A 12V battery stores energy in watt-hours, not watts. Watts measure power (how fast energy flows), while watt-hours measure capacity (how much energy is stored). The calculation is straightforward: multiply amp-hours by 12 volts. A 100Ah 12V battery stores 1,200Wh — enough to run a 100W device for 12 hours or a 600W device for 2 hours.
Example: Reading a 12V Battery Label
You are looking at a 12V battery in the shop. The label says:
- 12V 100Ah — this is the key spec. Total energy: 100 x 12 = 1,200Wh (1.2kWh).
- Nominal Voltage: 12.8V — this is a LiFePO4 battery. For calculations, use 12.8V instead of 12V for slightly more accurate results: 100 x 12.8 = 1,280Wh.
- Max Discharge: 100A — the battery can deliver up to 1,200W at 12V (100A x 12V). Going above this causes overheating.
- Cycle Life: 3,000 at 80% DoD — at 80% depth of discharge, the battery lasts 3,000 charge-discharge cycles before losing significant capacity.
The usable watt-hours depend on how deep you discharge. At 80% DoD: 1,200 x 0.80 = 960Wh usable. At 50% DoD: 1,200 x 0.50 = 600Wh usable. The full 1,200Wh is only accessible if you drain the battery completely, which shortens its lifespan for every chemistry.
Common 12V Batteries and Their Watt-Hours
| Battery | Ah Rating | Total Wh | Usable Wh (typical DoD) |
|---|---|---|---|
| UPS battery (12V 7Ah SLA) | 7 Ah | 84 Wh | 42 Wh (50%) |
| Motorcycle battery | 8-14 Ah | 96-168 Wh | 48-84 Wh (50%) |
| Compact car battery | 40-55 Ah | 480-660 Wh | 240-330 Wh (50%) |
| Full-size car battery | 60-80 Ah | 720-960 Wh | 360-480 Wh (50%) |
| Marine deep cycle | 75-125 Ah | 900-1,500 Wh | 450-750 Wh (50%) |
| LiFePO4 (standard) | 100 Ah | 1,200 Wh | 960 Wh (80%) |
| LiFePO4 (large RV) | 200 Ah | 2,400 Wh | 1,920 Wh (80%) |
| Golf cart bank (2x6V) | 225 Ah | 2,700 Wh | 1,350 Wh (50%) |
The usable energy column is what you can actually use before the battery needs recharging. This is the number that matters for runtime calculations.
Frequently Asked Questions
Pairing your 12V battery with solar panels? The <a href="/solar/solar-panel-output-calculator">solar panel output calculator</a> shows daily energy generation for comparison.
A 12V battery's usable watts depend on its amp-hour rating, chemistry, and how deep you discharge it. Multiply Ah by 12 for total watt-hours, then apply your safe DoD percentage for the usable figure. That usable number is what your runtime calculations should be based on.
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Written and maintained by Dan Dadovic, Developer & Off-Grid Energy Enthusiast. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, and wind power — all from building and maintaining his own systems..
Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.