Battery bank sizing calculator (off-grid and backup)
Find the battery capacity you need in Ah and kWh, how many batteries and in what series–parallel layout, starting from a daily total or a list of loads.
How a battery bank is sized
E_covered = E_day · days / η C_nominal = E_covered / DoD C_Ah = C_nominal / U_system series = U_system / U_battery parallel = ⌈ C_nominal / (series · U_battery · Ah_battery) ⌉
- E_day = daily consumption (Wh), the sum of power × hours/day × quantity
- η = system efficiency (inverter + battery)
- DoD = permitted depth of discharge
Worked example
3,000 Wh/day, 2 days of autonomy, 48 V system, LiFePO4 (DoD 80%, η 90%), 12.8 V / 100 Ah batteries:
E_covered = 3000 · 2 / 0.9 = 6,667 Wh C_nominal = 6,667 / 0.8 = 8,333 Wh = 8.33 kWh C_Ah = 8,333 / 48 = 173.6 Ah series = 48 / 12.8 ≈ 4 → 51.2 V parallel = ⌈ 8,333 / (4 · 12.8 · 100) ⌉ = ⌈1.63⌉ = 2 total = 4S × 2P = 8 batteries = 10.24 kWh installed
Example daily consumption
| Load | Power (W) | Hours/day | Energy (Wh/day) |
|---|---|---|---|
| Efficient fridge (equivalent to 8 h at full power) | 100 | 8 | 800 |
| LED lighting, 6 bulbs | 60 | 5 | 300 |
| TV and router | 100 | 4 | 400 |
| Laptop | 60 | 6 | 360 |
| Water pump | 500 | 0.5 | 250 |
| Total | 2,110 |
Default values used
| Chemistry | Depth of discharge | System efficiency |
|---|---|---|
| LiFePO4 | 80% | 90% |
| Lead-acid (AGM / gel) | 50% | 80% |
Typical values, not datasheet figures. Use your battery manufacturer's numbers.
Frequently asked questions
How many days of autonomy should I choose?
For an off-grid system with solar panels, 1–3 days usually covers cloudy spells; in areas with long winters, aim for 3 or add a generator. For grid-outage backup, autonomy is a few hours, i.e. 0.1–0.5 days.
Why can I not use the full battery capacity?
Deep discharge shortens battery life. Lead-acid is normally limited to about 50% depth of discharge, while LiFePO4 typically allows 80–90% or more, per the manufacturer datasheet. The calculator divides the required energy by this value.
What is system efficiency?
It is the ratio of energy delivered to the loads to energy drawn from the battery, after inverter and battery losses. 85–95% is typical for LiFePO4 systems and 75–85% for lead-acid.
What does 4S × 2P mean?
4S is four batteries in series (raising voltage to the system voltage) and 2P is two such strings in parallel (raising capacity). The total number of batteries is the product: 8.
Does the calculator also check the inverter or the solar panels?
No. It sizes the battery bank only. Inverter rating, battery maximum discharge current and the panels needed to recharge are checked separately; use the solar PV sizing calculator for panels.