the time: average power (kW) = energy (kWh) ÷ time (hours). 30 kWh used in 5 hours is an average of 6 kW.

Meters count energy, so a meter's register tells you the energy and not the load. A counter that goes from 12,450.0 kWh at 09:00 to 12,454.5 kWh at 09:15 shows 4.5 kWh in 0.25 h, an average of 18 kW:

That is the average for the interval. The load can be higher for part of it. Compare like with like: a one-second peak is not the same value as a 30-minute demand figure on a bill.

Before you subtract counter values, check for a counter reset, a rollover, a replaced meter and the direction of flow on sites that export. A negative difference is a data fault to investigate, not negative consumption.

Related units: kVA, kWp and kW/h

kWp (kilowatt peak) is the DC power that the manufacturer declares for a PV array at standard test conditions: 1,000 W/m² of irradiance on the array at 25 °C. It is a power rating. What the array generates is energy, in kWh, and the kWh per kWp over a year depend on the location, orientation and shading. The Solar PV generation estimate gives the monthly kWh for a location from PVGIS data.

"kW/h" is a rate of change of power. It is not a unit of energy, but charts and exports often use it by mistake for kW or kWh. Check the source field before you relabel historical data. The energy unit converter converts kWh to MJ, therms and Btu, and kW to horsepower and tons of refrigeration.

kW and kWh on a bill: energy and demand charges

Many commercial tariffs have separate charges for energy and for demand. The U.S. Department of Energy guide to rate options explains the usual parts:

• energy charge: a price per kWh, often different by time of day;
• demand charge: a price per kW (or kVA) of the highest average demand in one interval of the billing period;
• fixed charges: a standing charge per day or month, independent of use.

Two sites can use the same kWh in a month and draw very different peaks. Load factor compares average demand with peak demand, and the peak shaving and load shifting guide explains two ways to reduce a peak. The electricity cost calculator adds energy, demand and fixed charges for one bill.

A transformer illustrates why energy loss depends on the operating profile. No-load loss continues while the transformer is energised, while the load-dependent component rises approximately with current squared at the test reference temperature. Use the manufacturer's loss-test figures and sum changing operating points by interval, rather than squaring an annual average load.

What to measure

For a monitoring project, ask for:

• cumulative import energy (kWh), and export energy on sites with generation;
• interval power (kW) at the interval that your tariff or analysis uses;
• timestamps, units and a data-quality state on every value;
• the raw counter values as well as the calculated interval values, so that each result can be checked later.

Decide early whether you need whole-site totals, individual circuits or both. The submetering guide shows how to choose the circuits, and the three-phase power guide explains how a meter measures kW. A ZEM electricity monitor records kWh and kVAh on each three-phase circuit and sends the readings to a Gateway running Edge.