Estimate required battery bank watt-hours and amp-hours for backup systems, solar storage, and off-grid power planning.
This result is an estimate for required battery-bank size. Final sizing should consider battery chemistry, temperature, discharge rate, autonomy margin, aging allowance, and charging strategy.
A battery bank sizing calculator helps you work backwards from the required backup load and runtime to the amount of stored energy the system must provide. This is a common design step for home backup systems, solar storage banks, telecom cabinets, off-grid systems, UPS planning, and emergency power design. Instead of guessing ampere-hours directly, good sizing starts with energy in watt-hours and then converts that requirement to the needed battery-bank ampere-hour rating at the selected DC voltage.
This calculator includes efficiency and usable battery capacity so the result is more realistic than a simple load-power times hours estimate. That matters because inverter losses, wiring losses, and battery depth-of- discharge limits all affect the amount of installed storage needed.
It is especially useful when comparing 12 V, 24 V, and 48 V systems or when deciding whether a proposed battery bank can support a given load for the required number of hours without being over-discharged.
| Variable | Meaning | Typical Unit |
|---|---|---|
| Eload | Energy the load must receive during the backup period | Wh |
| Estored | Total battery energy required after losses and discharge limits | Wh |
| Ah | Required battery-bank capacity | Ah |
| Efficiency | Fraction of stored battery energy delivered to the load | % |
| Usable capacity | Allowed battery discharge fraction | % |
Enter load power in watts, backup time in hours, battery-bank voltage in volts, and percentages as whole percentages. The result is shown as required battery watt-hours and required ampere-hours at the chosen system voltage.
This is useful because battery-bank voltage strongly affects ampere-hour size. A higher-voltage system can provide the same stored energy with fewer ampere-hours than a lower-voltage system.
Suppose a load needs 600 W for 8 h on a 48 V battery bank, with 85% efficiency and 80% usable capacity.
Eload = 600 x 8 = 4800 Wh
Estored = 4800 / (0.85 x 0.80) = 7058.82 Wh
Ah = 7058.82 / 48 = 147.06 Ah
A practical bank would be sized above 147 Ah at 48 V.
Suppose a 24 V system must support 250 W for 12 h, with 90% efficiency and 70% usable capacity.
Eload = 250 x 12 = 3000 Wh
Estored = 3000 / (0.90 x 0.70) = 4761.90 Wh
Ah = 4761.90 / 24 = 198.41 Ah
This shows how lower-voltage systems often require a larger ampere-hour bank for the same energy target.
Real battery-bank sizing usually includes more margin than the bare energy calculation. Designers often add allowance for future load growth, battery aging, low-temperature performance, charge reserve, surge load, and maintenance strategy. Lead-acid systems and lithium systems also behave differently under discharge, so final storage selection should always be checked against the battery data sheet and the intended operating profile.
This calculator is best used as a fast sizing estimate before choosing the number of batteries in series and parallel, battery chemistry, charging source, and protection hardware.