Quick answer
Energy in watt-hours = watts × hours, or amp-hours × volts. A 60 W device running for 5 hours uses 300 Wh, which is 0.3 kWh or 1,080,000 joules.
Choose a mode, then enter the power and time or the amp-hours and voltage. The calculator returns Wh, kWh and joules.
Related tools: Ohm's Law Calculator: Solve voltage, current or resistance. Parallel Resistor Calculator: Equivalent resistance of resistors in parallel and in series. LC Resonant Frequency Calculator: Resonant frequency of an LC circuit.
How to use the Watt-Hours Calculator
- Choose use: Watts × hours and Amp-hours × volts.
- Optionally, enter power (W) (above 0).
- Optionally, enter time (h) (above 0).
- Optionally, enter capacity (Ah) (above 0).
- Optionally, enter voltage (V) (above 0).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Use | Choose one: Watts × hours; Amp-hours × volts | Starts at Watts × hours |
| Power (optional) | A number, in W | > 0 |
| Time (optional) | A number, in h | > 0 |
| Capacity (optional) | A number, in Ah | > 0 |
| Voltage (optional) | A number, in V | > 0 |
Formula and method
Formula
Wh = W × h = Ah × V
IEEE-754 double precision; shown to the decimals indicated.
1 Wh = 3,600 J.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Use = Watts × hours; Power = 60; Time = 5.
| Result | Value |
|---|---|
| Energy | 300 Wh |
| Kilowatt-hours | 0.3 |
| Joules | 1,080,000 |
Example 2
Inputs: Use = Amp-hours × volts; Capacity = 100; Voltage = 12.
| Result | Value |
|---|---|
| Energy | 1,200 Wh |
| Kilowatt-hours | 1.2 |
| Joules | 4,320,000 |
Example 3
Inputs: Use = Watts × hours; Power = 1500; Time = 0.5.
| Result | Value |
|---|---|
| Energy | 750 Wh |
| Kilowatt-hours | 0.75 |
| Joules | 2,700,000 |
The first line is the headline result (energy). Below it the watt-Hours Calculator lists kilowatt-hours and joules, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
From watts to the electricity bill
Electricity is billed in kWh. 0.3 kWh at $0.15 per kWh costs 4.5 cents. A 60 W bulb left on for 24 hours uses 1.44 kWh. Divide Wh by 1,000 for kWh and multiply Wh by 3,600 for joules.
Battery capacity
Battery sizes are in Wh or Ah. A 3.7 V, 3,000 mAh phone battery stores 11.1 Wh. A 100 Wh limit applies to spare lithium batteries in carry-on luggage on many airlines.
How the result changes with power
Holding every other input at the first worked example, this table changes only power. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Power | Energy | Kilowatt-hours | Joules |
|---|---|---|---|
| 30 | 150 Wh | 0.15 | 540,000 |
| 60 | 300 Wh | 0.3 | 1,080,000 |
| 120 | 600 Wh | 0.6 | 2,160,000 |
| 240 | 1,200 Wh | 1.2 | 4,320,000 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Power: left empty shows "Enter the power." beside that field and no result.
- Voltage: left empty shows "Enter the voltage." beside that field and no result.
Choosing between the Watt-Hours Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Ohm's Law Calculator](/engineering/ohm-s-law-calculator/) | Current | |
| [Parallel Resistor Calculator](/engineering/parallel-resistor-calculator/) | resistances | Parallel resistance |
| [LC Resonant Frequency Calculator](/engineering/resonant-frequency-lc/) | inductance and capacitance | Resonant frequency |
| [Wattage Calculator](/engineering/wattage/) | Power | |
| [Battery Runtime Calculator](/engineering/battery-runtime-calculator/) | battery capacity, battery voltage and load | Runtime |
Background: electrical circuits
Ohm's law links voltage, current and resistance: V = I × R, so I = V ÷ R and R = V ÷ I. Power is P = V × I, which also equals I² × R and V² ÷ R.
These relations hold for ohmic components, where resistance is constant. Diodes and transistors are non-linear, and in AC circuits with reactive parts impedance (Z) takes the place of resistance. Check component ratings and local electrical code before relying on any calculation.
Assumptions and limitations
- Only the fields for the chosen mode are used
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for power and voltage to confirm this.
Frequently asked questions
How many Wh in a kWh?
There are 1,000 watt-hours in a kilowatt-hour.
How do I find Wh from mAh?
Multiply amp-hours by volts: 3 Ah × 3.7 V = 11.1 Wh.
Is Wh a unit of power?
No, it is a unit of energy.
What does the Watt-Hours Calculator calculate?
Energy in watt-hours from power and time, or from amp-hours and voltage. It returns Energy, Kilowatt-hours and Joules.
What formula does the Watt-Hours Calculator use?
Wh = W × h = Ah × V. 1 Wh = 3,600 J.
What do I need to enter in the watt-Hours Calculator?
You enter Use. Optional: Power (W), Time (h), Capacity (Ah) and Voltage (V).
What does the “Use” option change?
It sets how the watt-Hours is worked out. The choices are Watts × hours and Amp-hours × volts.
What is an example of a watt-Hours result?
With use Watts × hours, power 60 and time 5, the energy is 300 Wh.
Where does the method come from?
The method follows: SI definitions.
Can I rely on this calculator for a decision?
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them. For this calculator in particular: Only the fields for the chosen mode are used.
Related calculators
- Ohm's Law Calculator — Solve voltage, current or resistance.
- Parallel Resistor Calculator — Equivalent resistance of resistors in parallel and in series.
- LC Resonant Frequency Calculator — Resonant frequency of an LC circuit.
- Wattage Calculator — Solve power, voltage or current for a DC or resistive load.
- Battery Runtime Calculator — Runtime of a battery under a constant load.
- Hours Calculator — Hours worked between a start and end time, minus breaks.
Sources
- SI definitions
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.