Electronics · battery dc power
Battery Capacity Calculator
Calculate the nominal Wh and Ah required for a target runtime from explicit AC/DC loads, losses and usable-capacity factors.
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Calculate required battery capacity
Reverse the energy balance using only explicit load and battery-planning factors.
Example arithmetic values are preloaded for runtime and DC load. Replace them with measured or documented project inputs; no battery factor or inverter efficiency is recommended.
83.3333 Ah at the entered bank voltage
Usable energy needed
1,000 Wh
Battery-side load
100 W
Show calculation steps
| Step | Expression | Raw value |
|---|---|---|
| Battery-side power | DC W + AC W ÷ efficiency + inverter idle W | 100 W |
| Required usable energy | target hours × battery-side W | 1,000 Wh |
| Required nominal energy | usable Wh ÷ (DoD × SoH × temperature) | 1,000 Wh |
How to use the battery capacity calculator
- Enter the target runtime in hours and the average direct-DC and AC loads separately.
- If an AC load is present, enter the inverter efficiency at the relevant load and its idle draw. A percentage from a different operating point may be misleading.
- Enter only usable-depth, condition and temperature factors supported by your operating plan, measurements or exact product documents.
- Add bank voltage if you need the equivalent Ah result. Add one battery’s Wh only if you want a rounded-up arithmetic count.
- Verify current, surge, cutoff, BMS, wiring, fusing, charging and installation separately.
The calculator runs locally in your browser. It stores no project or battery data on a calculation server.
Formula and derivation
The AC load is first converted to battery-side power:
battery W = DC W + AC W ÷ inverter efficiency + inverter idle W
The target usable energy is hours × battery W. Required nominal energy is:
nominal Wh = usable Wh ÷ (depth factor × health factor × temperature factor)
When bank voltage is known, required Ah = nominal Wh ÷ bank V. This is the algebraic inverse of the battery runtime calculator. It is an energy-balance estimate, not a product-sizing recommendation.
Worked examples
A constant 100 W DC load for 10 hours needs 1,000 usable Wh. If the planned usable depth is 80% and the other explicit factors remain 100%, nominal capacity is 1,000 ÷ 0.8 = 1,250 Wh. At 12 V that is 104.17 Ah.
For a 100 W AC appliance, 90% inverter efficiency and 5 W idle draw, battery-side power is 100 ÷ 0.90 + 5 = 116.111 W. Ten hours at an 80% usable-depth factor requires 1,451.39 nominal Wh before any health or temperature reduction.
What the count means
When unit-battery Wh is entered, the tool divides required nominal Wh by that value and rounds up. This count is arithmetic only. It does not establish that the product supports the proposed voltage, current, series/parallel topology, charger, BMS or environment.
Common mistakes and troubleshooting
- Do not confuse Wh with Ah; Ah needs a voltage basis.
- Do not use inverter peak efficiency when the relevant load has a different efficiency.
- Do not add idle draw twice if it is already included in a measured battery-side load.
- Do not treat a chemistry name as evidence for a universal depth, condition or temperature factor.
- A zero total load has no finite capacity requirement in this model, so the tool withholds the result.
Frequently asked questions
Does this include a reserve margin?
No hidden reserve is added. Any explicit operating reserve must be represented in inputs and documented for the project.
Can it select a battery?
No. The optional unit-Wh count is not a product recommendation or compatibility check.
Why could actual capacity need to be higher?
Cutoff voltage, transient current, discharge rate, temperature, age, imbalance and conversion behavior can all matter. Use exact-product discharge information when the simple energy model is insufficient.
Is this a wire, fuse or BMS sizing tool?
No. It performs an energy calculation only.
References and further reading
These sources define or explain the calculation. Source editions and model limits are part of the result—not decorative citations.
- Battery capacity and Peukert exponent, Victron Energy; accessed 2026-08-22. BAT-C01
- DOE Explains...Batteries, U.S. Department of Energy; accessed 2026-08-22. BAT-C03
- NIST Guide to the SI, National Institute of Standards and Technology; accessed 2026-08-22. BAT-C04