How the conversion works
Amp-hours measure charge, not energy. To turn charge into energy you have to know the voltage it sits at.
This is why watt-hours are the honest unit for comparing batteries. A 100 Ah 12 V battery and a 50 Ah 24 V battery store exactly the same energy, 1,200 Wh, even though one advertises twice the amp-hours. Sellers quoting amp-hours without voltage are giving you half the information.
Rated versus usable
The second number that matters is how much of that energy you can actually take out. Lead-acid batteries should not be drained past about half without shortening their life dramatically, so a 1,200 Wh lead-acid battery realistically offers 600 Wh. LiFePO4 comfortably delivers 80% or more, giving roughly 960 Wh from the same nameplate. Judged on usable energy per unit of cost and per cycle, lithium usually wins even at a higher purchase price.
Worked example
Comparing two batteries that look different but are not:
Once you convert to watt-hours, comparisons across chemistry, voltage and format become straightforward, and so does matching a bank to a daily load figure expressed in watt-hours.
Common mistakes
- Comparing amp-hours across voltages. Always convert to watt-hours first; it is the only fair comparison.
- Confusing rated with usable. The printed capacity is a laboratory figure at gentle discharge. Real usable energy depends on chemistry and how hard you draw.
- Forgetting nominal voltage differs from actual. A 12 V lead-acid battery sits near 12.7 V full and 11.8 V nearly empty; a 12 V LiFePO4 pack holds around 13.2 V for most of its discharge. Nominal voltage is fine for planning, but instantaneous readings will differ.
- Adding amp-hours in series. Wiring batteries in series adds voltage and keeps amp-hours the same; parallel adds amp-hours and keeps voltage. Either way, total watt-hours add up the same.
Frequently asked questions
How many watt-hours is a 100 Ah battery?
1,200 Wh at 12 V, 2,400 Wh at 24 V, and 4,800 Wh at 48 V. Usable energy is lower: about 960 Wh for a 12 V lithium unit, about 600 Wh for lead-acid.
How do I go the other way, from watt-hours to amp-hours?
Divide by voltage. A daily load of 2,000 Wh on a 24 V system needs 2,000 / 24, or about 83 Ah of usable capacity, which then has to be divided again by the usable fraction to get the rating you must buy.
Why do power stations advertise watt-hours instead?
Because they contain their own inverter and internal voltage, so amp-hours would be meaningless to a buyer. Watt-hours let you compare a portable power station directly against a battery bank, which is exactly the comparison most people want to make.