Educational explanation, not professional design advice. Last reviewed 11 November 2026. Disclaimer
In plain English
Power is how fast energy is used or produced at a moment. Energy is how much is used over a period of time. A 2 kW electric heater always draws 2 kW while it is on — that is its power. If it runs for three hours, it uses 6 kWh of energy, and that is what the electricity meter counts and the bill charges for.
Think of water: power is like the flow rate from a tap, and energy is the amount of water collected in a bucket. A high flow for a short time can fill the same bucket as a low flow for a long time.
Technical definition
Power P is the rate of energy transfer, measured in watts (W); 1 W = 1 joule per second, and 1 kW = 1000 W. Energy E is power integrated over time; for a constant power, E = P × t. The SI unit of energy is the joule (J), but electricity is sold in kilowatt-hours: 1 kWh is the energy of 1 kW for one hour, equal to 3.6 million joules (3.6 MJ). Batteries are rated in ampere-hours (Ah) at a nominal voltage, and their energy in watt-hours is E = V × Ah.
How it works and where it is used
Most devices do not run at full power all the time. A refrigerator compressor cycles on and off, and a motor draws less than its rating when lightly loaded. Energy use is therefore power × hours × average load (the duty cycle). For alternating current, there is one more distinction: equipment that supplies power, such as generators, transformers and UPS units, is rated in kilovolt-amperes (kVA), because its size is limited by current. The useful (real) power in kW equals the kVA multiplied by the power factor of the load.
| Quantity | Unit | Used for |
|---|---|---|
| Real power | W, kW | Device ratings, motor output, heater power |
| Apparent power | VA, kVA | Generator, transformer and UPS ratings |
| Energy | Wh, kWh (or MJ) | Electricity bills, battery capacity, solar production |
| Charge | Ah | Battery capacity at a stated voltage |
Formula
E (kWh) = P (kW) × t (h) × load factor Cost = E × price per kWh
Battery energy (Wh) = V × Ah P (kW) = S (kVA) × power factor
P = power, t = time, load factor = average share of full power (1.0 = always at full power), V = nominal battery voltage, Ah = rated capacity, S = apparent power. 1 kWh = 3.6 MJ.
Worked example
Example 1 — energy and cost. A 2 kW heater runs for 3 hours: E = 2 × 3 = 6 kWh. At a price of 50 per kWh, the cost is 6 × 50 = 300 (in your currency).
Example 2 — battery energy. A 12 V, 100 Ah battery stores 12 × 100 = 1200 Wh = 1.2 kWh nominal. If only 80% may be used to protect the battery, the usable energy is 0.96 kWh — enough to run a 100 W load for a little under 10 hours, before inverter losses.
Example 3 — generator rating. A workshop load of 20 kW with a power factor of 0.8 needs at least 20 ÷ 0.8 = 25 kVA of generator capacity, before any margin for motor starting.
In the workshop and in practice
Understanding kW and kWh helps to read electricity bills, to compare the running cost of machines, to size batteries and solar systems, and to choose generators. A high-power device used briefly (a 2 kW kettle for 3 minutes, 0.1 kWh) can use less energy than a small device that runs all day (a 50 W router for 24 hours, 1.2 kWh). For energy saving, look at both the power and the hours of use.
Common mistakes and misconceptions
- Writing “kW per hour”. The unit of energy is kWh (kilowatt times hour), not kW/h.
- Comparing battery Ah at different voltages. 100 Ah at 12 V is 1.2 kWh, but 100 Ah at 48 V is 4.8 kWh.
- Sizing a generator in kW only. Its limit is kVA; low power factor and motor starting need extra capacity.
- Using nameplate power for energy. Real energy use depends on the average load and duty cycle.
Related calculators
- Electricity Cost Calculator — Energy use (kWh) and running cost of any appliance or machine.
- Battery Bank Sizing Calculator — Battery capacity (Ah and kWh) and the series/parallel layout for your loads and days of autonomy.
- Generator Sizing Calculator — Running kW and kVA of a list of loads, the worst motor-starting demand, demand factor and design margin, and an estimated generator rating in kVA and kW.
- Solar Panel Output Estimator — Estimated DC and AC output of solar panels at a given sun and temperature, or their daily energy from peak sun hours.
Related topics
- Solar PV System Sizing Basics — Matching the size of a solar array, and of its batteries, to the energy your loads need each day.
References
- BIPM: The International System of Units (SI Brochure) — watt and joule
- IEC 60050-131: International Electrotechnical Vocabulary — active, reactive and apparent power
- Hughes Electrical and Electronic Technology