A nominal 12.8 V, 100Ah lithium battery stores about 1,280 Wh. After allowing for usable depth and conversion losses, roughly 1,000–1,150 Wh may be available for planning, depending on the battery and installation.
Runtime formula
Runtime in hours = nominal voltage × amp-hours × usable fraction × efficiency ÷ load watts.
With 12.8 V, 100 Ah, 90% usable depth and 90% system efficiency, the planning value is about 1,037 usable Wh. A steady 100 W load would therefore run for about 10.4 hours under those assumptions.
Try your own inputs in the battery runtime calculator.
Example runtimes for common loads
| Average load | Estimated runtime | Typical interpretation |
|---|---|---|
| 25 W | About 41 hours | Small DC loads |
| 50 W | About 21 hours | Efficient refrigerator average |
| 100 W | About 10 hours | Mixed light loads |
| 500 W | About 2.1 hours | Medium inverter load |
| 1,000 W | About 1 hour | Heavy short-duration load |
These examples use the same 1,037 Wh planning value. Real appliances cycle, surge and change power with operating conditions.
Why a refrigerator is not a simple watts calculation
A compressor refrigerator may draw a higher power while running but cycle off for part of the day. Ambient temperature, ventilation, thermostat setting, door openings and food temperature all change the duty cycle. The best estimate comes from measuring daily energy with a battery monitor or energy meter.
What reduces real runtime
- Inverter idle consumption and conversion loss.
- Cold temperature and battery protection limits.
- High current that triggers BMS or inverter limits.
- Cable voltage drop and loose or undersized connections.
- Age, imbalance or an incomplete charge.
- Loads that cycle more often than expected.
When AC loads are involved, use their measured input power through the inverter rather than the appliance’s idealized output rating.
Can a 100Ah battery run a 2,000W inverter?
The inverter rating alone does not answer the question. At full load, a 2,000 W inverter on a 12 V system may demand roughly 175–200 A after losses. Many individual 100Ah batteries cannot supply that continuously, even if they hold enough energy for a short run. Check the battery’s continuous and surge discharge limits, the inverter manual, and every current-carrying component.
When to use two 100Ah batteries
Two batteries can increase energy and, in a correctly designed parallel bank, available current. But parallel batteries should be compatible, similarly charged and connected for balanced current sharing. Increasing capacity also increases the charging time unless charging capability grows with it.
For whole-system sizing, continue with our RV battery bank guide.
Planning note: Treat these calculations as a design starting point. Confirm final equipment, conductor and protection choices against current manufacturer instructions and applicable codes.
Frequently asked questions
Will a 100Ah lithium battery run a fridge overnight?
Often yes for an efficient RV refrigerator, but calculate from the refrigerator’s measured daily watt-hours and include all other overnight loads.
How long will a 100Ah battery run a 500W appliance?
Using 12.8 V, 90% usable depth and 90% efficiency gives about 2.1 hours at a steady 500 W. High-current limits may shorten or prevent operation.
Can I use all 100Ah?
Do not assume so. Use the battery manufacturer’s allowed depth of discharge and BMS limits, then include conversion and wiring losses.