What Are Watt-Hours? Wh, Watts and Battery Runtime

A watt-hour, written Wh, is a unit of energy. One watt-hour means using one watt for one hour, so a 60W device running for three hours consumes 180Wh.

Watts tell you how fast power is being used. Watt-hours tell you how much energy is used over a period, which is why batteries and portable power stations are rated in Wh.

TL;DR

Watt-hours = watts × hours. A 100W device uses 400Wh in four hours. A battery needs more rated watt-hours than the device calculation because conversion losses, inverter idle draw and reserve reduce what reaches the plug.

Quick Answer

10W for 8 hours = 80Wh
60W for 3 hours = 180Wh
100W for 4 hours = 400Wh
250W for 2 hours = 500Wh
500W for 2 hours = 1000Wh

The basic formula is:

Wh = W × hours

Rearranged for ideal runtime:

hours = Wh ÷ W

The word “ideal” matters. The division describes energy at the load, not guaranteed runtime from a labelled battery.

Watts vs Watt-Hours

An easy analogy is flow and volume. Watts are like the rate at which water leaves a tank; watt-hours are the total amount delivered over time.

Unit	Measures	Question it answers
Watts (W)	Power at a moment	Can the inverter run this load?
Watt-hours (Wh)	Energy over time	How much battery energy does the job require?
Kilowatt-hours (kWh)	1000 watt-hours	How much larger-scale energy was used or stored?
Volt-amps (VA)	Apparent AC power	Does the load stay within a UPS VA limit?

A 1000W inverter and a 1000Wh battery are not the same thing. The inverter may be able to supply 1000W, while the battery stores enough labelled energy for one ideal hour at that load. Conversely, a 2000Wh battery with only a 500W inverter stores plenty of energy but cannot run a 700W appliance.

Watt-Hour Examples

Device or setup	Planning draw	Time	Device energy
Router	10W	8 hours	80Wh
Laptop	90W	2 hours	180Wh
Laptop + monitor + router	145W	4 hours	580Wh
Office desktop + monitor + router	355W	2 hours	710Wh
500W constant load	500W	2 hours	1000Wh

Darker cells indicate more energy demand. These are device-energy calculations before battery and inverter losses.

If the laptop setup needs 580Wh at the devices, a 600Wh-labelled power station is too tight. The battery must also cover conversion losses and keep some practical reserve.

What Will a 1000Wh Battery Run?

On paper, 1000Wh supports any combination whose watts multiplied by hours equals 1000Wh:

Constant load	Ideal mathematical runtime	Why real runtime is shorter
10W	100 hours	AC inverter idle draw can rival the tiny load
100W	10 hours	Conversion and battery losses
250W	4 hours	Conversion losses and reserve
500W	2 hours	Conversion losses and higher battery current
1000W	1 hour	Inverter efficiency, heat and battery limits

PowerLasts uses an 85% moderate-load AC inverter-efficiency default when no better product-specific figure is available, plus chemistry and idle-draw assumptions described in the methodology. That does not mean every 1000Wh station always delivers exactly 850Wh. Low loads can lose proportionally more to inverter overhead, and actual products vary.

For a 100W load, a rough first pass using 85% conversion is 8.5 hours before accounting for idle draw:

1000Wh × 0.85 ÷ 100W = 8.5 hours

Use the calculator for the fuller model rather than treating that shortcut as a promise.

Why Rated Wh Is Not Usable Wh

The number on a portable power station describes rated battery energy. Several factors reduce energy available to an AC device:

inverter conversion losses;
power used just to keep the inverter on;
battery-management cut-offs;
temperature and battery condition;
DC conversion losses when using USB or regulated DC outputs;
very high or very low load behaviour.

The correct adjustment is not always “take off 15%”. At a moderate AC load, conversion efficiency may dominate. At a 10W AC load, an inverter drawing several watts by itself can dominate instead. That is why our model includes idle draw and labels it as an estimate when no measured product figure exists.

Read You Only Get Half the Battery for the broader reasons that label capacity and delivered energy differ.

Convert Amp-Hours to Watt-Hours

Amp-hours need a voltage before they describe energy:

Wh = Ah × V

A 12V, 100Ah battery contains 1200Wh in nominal terms:

100Ah × 12V = 1200Wh

That still does not promise 1200Wh at an AC outlet. Battery chemistry, allowed depth of discharge, wiring and inverter efficiency all affect delivered energy.

This is also why comparing batteries by Ah alone is misleading. A 100Ah battery at 12V stores half the nominal energy of a 100Ah battery at 24V.

Convert mAh to Watt-Hours

Power banks and small batteries often use milliamp-hours. Convert with:

Wh = mAh × V ÷ 1000

A 20,000mAh power bank rated at a nominal 3.7V contains 74Wh:

20,000 × 3.7 ÷ 1000 = 74Wh

It is not a 20,000Wh battery. The large mAh number reflects the smaller unit and the cell voltage used for the rating. Output at 5V, 9V or 20V goes through conversion, so delivered energy is again lower than nominal.

Capacity label	Nominal voltage	Nominal energy
10,000mAh	3.7V	37Wh
20,000mAh	3.7V	74Wh
27,000mAh	3.7V	99.9Wh
100Ah	12V	1200Wh

Watt-Hours and Appliances That Cycle

A fridge does not normally draw its compressor wattage every minute. It cycles on and off, so long-duration energy depends on average draw or measured kWh over time. It also has a higher startup demand that the inverter must support.

These are still two separate checks:

output watts and surge support: can the power station start and run the compressor?
watt-hours: can the battery sustain the average energy demand for the required time?

Our calculator models refrigeration with average runtime draw while retaining a separate startup requirement. Read What Is Startup Surge? and How Long Will a 1000Wh Power Station Run a Fridge? for that case.

Watt-Hours and UPS Runtime

Traditional UPS products are usually marketed by VA and output watts, not accessible rated watt-hours. Even if you know the internal battery voltage and amp-hours, simple multiplication does not reproduce runtime well because lead-acid delivery changes strongly with load.

Use a manufacturer runtime curve for the exact UPS. The curve tells you expected minutes at a stated watt load and captures more of the battery’s high-rate behaviour. How Long Will a UPS Run? has published examples and the full method.

Try It in the Calculator

Setup	Shorter run	Longer run
Laptop	2 hours	4 hours
Router	4 hours	8 hours
Laptop + monitor + router	2 hours	4 hours
Small fridge	4 hours	8 hours

What People Miss

Watts are not watt-hours. A 1000W inverter describes power delivery. A 1000Wh battery describes stored energy. One cannot replace the other.

mAh is incomplete without voltage. The same amp-hour number can represent very different energy at different voltages.

Ideal division ignores losses. Rated Wh divided by load W gives a theoretical ceiling, not a reliable AC runtime.

Tiny AC loads can be surprisingly inefficient. Inverter idle draw matters when the device itself uses only a few watts. DC output can be a better route when voltage and connectors are compatible.

Cycling appliances need average energy and startup power. One number cannot safely answer both questions.

UPS runtime is model-specific. A guessed internal Wh figure is weaker evidence than the manufacturer’s curve at your measured load.

Bottom Line

A watt-hour is one watt used for one hour. Multiply watts by hours to find device energy, or divide watt-hours by watts for an ideal runtime before losses.

For real backup sizing, add conversion losses, idle draw and reserve, then check that the output can carry the load. Use the calculator for portable power stations and the UPS runtime comparison for traditional UPS models.