kW vs kWh: the difference between power and energy

A meter that reads 12,450.0 kWh at 09:00 and 12,454.5 kWh at 09:15 has not told you the load. It has told you the energy. Divide the change by 0.25 h and the average load in that quarter hour was 18 kW. The rest of this guide shows each conversion and where each unit appears on a bill.

Convert kW to kWh

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.

Convert kWh to kW: average power from a meter

Meters count energy. To find the average power in an interval, divide the change in the energy counter by the length of the interval in hours.

A counter that goes from 12,450.0 to 12,454.5 kWh in 15 minutes 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.

Units side by side

"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.

Energy charges 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.

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.