Amp-hours are meaningless without a voltage attached, because watt-hours are amp-hours times voltage. A 100 Ah battery holds 1,200 Wh at 12 V and 4,800 Wh at 48 V — the same nameplate, four times the energy. Comparing two batteries by amp-hours alone compares nothing at all unless they are on the same system voltage.
And even watt-hours overstate what you can use, because none of the nameplate is available in practice.
How much of the nameplate is usable?
Between half and nine tenths, depending on chemistry, and it is a decision about battery life rather than a physical limit.
| Chemistry | Usual depth of discharge | Usable from 100 Ah at 12 V |
|---|---|---|
| Lead-acid | 50% | 600 Wh |
| AGM | 50–60% | 600–720 Wh |
| Lithium iron phosphate | 80–90% | 960–1,080 Wh |
Lead-acid is not physically incapable of deeper discharge; it is that doing so repeatedly shortens its life sharply. Lithium tolerates deeper cycling, which is most of why it displaces lead in anything cycled daily despite costing more per nameplate amp-hour.
On usable energy the gap narrows considerably. A 100 Ah lithium battery delivers roughly the same as a 180 Ah lead-acid one, and weighs a third as much.
What else comes off before the load sees it?
Conversion, and it is easy to forget. An inverter turning 12 V DC into 230 V AC is typically 85 to 92 per cent efficient, and it draws idle current whether or not anything is switched on.
A 100 Ah 12 V lithium battery at 80 per cent depth of discharge and 90 per cent inverter efficiency has 864 usable watt-hours, which runs a 120 W load for 7.2 hours. The nameplate said 1,200 Wh and ten hours.
The idle draw matters on long, light loads. An inverter idling at 20 W consumes 480 Wh a day doing nothing, which is more than half the usable capacity of that battery — which is why off-grid systems switch inverters off rather than leaving them on standby.
Why does a fast discharge give less?
Because capacity ratings assume a slow discharge, typically over twenty hours. Pull the same battery hard and you get measurably less out of it — an effect that is pronounced in lead-acid and small in lithium.
The practical version: a lead-acid battery rated 100 Ah over twenty hours might deliver only 60 to 70 Ah if drained in two. A lithium battery of the same rating loses very little over the same range, which is another reason it suits high-draw applications.
This is also why a power bank delivers less than its printed rating. The cells are rated at 3.7 V and the output is 5 V, so a 10,000 mAh bank holds 37 Wh and delivers maybe 28 Wh after conversion — about 5,600 mAh at 5 V.
What is C-rate?
A way of expressing current relative to capacity, so it means the same thing on any battery. A 1C discharge empties the pack in an hour, 0.5C in two, and 0.05C is the twenty-hour rate the capacity was measured at. A 100 Ah battery at 0.5C is being pulled at 50 A.
It is the useful shorthand because every specification that depends on rate — maximum continuous discharge, charge current, the derating curve — is quoted in C rather than amps. That way a single figure applies across a product range instead of one number per size.
How long does charging take?
The amp-hours to replace, divided by the charge current, divided again by charging efficiency. A 100 Ah battery at 20 per cent charged with a 10 A charger at 85 per cent efficiency takes about 9.4 hours.
That covers the bulk phase, where the charger delivers full current. Real chargers then move to an absorption phase at falling current and, for lead-acid, a float phase — and the last 20 per cent of a lead-acid charge can take as long as the first 80.
Lithium behaves far better here, holding near full current until it is nearly full. It is the second large practical difference between the chemistries and it rarely appears in a specification comparison.
Questions people ask
How do I convert Ah to Wh? Multiply by the nominal voltage. A 100 Ah 12 V battery is 1,200 Wh, and that figure can be compared with any other battery regardless of its voltage.
Is a higher system voltage better? For anything substantial, yes. The same energy at a quarter of the current means thinner cable and lower losses, which is why larger installations run at 24 or 48 V rather than 12.
Should I run a battery flat? No, and particularly not lead-acid. Depth of discharge is the largest single influence on cycle life, and a battery cycled to 50 per cent lasts several times longer than the same one cycled to 90.
Does temperature matter? Substantially. Cold reduces available capacity in every chemistry, and charging lithium below freezing damages it — which is why quality lithium batteries have a low-temperature charge cut-off built in.
Convert to watt-hours, then take off the depth of discharge and the conversion losses before believing anything. The battery capacity calculator handles the Ah-to-Wh step and the usable fraction, the battery runtime calculator adds the inverter, and the battery charge time calculator covers the way back.