Battery Bank Sizing Calculator
Size a battery bank in Ah from load power, backup time, bank voltage, depth of discharge and inverter efficiency, with the formula and a worked example.
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Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
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About this calculator
Use this to find the battery capacity needed to carry a constant load for a set backup time. It allows for the usable depth of discharge and the inverter efficiency, and gives the energy drawn from the battery as well as the capacity in amp-hours.
Limits of this calculator
No ageing, temperature or Peukert allowance is included. Add an ageing margin (typically 20 to 25%) and a temperature factor before selecting the battery, and use the capacity quoted at the discharge rate of your backup time.
Cross-check: 6 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Indicative
Indicative. Estimation only; further engineering checks are required. Basis: Sizing estimate from simple derating factors.
What this calculator checks
- The arithmetic of the simplified model for the values you entered.
What remains unverified
- Whether the simplifying assumptions listed on this page hold for your case.
- Any standard, manufacturer or site data. Treat the result as an estimate and confirm it by a further engineering check.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | IEEE 485; IEC 62485-2, IEEE 485-2020 (active); IEC 62485-2:2010 |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | Stationary lead-acid battery sizing and stationary battery safety are relevant. IEEE 485 applies to lead-acid float-service applications; the calculator is a simplified energy estimate and does not implement duty-cycle sizing. |
| Cross-check test cases | 6 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Indicative |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; licensed clause-by-clause standard review; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Load power | 500 W |
| Backup time | 4 h |
| Battery bank voltage | 48 V |
| Usable depth of discharge | 0.8 |
| Inverter efficiency | 0.9 |
- Energy = P × t / η = 500 × 4 / 0.9 = 2,222.2 Wh
- Ah = 2,222.2 / (48 × 0.8) = 57.87 Ah
Assumptions
- Constant load for the whole backup time
- Capacity quoted at the discharge rate of the backup time
- No ageing, temperature or Peukert allowance
Add an ageing margin (typically 20 to 25%) and a temperature factor before selecting the battery.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Load power | W | greater than 0 W | |
| Backup time | h | greater than 0 h | |
| Battery bank voltage | V | greater than 0 V | |
| Usable depth of discharge | (number) | greater than 0, up to 1 | |
| Inverter efficiency | (number) | greater than 0, up to 1 |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Required capacity | Ah | Shown to 1 decimal place | |
| Energy from battery | kWh | Shown to 3 decimal places |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
Typical values
| Backup time (h) | Required capacity (Ah) | Energy from battery (kWh) |
|---|---|---|
| 1 | 14.5 | 0.556 |
| 2 | 28.9 | 1.111 |
| 4 | 57.9 | 2.222 |
| 8 | 115.7 | 4.444 |
| 12 | 173.6 | 6.667 |
Other inputs are the same as in the worked example above.
Standards and further reading
- IEC 60909-0 (short-circuit currents)
- IEEE Std 519 (harmonic limits)
Check results against your utility's connection requirements and the equipment data sheets. These are named as background reading. Elecmetric does not embed clause text or rating tables, and the reviewer record for this calculator is separate from this list.
Frequently asked questions
What do I need to enter?
Enter load power, backup time, battery bank voltage, usable depth of discharge and inverter efficiency. Change the unit of a value with its selector and the result stays the same.
Why is the Ah more than the energy divided by the voltage?
Only part of the battery can be used, set by the depth of discharge, and the inverter loses some energy. The formula divides by both, so the bank must be larger than the plain energy figure.
Should I add a margin to the result?
Yes. The result is the capacity needed when new, at the rated temperature. Batteries lose capacity with age and in the cold, so add an ageing margin and a temperature factor.
How is this different from battery runtime?
This works from the backup time you need to the capacity required. The UPS runtime calculator goes the other way, from a given battery to the time it can carry the load.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 6 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- Battery Formulas: Capacity and Runtime: Battery formulas for energy in Wh, bank capacity from load and depth of discharge, runtime
- Battery Pack Formulas: Series and Parallel: Battery pack formulas: cells in series for voltage, strings in parallel for capacity
- Peukert Law and Battery Runtime Formulas: Peukert law formulas: the runtime of a battery at a given current
- Battery C-Rate and Cycle Life Formulas: Battery C-rate formulas: the current for a C-rate, the ideal run time
- Supercapacitor Energy Formulas: Supercapacitor formulas: the usable energy between two voltages
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