kW vs kWh: The Difference Between Power and Energy
kW measures power, the rate of electricity use. kWh measures energy, power used over time. Learn the formulas, see worked examples, and convert between them.
Common conversions
| Input | Result |
|---|---|
| 1 kW for 1 hour | 1 kWh |
| 2 kW for 3 hours | 6 kWh |
| 0.5 kW for 8 hours | 4 kWh |
| 5 kW EV charger for 2 hours | 10 kWh |
| 1.5 kW heater, 5 h/day for 30 days | 225 kWh |
| 10 kW generator for 1 hour | 10 kWh |
| 3 kW oven for 45 minutes | 2.25 kWh |
| 100 kW industrial load for 1 hour | 100 kWh |
The math behind it
- kWh = kW × hours/day × days
- kWh = 1.5 × 5 × 30 = 225 kWh
- Cost = kWh × price = 225 × 0.16
- Cost = $36.00
Everything you need to know
A kilowatt (kW) and a kilowatt-hour (kWh) are the two units people mix up most often on this site, and the mix-up is expensive. kW is power, the rate at which electricity is used at one instant. kWh is energy, the total amount used over a stretch of time. Confusing a rate with a total leads to bad generator sizing, bad solar estimates, and bad guesses about a utility bill.
Rate versus total: the mental model
Think of kW as speed and kWh as distance traveled. A car going 60 mph for 2 hours covers 120 miles; the speed (60 mph) never changes even though the distance keeps climbing. A 2 kW heater running for 3 hours uses 6 kWh; the heater's power draw stays at 2 kW the whole time, but the energy total grows with every minute it runs. An appliance's nameplate lists kW (how hard it pulls at any moment). A utility bill lists kWh (how much it pulled in total over the billing period).
Converting between the two
The formula only needs one extra variable: time. kWh = kW × hours turns a power rating into total energy once you know the runtime. Reverse it with kW = kWh / hours to find the average power behind a known energy total, useful when a bill shows total kWh for the month but not the load's instantaneous draw. Both directions assume the load runs at a constant kW; for a load that cycles on and off, like a refrigerator or an air conditioner, use the average draw across the full period, not the nameplate peak.
Where kW shows up on a bill: demand charges
Most residential bills only show kWh, the energy charge. Many commercial and industrial bills add a separate demand charge, billed in kW, based on the highest average power draw recorded in any 15- or 30-minute window during the billing period. A facility can use the same total kWh in a month but pay very different demand charges depending on whether that energy was spread evenly or spiked all at once, which is why commercial energy managers care about kW peaks as much as kWh totals.
Why two loads with the same kW use different kWh
Runtime is the variable that separates them. A 1 kW space heater run for 10 hours uses 10 kWh; the same 1 kW heater run for 2 hours uses 2 kWh, a fifth of the energy, even though the power rating never changed. This is also why duty cycle matters for cyclic loads: a 0.15 kW refrigerator compressor that only runs 30% of the time uses about 0.15 × 0.30 × 24 = 1.08 kWh per day, not the 3.6 kWh you'd get by assuming it runs continuously.
Common applications
Residential bills charge for total kWh used. Commercial and industrial bills often add a demand charge in kW based on the highest average draw in a 15- or 30-minute window, so energy managers watch both numbers separately.
A solar array's kW rating describes its instantaneous output under full sun. Annual kWh production depends on that kW rating plus local irradiance, panel orientation, and shading, which is why identical kW systems in Phoenix and Seattle produce very different annual kWh totals.
Multiplying an appliance's kW rating by its daily runtime and your utility rate turns a nameplate number into an actual monthly cost, letting you compare a space heater against a heat pump on the same basis.
A generator or battery's kWh capacity divided by the connected kW load gives the number of hours it can run before it needs refueling or recharging, the core calculation behind any backup power plan.
Common mistakes
Appliance and equipment nameplates rate power in kW (or W), not kWh. Labeling a spec sheet or a calculation with the wrong unit turns a rate into a total and throws off every downstream number.
A high-kW appliance used briefly can use less energy than a low-kW appliance left running for hours. Only kWh, power multiplied by time, determines the energy charge on a bill.
A battery rated for 5 kW of continuous output and 13.5 kWh of storage can't deliver 13.5 kW even briefly. The kW number caps instantaneous draw; the kWh number caps total runtime.
Refrigerators, air conditioners, and well pumps don't run continuously at their nameplate kW. Estimating daily kWh from the nameplate rating without accounting for the actual on-time overstates energy use, sometimes by 3x or more.
Frequently asked questions
Is a kilowatt-hour bigger than a kilowatt?+
No, they measure different things and can't be compared by size. kW is a rate, kWh is a total. 1 kW sustained for exactly 1 hour equals 1 kWh, but a kW figure alone says nothing about total energy used.
How do I convert kW to kWh?+
Multiply by hours: kWh = kW × h. A 4 kW oven run for 1.5 hours uses 4 × 1.5 = 6 kWh.
How do I convert kWh back to kW?+
Divide by hours: kW = kWh / h. If a device used 12 kWh over a 4-hour run, its average power was 12 / 4 = 3 kW.
Which unit shows up on my electric bill?+
Almost always kWh, the energy charge. Larger commercial and industrial accounts often add a separate demand charge billed in kW, based on the highest average power draw during the billing period.
Does a higher kW rating always mean a higher electric bill?+
No, kWh drives the bill, not kW by itself. A 5 kW oven run for 10 minutes uses about 0.83 kWh, while a 0.1 kW lamp left on for 100 hours uses 10 kWh, twelve times more energy despite the much lower power rating.
How many kWh does a 2 kW appliance use in a day?+
It depends entirely on runtime. Running 5 hours a day, a 2 kW appliance uses 2 × 5 = 10 kWh/day. Running 24 hours a day, the same appliance would use 48 kWh/day.
What's the difference between a battery's kW rating and its kWh capacity?+
kW is how fast the battery can deliver power; kWh is how much total energy it stores. A Tesla Powerwall stores 13.5 kWh but outputs at most 5 kW continuously, so it can run a 5 kW load for up to 2.7 hours before it's empty.
Is 1,000 Wh the same as 1 kWh?+
Yes, 1,000 watt-hours equal exactly 1 kilowatt-hour. The kilo prefix means 1,000, the same relationship as 1,000 W equaling 1 kW.
How much does 1 kWh cost in the US?+
About 16 cents at the 2025 US residential average, though rates range from roughly 11 cents/kWh in low-cost states like Idaho to 40 cents/kWh or more in Hawaii.
Why do two appliances with the same kW rating cost different amounts to run?+
Because runtime differs between them. Since kWh = kW × hours, a 1.5 kW appliance run for 2 hours a day costs a third as much per month as the identical appliance run for 6 hours a day.