How to Calculate Battery Capacity: Ah to kWh Converter Guide
How to Calculate Battery Capacity: Ah to kWh Converter Guide
Capacity is the most quoted and least understood battery number, because buyers mix amp-hours and watt-hours as if they were the same. They are not. Amp-hours describe charge at a voltage; watt-hours describe energy you can actually use. This guide gives you the conversion cold, a worked table, and the traps that make a spec sheet lie. For a lithium battery manufacturer, the conversion is routine, but for a procurement manager it is the difference between buying energy and buying a number.
The basic conversion
The formula is simple: watt-hours = amp-hours x volts. A 100 Ah pack at 12 V holds 1,200 Wh, or 1.2 kWh. At 48 V the same 100 Ah is 4,800 Wh, or 4.8 kWh. The amp-hour figure alone is meaningless without the voltage, which is why a “100 Ah” claim on its own tells you nothing about usable energy. Always convert to kWh before comparing packs, because that is the unit your load and your inverter speak. A buyer who compares two “200 Ah” packs at different voltages is comparing oranges to apples and will size the system wrong, a mistake that surfaces only when the bank runs out early.
Worked conversion examples
| Pack | Ah | V | Wh | kWh |
|---|---|---|---|---|
| Small portable | 20 | 12 | 240 | 0.24 |
| RV bank | 200 | 12 | 2,400 | 2.4 |
| Home storage | 100 | 48 | 4,800 | 4.8 |
| C&I rack | 300 | 51.2 | 15,360 | 15.36 |
The table shows why voltage matters as much as Ah. The 200 Ah RV pack at 12 V holds the same energy as a 50 Ah pack at 48 V—yet a buyer scanning only the Ah column would call the 200 Ah pack “four times bigger.” Convert everything to kWh first, then compare. The kWh column is the honest number, and it is the one your daily load and your autonomy math should be built on, not the headline Ah.
Usable vs nameplate capacity
The nameplate kWh is not what you get. Lithium is safely discharged to about 80% depth-of-discharge, so a 4.8 kWh pack delivers roughly 3.84 kWh usable. Lead-acid, by contrast, is often limited to 50% DoD, so its usable energy is half the nameplate—a key reason lithium looks smaller but performs larger. When you size, use usable kWh: nameplate divided by the allowed DoD. A supplier quoting “10 kWh” without stating DoD is hiding whether you get 8 kWh (lithium at 80%) or 5 kWh (lead at 50%). Insist on usable kWh at a stated DoD, because that is the number your load actually sees, and the only fair basis for a price comparison across chemistries.
Why C-rate changes the number
Capacity is stated at a test C-rate, often 0.2C or 0.5C. Pull harder and delivered Ah drops, because internal resistance and heating eat into what is available. A cell rated 100 Ah at 0.2C may deliver only about 90 Ah at 1C and less at 2C. So the converted kWh is only valid at the test rate; at your duty it can be lower. A battery application solutions buyer should request the discharge curve at the real C-rate and re-run the conversion against that delivered Ah, not the brochure Ah. The curve is the honest spec; the headline kWh is a best case measured at a gentle rate you may never run.
Converting for a solar bank
For a home energy storage system, the gentle 0.2C solar duty means capacity stays near nameplate, so the simple Ah x V conversion holds. Take a 5 kWh (about 104 Ah at 48 V) pack and three days of autonomy, and you can cover roughly 15 kWh of load before hitting the 80% floor—about 12 kWh usable. The conversion is the bridge between your daily load in Wh and the bank’s nameplate in Ah, and at this low rate it is reliable. Resist bolting on a high-C inverter that never fires, because it adds cost and heat with no benefit at a 0.2C duty where the simple math already fits.
Chemistry effects on the conversion
The conversion formula does not care about chemistry—Ah x V is Ah x V—but the usable fraction does. LFP and a sodium-ion battery both reach ~80% usable DoD; NMC is similar but trades life for density; lead-acid lags at 50%. The voltage also differs by cell count: LFP is 3.2 V per cell (16 cells for 51.2 V), NMC about 3.7 V (14 cells for ~51.8 V). So two “48 V” packs can have different cell counts and slightly different nameplate Wh for the same Ah. Read the nominal voltage on the datasheet, not the marketing “48 V,” because the real V is what the conversion uses, and a 3.7 V versus 3.2 V per cell gap shifts the kWh more than buyers expect.
Power vs energy: the other axis
Capacity in kWh is energy; the C-rate is power. A 5 kWh pack can be a 0.5C (2.5 kW) bank or a 1C (5 kW) bank, delivering the same energy at different speeds. Confusing the two leads to a bank that holds enough energy but cannot deliver it fast enough for the load, or vice versa. When you convert Ah to kWh you have answered “how much”; you still must answer “how fast” with the C-rate and the BMS peak rating. A semi-solid-state battery packs more Wh per kg, so for a fixed energy a semi-solid pack is lighter, but the Ah-to-kWh math is identical—only the weight and the price change, not the conversion.
Series and parallel: how configuration shifts Ah and V
How cells are wired changes both numbers, and the datasheet assumes you know which. Cells in series raise voltage and keep Ah the same; four 3.2 V 100 Ah LFP cells in series make a 12.8 V 100 Ah module (1.28 kWh), not 400 Ah. Cells in parallel keep voltage and add Ah; four in parallel make 3.2 V 400 Ah (1.28 kWh), not 12.8 V. The energy is identical, but the voltage decides your inverter and your cabling. A buyer who misreads a “100 Ah” module as usable at their 48 V system double-counts or halves the bank. Always convert the module to kWh first—Ah x nominal V—then sum modules, because kWh is the only figure that survives the wiring. This single check catches more spec errors than any other, because series and parallel confuse even experienced buyers skimming a cell-level sheet, and the error only appears when the bank is built and will not match the load it was supposed to carry.
A practical conversion worksheet
Before any quote, write one page: list each load in watts and hours, sum to daily Wh, pick system voltage, convert to daily Ah (Wh / V), multiply by autonomy days, divide by allowed DoD for nameplate Ah, then convert back to kWh (Ah x V) to sanity-check against your load. Hand that to the supplier and ask for a pack whose usable kWh at 80% DoD covers your number with margin. The worksheet exposes the liar: a “10 kWh” pack that only delivers 6 kWh usable at your C-rate is a 6 kWh pack in practice, and pricing it as 10 kWh is where margins leak. Ten minutes of conversion prevents a mis-specification that only shows up after install, when the bank quits at 70% of the promised energy.
Reading the datasheet line by line
When the datasheet arrives, find the nominal voltage, the rated Ah, the test C-rate, the DoD at which capacity is stated, and the end-of-life threshold. Convert rated Ah x nominal V, then apply the stated DoD to get usable kWh, then discount for your real C-rate using the discharge curve. If the sheet states capacity at 0.2C but you run 1C, subtract the drop the curve shows. A sheet that omits the test C-rate or the DoD is not a spec, it is an ad; reject it. The conversion is only as good as the conditions attached, and a disciplined buyer converts against the same conditions for every bid so the comparison is honest, not a contest of who buried the worst assumptions deepest.
Frequently asked questions
How do I convert Ah to kWh? Multiply amp-hours by nominal volts, then divide by 1000; a 100 Ah 48 V pack is 4.8 kWh.
Why is usable capacity less than nameplate? Lithium is capped near 80% DoD, so a 4.8 kWh pack gives about 3.84 kWh usable.
Does C-rate change the kWh? Yes—capacity drops as you pull harder, so convert against your real discharge rate, not the test rate.
Do chemistries convert differently? The formula is the same; usable fraction and nominal voltage per cell differ, so read the sheet closely.
Written by Karl at China Battery Technology. Send us your loads and we will convert them to a real bank—request a quote for a correctly sized pack.
Related Reading
- battery application solutions hub
- How to Choose a BMS: Topology, Comms and Protection
- How to Read a Battery Datasheet: Specifications Decoded
