Estimated battery capacity
125 Ah
Uses an 80% combined usable-energy allowanceHow this calculator helps
Size a starting battery bank from the appliances that must remain on and the desired backup duration. The calculation includes a conservative usable-energy allowance before converting required watt-hours into amp-hours.
How to use it
- 1
Add the running watts of appliances that must operate together.
- 2
Choose realistic backup hours and the voltage of the planned battery bank.
- 3
Confirm usable depth of discharge, current limits and compatible capacity with the manufacturer or installer.
Formula and methodology
Required nominal watt-hours = load watts × backup hours ÷ usable-energy factor. Required amp-hours = nominal watt-hours ÷ system voltage.
Worked calculation example
A 600 W essential load for four hours requires 2,400 usable Wh. At 80% usable energy on a 24 V bank, the estimate is about 125 Ah.
What can affect the result?
Battery chemistry
Lithium and lead-acid systems allow different usable proportions and charge cycles.
Surge demand
Motors and compressors may require inverter power beyond the running load.
Age and temperature
Available capacity can fall with battery age, heat and repeated deep discharge.
Limits of this estimate
This energy estimate uses a fixed 80% usable allowance and does not design battery strings, protection, cables, charging or inverter surge capacity.
Frequently asked questions
Is the solar battery size calculator free?＋
Yes. SolvePilot tools are free, work in your browser, and require no account.
Does SolvePilot store my values?＋
No. Calculations run locally in your browser and the values you enter are not sent to a server.
Should I treat the result as an exact bill or quotation?＋
No. Results are informed estimates. Final charges can vary because of taxes, tariffs, lender terms, usage patterns, or provider rules.
How can I improve the accuracy?＋
Use recent values from your own bill, statement or institution, review every assumption, and confirm important results with the relevant official source.
Can lead-acid and lithium use the same allowance?＋
No. Safe depth of discharge and efficiency differ, so confirm the factor with the battery manufacturer.