Battery Capacity Explained: Wh, Ah, kWh and Real Runtime
Learn how battery energy ratings translate into appliance runtime, why amp-hours alone can mislead, and how to estimate usable capacity without confusing watts with watt-hours.
This guide explains standard electrical relationships and uses clearly labeled hypothetical loads. It does not claim hands-on battery discharge testing. Published capacity is a nominal rating, not a guarantee of usable AC energy under every condition.
Watt-Hours Tell You How Much Energy Is Stored
Wh (watt-hours) measures energy; W (watts) measures instantaneous power. A 1,000Wh battery can theoretically supply 100W for 10 hours, before losses and operating limits. Ah (amp-hours) is charge capacity and needs the battery voltage to become an energy comparison. A 100Ah battery at 12.8V stores about 1,280Wh nominally, whereas 100Ah at 25.6V stores about 2,560Wh.
Real AC runtime is shorter because of inverter losses, battery reserve, idle consumption, temperature and load variation. Check the continuous output rating separately: a large battery can still have an inverter too small for a high-wattage appliance.
Battery Capacity Units: W, Wh, Ah and kWh
| Unit | What it measures | How to interpret it |
|---|---|---|
| W (watts) | Power at a moment | A 700W microwave input draws energy at about 700W while operating |
| Wh (watt-hours) | Energy stored or consumed | A 1,000Wh battery stores 1kWh nominal energy |
| kWh (kilowatt-hours) | 1,000Wh | A 2.5kWh system is rated at 2,500Wh |
| Ah (amp-hours) | Electric charge | Requires nominal voltage for meaningful energy comparison |
| V (volts) | Electrical potential | Voltage changes the Wh represented by the same Ah rating |
Common trap: 100Ah is not automatically more energy than 50Ah. A 12.8V 100Ah battery is about 1,280Wh, while a 51.2V 50Ah battery is about 2,560Wh. Compare watt-hours at the same measurement basis.
How to Convert Amp-Hours to Watt-Hours
| Example battery | Calculation | Nominal energy |
|---|---|---|
| 12.8V, 50Ah | 12.8 × 50 | 640Wh |
| 12.8V, 100Ah | 12.8 × 100 | 1,280Wh |
| 25.6V, 100Ah | 25.6 × 100 | 2,560Wh |
| 51.2V, 100Ah | 51.2 × 100 | 5,120Wh |
These calculations describe nominal battery energy, not energy guaranteed at the wall outlet. Manufacturers may rate a pack at cell level, specify usable capacity separately, or restrict discharge to protect battery life. For an assembled power station, use its documented Wh rating rather than reverse-engineering the battery from marketing claims.
Estimate Battery Runtime for Real Appliances
For a nominal 1,024Wh battery, the example assumptions give about 783Wh delivered to the appliance (1,024 × 0.90 × 0.85). This is an illustrative planning budget, not a manufacturer runtime promise.
| Illustrative load | Energy consumed per hour | Estimated runtime from 1,024Wh |
|---|---|---|
| 20W networking equipment | 20Wh | 39.2 hours |
| 60W laptop charging average | 60Wh | 13.1 hours |
| 100W television average | 100Wh | 7.8 hours |
| 150W portable refrigerator average while compressor runs | 150Wh | 5.2 hours if continuously drawing 150W |
| 500W appliance | 500Wh | 1.6 hours |
| 1,000W appliance | 1,000Wh | 0.8 hours |
Example: a 60W device running for eight hours
The appliance consumes 60W × 8h = 480Wh. With the illustrative 90% usable and 85% conversion assumptions, required nominal battery energy is 480 ÷ (0.90 × 0.85) = approximately 627Wh. Add reserve for battery aging, colder conditions and unexpected loads rather than choosing exactly 627Wh.
Example: a 1,500W kettle for six minutes
1,500W × (6 ÷ 60)h = 150Wh appliance energy. The battery might have enough stored energy, but a station with only a 1,000W continuous AC inverter still cannot reliably power that kettle. Energy and inverter output are separate constraints.
Why Rated Capacity and Usable Capacity Differ
DC battery energy must be converted to AC, and efficiency varies by load and inverter design.
Protection systems may reserve energy rather than allowing full nominal discharge.
Cold weather, battery wear and heavy discharge can reduce delivered energy.
AC inverter standby draw can be material for very small loads over long periods. DC outputs may avoid some conversion losses, but USB-C, 12V and other DC circuits also have their own conversion and operating losses. Battery chemistry affects cycle life, operating temperature and maintenance; chemistry alone does not determine usable energy.
Battery Capacity Buying and Sizing Checklist
- Measure typical device watts and daily Wh with a suitable meter when possible.
- Compare nominal Wh or kWh, not Ah alone.
- Check continuous AC watts and short-duration surge separately.
- Plan for conversion losses, reserves and seasonal variation.
- Confirm ports, solar input limits and expansion-battery compatibility.
- Do not equate 1,000W with 1,000Wh.
- Do not treat surge ratings as sustained power.
- Do not assume every advertised Wh is available through AC outlets.
- Do not calculate a cycling fridge from compressor watts alone.
- Do not backfeed household wiring without approved transfer equipment.
For a multi-device backup plan, sum each appliance's expected daily Wh, multiply by desired days, and then allow for realistic usable capacity and conversion losses. Separately sum the maximum simultaneous watts and account for startup surge. This avoids buying a battery that has adequate energy but insufficient inverter output.
For a step-by-step input tool, see our battery capacity calculator. That page serves the interactive calculation intent; this guide explains the underlying units, assumptions and trade-offs.
Final Verdict: Size Energy and Output Separately
Use Wh or kWh to compare energy capacity, W to check whether your appliances can operate, and Ah × nominal voltage only when converting battery charge into nominal energy. Estimate real runtime with an explicit usable-energy and efficiency allowance. For dependable backup, prioritize measured consumption and a margin for real operating conditions rather than headline battery numbers.
Frequently Asked Questions
Is 1,000Wh the same as 1,000W?
No. 1,000Wh is energy capacity; 1,000W is a rate of power use. A 1,000W appliance running one hour uses 1,000Wh before conversion losses.
How many watt-hours are in a 100Ah battery?
Multiply amp-hours by nominal voltage. At 12.8V, 100Ah equals approximately 1,280Wh; at 25.6V, it equals approximately 2,560Wh.
How long will a 1kWh battery run a 100W appliance?
The ideal calculation is 10 hours, but real AC runtime is lower because of usable-capacity limits, inverter losses and standby consumption.
Why does a 2,000Wh station not always deliver 2,000Wh through AC?
The inverter consumes energy, the battery management system may hold reserve, and delivered energy depends on load, temperature and battery condition.
Is a higher Ah rating always better?
No. Ah alone is not a fair energy comparison unless voltage is the same. Compare Wh and verify usable capacity.
Does a bigger battery mean a more powerful inverter?
Not necessarily. The battery Wh rating and continuous AC watt rating are separate specifications.
Can I connect a battery directly to household wiring?
Do not improvise a connection or backfeed outlets. Home integration requires manufacturer-approved equipment and code-compliant installation.
Sources and Editorial Methodology
Electrical conversions use standard definitions of watts, watt-hours, amp-hours and nominal voltage. All runtime numbers are calculated illustrations with assumptions stated explicitly, not laboratory tests or product-specific guarantees. Review each battery maker's manual for actual rated energy, usable capacity, inverter efficiency and operating limits.