Free Watt-hour calculator
Enter the watts a device draws and the hours it runs — or a battery's voltage and amp-hour rating — and this calculator returns the energy in watt-hours (Wh) and kilowatt-hours (kWh), updated live, as you type.
On this page13 sections
Ideal energy from the values you enter. Real battery runtime is lower after losses.
Results are estimates. Consult a professional.
How the watt-hour calculator works
A watt-hour (Wh) is a unit of energy: the energy a one-watt load uses in one hour. Energy is power added up over time, so this calculator finds it two ways. Pick "Power × time" and enter the watts a device draws and how many hours it runs. Or pick "Battery V × Ah" and enter a battery's voltage and amp-hour rating to read how much energy it actually stores. Either way you get the answer in watt-hours and in kilowatt-hours (kWh), the unit your electricity bill is measured in.
What a watt-hour is — watt vs watt-hour vs amp-hour
Three units sound alike but mean different things. Getting them straight is the whole point of the calculator.
Watt (W) — the rate
A watt is power: how fast energy is being used right now. A 100 W bulb draws energy at 100 watts the whole time it is on. It says nothing, on its own, about how much energy you use — that depends on how long it runs.
Watt-hour (Wh) — the amount
A watt-hour is energy: the running total of power over time. One watt for one hour is one watt-hour. The 100 W bulb left on for 10 hours uses 1000 Wh, or 1 kWh — which is what the utility bills you for. Watts are the speedometer; watt-hours are the odometer.
Amp-hour (Ah) — charge, not energy
An amp-hour measures electric charge — how many amps a battery can deliver for how many hours — but it leaves out voltage, so it is not energy by itself. A 100 Ah battery holds very different amounts of energy at 12 V versus 48 V. Multiply amp-hours by voltage and you get watt-hours, the apples-to-apples energy figure.
The two watt-hour formulas
Use whichever formula fits what you already know. Both return the same kind of answer: energy in watt-hours.
From power and time
When you know how many watts a device draws and how long it runs, multiply: Wh = W × hours. This answers "how much energy will this appliance use?" Divide by 1000 for kilowatt-hours to compare against your bill.
From a battery's voltage and capacity
Batteries are rated in volts and amp-hours, so their stored energy is Wh = V × Ah. A 12 V, 100 Ah battery stores 12 × 100 = 1200 Wh. This is the number to use when you compare batteries or size a system, because it already folds voltage in.
Worked examples using the watt-hour calculator
You run a 100 W appliance for 5 hours and want to know the energy it uses.
Multiply power by time: Wh = 100 × 5 = 500 Wh. In kilowatt-hours that is 500 ÷ 1000 = 0.5 kWh. At a US-average rate near $0.15/kWh, that run costs about 7–8 cents.
You have a 12 V battery rated at 100 Ah and want its energy in watt-hours.
Multiply voltage by capacity: Wh = 12 × 100 = 1200 Wh, which is 1.2 kWh. That is the energy stored — separate from how fast you can draw it.
Watt-hour quick-reference table
Common loads and batteries, with their energy worked out from the formulas above.
| Source | Calculation | Energy |
|---|---|---|
| 60 W bulb for 10 h | 60 × 10 | 600 Wh (0.6 kWh) |
| 100 W device for 5 h | 100 × 5 | 500 Wh (0.5 kWh) |
| 1000 W heater for 1 h | 1000 × 1 | 1000 Wh (1 kWh) |
| 1500 W kettle for 0.25 h | 1500 × 0.25 | 375 Wh (0.375 kWh) |
| 12 V, 100 Ah battery | 12 × 100 | 1200 Wh (1.2 kWh) |
| 48 V, 25 Ah battery | 48 × 25 | 1200 Wh (1.2 kWh) |
| 3.7 V, 2 Ah phone cell | 3.7 × 2 | 7.4 Wh (0.0074 kWh) |
From Wh = W × h and Wh = V × Ah. Note the 12 V/100 Ah and 48 V/25 Ah batteries store the same 1200 Wh even though their amp-hour ratings differ fourfold.
Wh ↔ Ah, and why watt-hours compare batteries fairly
Battery shops advertise amp-hours, but amp-hours alone hide the voltage. The same energy can wear very different amp-hour labels.
A 12 V, 100 Ah battery and a 48 V, 25 Ah battery both store 1200 Wh, even though one shows "100 Ah" and the other "25 Ah". The 48 V pack is not a quarter of the battery — it just delivers the same energy at four times the voltage and a quarter of the current. That is why watt-hours, not amp-hours, are the right number for comparing batteries, power stations and EVs.
Estimating battery runtime from watt-hours
Once you know a battery's watt-hours, you can estimate how long it will power a given load.
A 1200 Wh battery driving a 100 W load runs for about 1200 ÷ 100 = 12 hours in the ideal case. Real runtime is shorter: inverters waste 10–20%, you rarely drain a battery to zero (lead-acid is usually limited to about 50% depth of discharge), and high draw rates reduce usable capacity. Treat the calculated figure as an optimistic ceiling and plan for less.
Common watt-hour mistakes
- Quoting amp-hours without voltage. "100 Ah" is not an energy figure. Always multiply by the voltage to get watt-hours before you compare or budget.
- Confusing watts with watt-hours. A 1000 W appliance does not use 1000 Wh — it uses 1000 Wh only if it runs for exactly one hour. Watts are a rate; watt-hours need a time.
- Forgetting the ÷ 1000 for kWh. Utilities bill in kilowatt-hours. 2500 Wh is 2.5 kWh, not 2500 kWh.
- Treating ideal runtime as real runtime. Wh ÷ W is a ceiling. Subtract inverter losses and depth-of-discharge limits for a realistic figure.
- Mixing minutes and hours. The time in Wh = W × h must be in hours. Thirty minutes is 0.5 h, not 30.
For the cost side of the same math, the electricity cost calculator multiplies kilowatt-hours by your rate, and Ohm's law ties watts back to volts and amps.
How accurate is this watt-hour calculator
The arithmetic is exact. Wh = W × h, Wh = V × Ah, and kWh = Wh ÷ 1000 are simple definitions, computed here to full floating-point precision. For the numbers you type, the watt-hour and kilowatt-hour figures are exactly right.
What the formulas cannot capture is real-world derating. A battery's labelled amp-hour capacity is measured at a slow, standard discharge rate; draw harder and you get fewer usable watt-hours. Lead-acid batteries should not be fully discharged, inverters and chargers lose a slice to heat, and capacity fades with age and temperature. Use the calculated watt-hours as the nameplate energy, then apply your own efficiency and depth-of-discharge margins for system planning.
Frequently asked questions about the free Watt-hour calculator
About this Watt-hour calculator
This watt-hour calculator runs entirely in your browser — nothing you enter is sent anywhere. It applies the energy definition Wh = power × time, or Wh = voltage × amp-hours for a battery, and divides by 1000 for kilowatt-hours, updating the instant you change a value.
Built for anyone sizing batteries, solar setups or appliance running costs. Browse more tools in electronics calculators or see the full calculator library.