Batteries — especially in solar, off-grid, EV, and marine contexts — are often labeled in amp-hours (Ah), a measure of electrical charge, while system sizing and comparisons are more useful in kilowatt-hours, a measure of usable energy. Converting between them requires one extra piece of information beyond just the Ah rating: the battery's voltage.
The Formula
Worked Example
A 100Ah, 12V deep-cycle battery:
That same 100Ah rating at a higher system voltage looks very different — a 100Ah, 48V battery bank:
This is exactly why comparing batteries by amp-hours alone is misleading unless you're also confirming they're the same voltage — a "100Ah battery" could represent anywhere from just over 1 kWh to nearly 5 kWh of actual usable energy, depending on the system voltage.
Usable vs. Rated Capacity — A Critical Distinction
The Ah-to-kWh formula above gives you the battery's rated capacity — but for most chemistries, you can't (and shouldn't) actually use 100% of that without shortening the battery's lifespan or, in some chemistries, risking damage.
- Lead-acid batteries (flooded, AGM, gel): typically only 50% depth of discharge is recommended for reasonable lifespan — using more than half the rated capacity regularly accelerates wear significantly.
- Lithium (LiFePO4): much more forgiving, commonly rated for 80-100% usable depth of discharge without significant lifespan penalty.
Example: That same 100Ah, 12V (1.2 kWh) lead-acid battery, used at a conservative 50% depth of discharge:
This is a genuinely common sizing mistake — someone sizes a battery bank against its full rated kWh, then finds real-world usable capacity is half (for lead-acid) or less of what they planned around.
Quick Reference Table
| Ah Rating | 12V System | 24V System | 48V System |
|---|---|---|---|
| 50 Ah | 0.6 kWh | 1.2 kWh | 2.4 kWh |
| 100 Ah | 1.2 kWh | 2.4 kWh | 4.8 kWh |
| 200 Ah | 2.4 kWh | 4.8 kWh | 9.6 kWh |
| 300 Ah | 3.6 kWh | 7.2 kWh | 14.4 kWh |
Frequently Asked
Why do EVs and home batteries usually list kWh directly instead of Ah?
Because their internal voltage isn't a fixed, simple number the way a 12V lead-acid battery is — lithium battery packs use many cells in series/parallel configurations, so manufacturers list the resulting total kWh directly rather than an Ah figure that would need extra context to interpret.
How many 100Ah 12V batteries do I need for a 5 kWh system?
At 1.2 kWh usable per battery (before applying any depth-of-discharge limit) — roughly 4-5 batteries for 5 kWh of rated capacity, or more if you're accounting for a conservative depth of discharge on lead-acid chemistry.
Does temperature affect real capacity?
Yes — most battery chemistries lose usable capacity in cold temperatures, sometimes significantly (lead-acid can lose 20%+ of capacity near freezing). For off-grid or outdoor systems in cold climates, size with a margin beyond the basic Ah-to-kWh math above.
For sizing a solar array to charge a battery bank like this, see the solar kW to kWh calculator.
