What this calculator does
Batteries decide how many days you can stay off the grid. This calculator multiplies your daily demand by the autonomy you want, divides by the usable depth of discharge, and returns the bank energy and ampere-hours — plus the series and parallel arrangement needed to build it from your chosen battery.
How to use it
- Enter the daily energy demand and the autonomy you want — one day for essential loads, three to five days for a resilient system.
- Select the system voltage, which fixes the battery configuration: 12 V for a small system, 48 V for most installations.
- Set the depth of discharge: 80 % for lithium, about 50 % for lead-acid.
- If you know your battery size, the result includes the series and parallel counts needed.
Governing formula
E_bank = E_day × n_days / DoD ; Ah = E_bank / V Where:
-
E_day= Daily energy demand [Wh] -
n_days= Autonomy [days] -
DoD= Usable depth of discharge as a fraction [–]
Derived Equations:
series = ⌈V_system / V_battery⌉ parallel = ⌈Ah_required / Ah_battery⌉ Worked example — 7.4 kWh a day, two days autonomy, 48 V lithium
The figures below were produced by running this page's own engine with the example inputs (Daily energy demand (Wh), Autonomy (days), System voltage, Usable depth of discharge (%), Your battery capacity (Ah), Your battery voltage (V)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Required bank energy | 18500 Wh |
|---|---|
| Required bank capacity | 385.42 Ah |
| Series strings | 4 no |
| Parallel strings | 4 no |
| Assessment | 18.5 kWh bank ≈ 385 Ah at 48 V → 4S × 4P of 100 Ah 12 V batteries. Size cables and OCPD for the bank current. |
- Two days at 7.4 kWh is 14.8 kWh, and dividing by 80 % depth of discharge gives 18.5 kWh of bank.
- At 48 V that is 385 Ah.
- Four 12 V batteries in series reach 48 V, and four such strings in parallel reach about 400 Ah.
Assumptions & limitations
Engineering calculation. Storage from daily demand, autonomy days and depth of discharge. It excludes temperature derating, ageing, Peukert losses and parallel-string imbalance.
- Nameplate capacity overstates usable capacity as batteries age; lithium banks lose capacity more slowly than lead-acid.
- Cold weather reduces available capacity and charging acceptance — size for the winter month, not the annual average.
- Peukert effect and controller round-trip efficiency are ignored, so the real bank should be larger.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- IEC 62619 — industrial secondary lithium cells and batteries.
- NREL — off-grid system sizing guidance.