Watts and watt-hours answer two different questions
Watts tell you a rate of power. Watt-hours tell you an amount of energy. A 100 W charger and a 100 Wh battery can share the number 100 while describing completely different things.
Ideal energy-time calculator
Divide energy in watt-hours by a constant power in watts to get an idealized time in hours.
A watt is a rate
The watt is the SI derived unit of power. One watt is one joule per second. In practical electronics, power can also be calculated from voltage multiplied by current: volts × amps = watts.
If a device is using 20 W at a particular moment, that number describes the rate at which energy is being used at that moment. It does not tell you how long the device has been running or how much total energy has been consumed.
A watt-hour is an amount of energy
A watt-hour combines power with time. Using 1 W for 1 hour corresponds to 1 Wh of energy. Using 10 W for 2 hours corresponds to 20 Wh. Using 40 W for half an hour also corresponds to 20 Wh.
This is why battery capacity is often easier to compare in watt-hours when the devices operate at different voltages. Watt-hours describe an energy quantity directly, whereas amp-hours alone still need a voltage context before they can be converted to energy.
The simple relationship
The calculator above uses exactly that arithmetic. It is useful for understanding units and making a first-order estimate when power is constant. Real batteries, chargers, power supplies, and electronic devices are not perfectly constant or perfectly efficient, so the result should not be treated as a guaranteed runtime or charging time.
Examples that make the difference obvious
| Example | Power or energy? | What it describes |
|---|---|---|
| 65 W laptop charger | Power | A maximum power capability under supported operating conditions, not stored energy |
| 60 Wh laptop battery | Energy | An energy-capacity figure, not the laptop's constant power draw |
| 20 W device load | Power | An energy-use rate at that operating point |
| 100 Wh portable battery | Energy | An energy quantity; usable output can be lower after conversion losses and operating limits |
Why a 60 Wh battery does not mean six hours of runtime
It would mean six hours only if the device drew a constant 10 W and every joule represented by the rated capacity were available to the load with no losses or cutoffs. Real laptops and portable electronics change power draw continuously. The processor, display brightness, radio activity, cooling system, storage, and attached devices all change the load.
Battery-management systems also reserve operating margins and stop discharge at defined limits. Conversion circuitry is not perfectly efficient. Temperature, battery age, workload, and software power management can change the usable result further.
So the equation is still correct; the uncertain part is the real-world input. If average power over a workload is not known, a runtime calculation is only an estimate.
Why charger wattage does not equal charging speed
A charger's printed wattage is normally a source capability, not a promise that every connected device will continuously receive that power. With USB Power Delivery, the charger and device establish supported operating conditions, and the cable can impose another capability limit.
Charging power also changes over the course of a battery charge. Devices commonly reduce power as battery state, temperature, or other control limits change. Dividing a battery's Wh rating by the charger's headline W rating therefore produces an ideal mathematical lower bound under assumptions that usually do not hold through an entire charge.
For the compatibility side of that problem, see the USB-C charger wattage guide.
What about amp-hours?
Amp-hours measure electric charge, not energy. To convert a simple nominal battery rating into watt-hours, voltage is also needed:
For example, a nominal 3.7 V cell rated at 5 Ah corresponds to about 18.5 Wh by simple multiplication. A different battery rated at 5 Ah but operating at a different nominal voltage does not contain the same amount of energy.
Do not mix W, Wh, and W/h
W and Wh are commonly useful consumer-electronics units. W/h, watts per hour, is a different unit describing how quickly a power level itself changes over time. It is not another way to write watt-hours.
If a product page means battery energy but writes W/h, that notation should not be silently interpreted as equivalent to Wh. Check the manufacturer's technical documentation for the intended specification.
A practical reading rule
- If the label is W, ask: “What power rate is this product offering, using, or dissipating?”
- If the label is Wh, ask: “How much energy is this battery, pack, or workload describing?”
- If you divide Wh by W, treat the time result as ideal unless you actually know average power and relevant losses.
- For charging, separately verify charger, device, cable, protocol, and thermal behavior.
Primary sources
- NIST — SI units: electric current and derived electrical units
- NIST — SI Units information
- USB-IF — USB Charger (USB Power Delivery)
The calculator is an arithmetic tool, not a battery-health, charging-time, or runtime guarantee.