Battery capacity formula
A battery's capacity can be described two ways, and people mix them up constantly. The amp-hour (Ah) rating is a measure of charge — how much current the battery can supply over time — and on its own it says nothing about energy. The watt-hour (Wh) rating is the actual energy, and energy is what runs your appliances. To turn a charge rating into an energy capacity you multiply by the voltage. That single step is the whole formula for battery capacity:
Wh = V × Ah and kWh = Wh ÷ 1000
So to determine the capacity of a 12 V battery rated 100 Ah you calculate 12 × 100 = 1,200 Wh, which is 1.2 kWh. To go back the other way the equation rearranges to Ah = Wh ÷ V. If the rating is in milliamp-hours — common for phones, vapes and 18650 cells — divide by 1000 first, so the volt-to-mAh capacity calculation becomes Wh = V × (mAh ÷ 1000) A 3.7 V cell rated 3,000 mAh therefore holds 3.7 × 3.0 = 11.1 Wh. This is also how you calculate Ah from watts and watt-hours when you already know the energy: Ah = Wh ÷ V.
The amp-hour calculation at 12 V is the one most people reach for, because so many van, RV, marine and solar batteries are nominally 12 V. But the same battery ampere figure means very different energy at 24 V or 48 V, which is exactly why you should always size and compare in watt-hours.
Usable capacity
Rated capacity is not the same as usable capacity. Fully draining a battery shortens its life, so you only use part of what is printed on the label. The fraction you can safely use is the depth of discharge (DoD), and the usable energy is just the rated energy scaled by it:
Usable Wh = Wh × DoD
The calculator applies your chosen DoD automatically, so the 1,200 Wh battery above shows 960 Wh usable at 80% DoD.
Build a battery pack (series and parallel)
For a lithium-ion or LiFePO4 pack built from individual cells, you do not have a single Ah and voltage to plug in — you build them up from the cell. There are only two rules, and the pack builder above applies both:
- Cells in series add voltage. Pack voltage = series count × cell voltage.
- Cells in parallel add capacity. Pack capacity = parallel count × cell capacity.
pack V = S × cell V · pack Ah = P × cell Ah · pack Wh = pack V × pack Ah · cells = S × P
This is the standard lithium-ion battery capacity calculation, and it works the same for a LiPo battery, a LiFePO4 battery or a NiMH stick. A "13S4P" 18650 pack — 13 cells in series, 4 in parallel — built from 3.6 V, 3.0 Ah cells comes out at 13 × 3.6 = 46.8 V and 4 × 3.0 = 12 Ah, which is 46.8 × 12 ≈ 562 Wh from 52 cells. Change the chemistry preset and the per-cell voltage updates to match.
LiFePO4, Li-ion and lead-acid
The only chemistry-specific numbers you need are the nominal cell voltage and a sensible usable DoD. These are universal, well-known figures:
| Chemistry | Nominal cell voltage | Typical usable DoD |
|---|---|---|
| Li-ion / NMC | 3.6–3.7 V | 80–90% |
| LiPo | 3.7 V | 80–90% |
| LiFePO4 | 3.2 V | 80–100% |
| Lead-acid (flooded / AGM / gel) | 2.0 V | ~50% |
| NiMH / NiCd | 1.2 V | varies |
A "12 V" lead-acid battery is six 2.0 V cells in series; a "12 V" LiFePO4 battery is four 3.2 V cells, giving a 12.8 V nominal pack. LiFePO4 tolerates the deepest discharge, which is why a LiFePO4 battery of the same rated capacity gives you noticeably more usable energy than lead-acid.
C-rate and discharge current
The C-rate links capacity to current. It tells you how fast a battery is being charged or discharged relative to its capacity, and it is the basis of any battery load calculation or battery discharge rate formula. The maximum current a battery can sustain at a given C-rate is:
max current (A) = capacity (Ah) × C time at that rate = 1 ÷ C hours
At 1C a 100 Ah battery delivers 100 A and empties in one hour. At 0.5C it delivers 50 A and lasts two hours; at 2C it delivers 200 A and lasts half an hour. Cells have a maximum continuous C-rate set by the manufacturer — exceed it and the cell overheats — so this is the figure to check before drawing a heavy load through a small pack.
A worked example
You have a 12 V LiFePO4 battery rated 100 Ah and want its energy, usable energy and safe current.
- Energy capacity: 12 V × 100 Ah = 1,200 Wh (1.2 kWh)
- Usable at 100% DoD: 1,200 Wh — LiFePO4 can run nearly flat
- At a 1C discharge: 100 Ah × 1 = 100 A for one hour
So a "100 Ah" 12 V LiFePO4 battery is really about 1.2 kWh of energy that can deliver up to 100 A if the cells are rated for 1C — enough headroom for a small inverter and a 12 V fridge for the better part of a day.