A 12-volt car battery doesn't store a fixed number of watts; it stores energy measured in watt-hours (Wh), which represents its total capacity. To find this, you multiply its voltage by its amp-hour (Ah) rating. For a typical car battery with a 50Ah rating, the calculation is 12 volts x 50 amp-hours = 600 watt-hours. This means it could theoretically supply 600 watts for one hour, 60 watts for 10 hours, and so on.
However, this is a simplified theoretical maximum. In reality, a battery's usable capacity is less due to the Peukert's Law effect, which states that the higher the current draw, the less total energy you can extract. For example, a battery rated at 100Ah over 20 hours might only provide 80Ah if discharged in 5 hours.
Here's a comparison of common 12V battery types and their typical energy storage:
| Battery Type | Typical Ah Rating | Approximate Watt-Hours (Wh) | Common Use Case |
|---|
| Standard Flooded (Lead-Acid) | 40-60 Ah | 480 - 720 Wh | Gasoline car starting |
| AGM (Absorbent Glass Mat) | 50-100 Ah | 600 - 1,200 Wh | Start-stop vehicles, audio systems |
| Deep Cycle (Lead-Acid) | 70-120 Ah | 840 - 1,440 Wh | Trolling motors, RVs, solar storage |
| Lithium Iron Phosphate (LiFePO4) | 50-100 Ah | 600 - 1,200 Wh | High-performance, RVs, off-grid |
The key takeaway is that the "watts" a battery can handle at any single moment (its power output) is different from the total energy it stores. A battery must also deliver a very high wattage for a few seconds to start an engine—this is measured in Cold Cranking Amps (CCA), not watt-hours. For any application beyond starting your car, like powering a cooler or accessories, the watt-hour capacity is the crucial number to determine how long your gear will run.
