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 dropping below 80% of rated 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.
Worked Examples
Can a 100Ah Battery Run a 600W Microwave?
Context
Your 100Ah 12V battery stores a known amount of energy. You want to run a 600W microwave through an inverter. How long does it last?
Calculation
Total energy: 100 Ah x 12V = 1,200 Wh
Usable at 80% DoD, 88% inverter eff: 1,200 x 0.80 x 0.88 = 844.8 Wh
Runtime: 844.8 / 600 = 1.4 hours
Interpretation
About 84 minutes of microwave use. That is 14 heating cycles of 6 minutes each — more than enough for a day of reheating meals.
Takeaway
For a detailed runtime at your exact load and battery chemistry, use our LiFePO4 runtime calculator or lead-acid runtime calculator with chemistry-specific DoD and efficiency values.
Total Watt-Hours in a Parallel Battery Bank
Context
You wire two 200Ah 12V LiFePO4 batteries in parallel. What is the total energy, and how does it compare to a single 100Ah 24V battery?
Calculation
Parallel bank: 400Ah at 12V = 400 x 12 = 4,800 Wh
Single 100Ah 24V: 100 x 24 = 2,400 Wh
Interpretation
The parallel 12V bank stores exactly twice the energy of the single 24V battery. Parallel wiring adds capacity (Ah) while keeping voltage the same. Series wiring adds voltage while keeping Ah the same.
Takeaway
For help designing series-parallel battery configurations, see our battery pack calculator which handles multi-cell arrangements.
Frequently Asked Questions
Glossary
Watt-Hours
The total energy stored in a battery. For a 12V battery, Wh = Ah x 12. A 100Ah 12V battery holds 1,200 Wh — the same energy as running a 100W light for 12 hours or a 600W appliance for 2 hours.
Usable vs Total Energy
Total energy (Ah x V) is the theoretical maximum. Usable energy subtracts depth of discharge limits and efficiency losses. A 1,200 Wh battery at 80% DoD and 90% efficiency delivers only 864 Wh to your devices.
Parallel Wiring
Connecting batteries positive-to-positive and negative-to-negative. This adds amp-hours while keeping voltage the same. Two 100Ah 12V batteries in parallel = 200Ah at 12V.
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.
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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. For worked examples of turning those watt-hours into real runtime across different loads and battery types, read our battery runtime guide.
Last updated:
Written and maintained by Dan Dadovic, Commercial Director at Ezoic Inc. & PhD Candidate in Information Sciences. He works professionally as Commercial Director at Ezoic Inc., leading revenue strategy across digital publishing.
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.