This calculator sizes the battery bank from the result you need: a stated load for a stated number of backup hours. It first calculates nominal energy and amp-hour capacity, then converts that requirement into whole batteries or modules arranged in series and parallel.
Battery size depends on average energy use, not inverter watts alone
An inverter rating describes the load it can supply. Battery capacity describes how long that load can be supplied. A refrigerator, pump or air conditioner may cycle, while a router, fibre/fiber ONT, alarm system or essential lighting may remain on throughout an outage. The energy calculation therefore starts with average power over the backup period.
Two load-entry methods
| Method | Inputs | Best use |
|---|---|---|
| Known average load | Average watts and an optional highest simultaneous load | Measured systems, energy-monitor data or an existing load schedule |
| Appliance list | Power, quantity and share of backup time for each load | Home backup, off-grid cabins, RVs, caravans and small mixed-load systems |
How watts become battery-bank Ah
- The average AC load is increased by the selected capacity allowance.
- Inverter efficiency converts the AC requirement into a higher DC demand from the battery bank.
- Optional inverter idle consumption is added on the battery side.
- DC power is multiplied by the required runtime to obtain usable watt-hours.
- Usable watt-hours are divided by the permitted usable-capacity fraction to obtain nominal bank energy.
- Nominal watt-hours are divided by system voltage to obtain amp-hours at that voltage.
Amp-hours are meaningful only when voltage is stated. A 24 V 100 Ah bank stores roughly twice the nominal energy of a 12 V 100 Ah bank.
Series raises voltage; parallel raises amp-hour capacity
What each connection changes
| Connection | Voltage | Ah capacity | Example |
|---|---|---|---|
| Series | Adds | Stays equal to one battery | Two 12 V 100 Ah batteries form a 24 V 100 Ah string |
| Parallel | Stays the same | Adds | Two identical 24 V 100 Ah strings form a 24 V 200 Ah bank |
Average load is 500 W, required runtime is 4 hours, inverter efficiency is 90%, usable capacity is 80%, and the allowance is 15%. The chosen units are 12 V 100 Ah batteries. What is required?
Answer: The calculation needs about 3.19 kWh nominal, or roughly 133 Ah at 24 V. Whole-battery rounding requires two 12 V batteries in series and two parallel strings, for four batteries in total. The installed bank is 24 V 200 Ah, or 4.8 kWh nominal.
Explanation: A fractional parallel string cannot be purchased, and every series string must contain the full number of matching batteries required for system voltage.
Treat 12 V, 24 V and 48 V as nominal system classes
Select the nominal class required by the inverter DC input. A lithium module sold for a 48 V system may be labelled 51.2 V, while a nominal 12 V LiFePO₄ battery may be labelled 12.8 V. When one module is designed to match the complete inverter voltage, use the matching-system-module option rather than trying to divide it into 12 V batteries.
Usable capacity must come from the actual battery
AGM, gel, flooded lead-acid and LiFePO₄ are useful categories, but they do not provide one universal depth-of-discharge setting. Product design, temperature, discharge rate, BMS limits, desired cycle life and warranty conditions all affect the usable state-of-charge window. Keep the field editable and verify the selected battery data sheet.
Do not increase the usable-capacity percentage merely to make the recommended bank smaller. The setting assumes a deeper discharge; it does not create more stored energy.
Battery Ah and discharge-current capability are different limits
A bank can contain enough kWh yet still be unable to support a high-power inverter. The optional current check compares the estimated maximum DC current with the combined continuous rating of parallel batteries or BMS units. Confirm short-duration surge capability separately where motors, pumps or compressors are involved.
The calculator is designed for US, UK, Canadian and Australian users
Electrical capacity uses the same W, Wh, kWh, V, A and Ah units in all four markets. The wording includes both utility and mains backup, fibre and fiber equipment, and RV and caravan use cases. Optional prices can be entered in USD, GBP, CAD or AUD without relying on a country-specific electricity tariff or product database.
How to read the main results
Calculated requirement versus installed bank
| Result | Meaning |
|---|---|
| Minimum Ah | Theoretical capacity required at the selected system voltage |
| Minimum kWh | Nominal stored energy before whole-battery rounding |
| Batteries in series | Units required to reach the nominal DC voltage |
| Parallel strings | Complete series groups required to reach the Ah target |
| Installed capacity | Actual bank after rounding to whole batteries or modules |
| Estimated runtime | Runtime using average load, allowance, efficiency and usable capacity |
High-current DC systems can overheat and cause fire. Cable or conductor size and length, fuses, breakers or disconnects, busbars, terminal torque, ventilation and battery protection must suit the actual equipment and local rules. Fixed installations should be checked by an appropriately qualified or licensed professional in the relevant country or jurisdiction.