The formula
Ah and kWh measure two different things. Ah is charge (current over time); kWh is energy (power over time). The bridge between them is voltage.
Use the pack's nominal voltage — not its full-charge or empty value. LiFePO4 cells are nominally 3.2 V, so a 16S system is 51.2 V. NMC cells are nominally 3.7 V, so a 13S pack is 48.1 V.
What Ah, kWh and voltage mean
- Ah (amp-hours)
- Charge — one amp of current sustained for one hour. Independent of voltage. A 100 Ah pack at 12 V and a 100 Ah pack at 48 V hold the same charge but four times the energy difference.
- Wh / kWh
- Energy. 1 kWh = 1000 W × 1 h = the kilowatt-hour your utility bills you for. This is the unit you'll use for system sizing, runtime estimates and LCOS math.
- Voltage (V)
- The bridge between charge and energy. Wh = Ah × V. The same cell sits at different voltages depending on state of charge — engineering math uses the nominal value.
- SOC / DoD
- SOC is the percentage of capacity left; DoD is the percentage already discharged. Usable energy ≈ nameplate energy × usable DoD window enforced by the BMS.
Common pack specs
A few shortcut conversions you'll run into across residential, commercial and utility-scale projects.
| Application | Voltage | Capacity | Energy |
|---|---|---|---|
| 12V 100Ah lead-acid replacement | 12 V | 100 Ah | 1.2 kWh |
| 24V telecom backup | 24 V | 200 Ah | 4.8 kWh |
| 48V telecom rack | 48 V | 100 Ah | 4.8 kWh |
| Residential LiFePO4 module | 51.2 V | 100 Ah | 5.12 kWh |
| C&I storage module | 51.2 V | 280 Ah | 14.34 kWh |
| EV battery pack | 400 V | 150 Ah | 60 kWh |
Engineering notes
- Nameplate vs usable. The number this calculator gives you is nameplate energy. Subtract the BMS reserve window (typically 5–10%) and a temperature derate when sizing for real-world dispatch.
- Pick the right voltage platform. Residential and RV systems live at 12/24 V; home storage standardises around 48/51.2 V; commercial and utility-scale projects move to 400 V+ to keep wiring losses in check.
- Cycle life math. LiFePO4 typically delivers 4,000–6,000 cycles at 100% DoD. Limiting daily DoD to 80% can dramatically extend service life — factor this in when sizing nameplate capacity.
- Standards reference. For deployable systems, IEC 62619, UL 9540 and (in China) GB/T 36276 dictate testing and installation requirements. This calculator is for first-pass sizing only.
FAQ
What's the difference between Ah and kWh?
Ah (amp-hours) measures charge — how much current a battery can sustain over time, independent of voltage. kWh (kilowatt-hours) measures energy — the actual usable electrical work the battery can deliver. To bridge the two you multiply by the system voltage: kWh = Ah × V ÷ 1000.
What's the Ah → kWh formula?
kWh = (Ah × V) ÷ 1000. Example: a 100 Ah pack on a 51.2 V LiFePO4 system stores 100 × 51.2 ÷ 1000 = 5.12 kWh of nameplate energy.
Why does my datasheet's kWh look slightly different?
Datasheet figures are nameplate — calculated from nominal voltage at a standard discharge rate. Real usable energy depends on the BMS SOC window, discharge C-rate, temperature, age and round-trip efficiency. For engineering estimates, plan for 10–20% margin below nameplate.
Should I use cell voltage or system voltage?
Use the pack's nominal voltage (48 V, 51.2 V, 400 V…) when sizing a full system. Use cell voltage (3.2 V for LiFePO4, 3.7 V for NMC) when working out single-cell figures. Both presets are in this calculator.
When do I run it in reverse — kWh → Ah?
Whenever you already know the energy target (say, 10 kWh of nightly backup) and want to see how many Ah that translates to on each voltage platform. It's the quickest way to compare a 48 V vs 51.2 V vs 400 V solution side-by-side.