Battery capacity appears in different units on different labels — amp-hours (Ah), watt-hours (Wh), or sometimes milliamp-hours (mAh). Converting between them requires knowing the voltage. This calculator turns watt-hours into amp-hours so you can compare batteries rated in different units on equal footing.
How to Read Your Battery Label
- Find the capacity rating. Look for "Ah" (amp-hours), "Wh" (watt-hours), or "mAh" (milliamp-hours) on the label. Larger batteries usually list Ah. Power banks and laptops list mAh or Wh.
- Find the voltage. Look for "V" (volts) — this is the nominal voltage. A "12V" battery actually operates between 10.5V (empty) and 14.4V (charging), but 12V is the nominal used for calculations. LiFePO4 "12V" batteries are actually 12.8V nominal.
- Convert if needed. Ah to Wh: multiply Ah by voltage. Wh to Ah: divide Wh by voltage. mAh to Ah: divide by 1,000. Examples: 100Ah at 12V = 1,200Wh. 20,000mAh at 3.7V = 74Wh = 20Ah.
- Compare fairly. A 50Ah 24V battery stores the same energy as a 100Ah 12V battery — both are 1,200Wh. Always convert to watt-hours for apples-to-apples comparison across different voltages.
Quick Amp-Hour Conversion Table
| Watt-Hours | At 12V (Ah) | At 24V (Ah) | At 48V (Ah) |
|---|---|---|---|
| 600 Wh | 50 Ah | 25 Ah | 12.5 Ah |
| 1,200 Wh | 100 Ah | 50 Ah | 25 Ah |
| 2,400 Wh | 200 Ah | 100 Ah | 50 Ah |
| 4,800 Wh | 400 Ah | 200 Ah | 100 Ah |
| 9,600 Wh | 800 Ah | 400 Ah | 200 Ah |
| 14,400 Wh | 1,200 Ah | 600 Ah | 300 Ah |
Notice the pattern: doubling the voltage halves the amp-hours for the same energy. This is why higher-voltage systems use smaller cables — the current is lower for the same power.
Frequently Asked Questions
Sizing a solar system? The <a href="/solar/solar-battery-bank-size-calculator">solar battery bank size calculator</a> uses amp-hour figures to match storage to your panels.
Converting between amp-hours and watt-hours is the foundation of every battery sizing calculation. Get this step right — and always compare at the same voltage — and the rest of your system design falls into place.
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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.