Free energy calculator
Solar Battery Size Calculator
Size a nominal battery bank for a daily energy target and desired days without charging.
Calculate your estimate
Change the example values to match your system.
Estimated result
Ideal—
Typical—
Conservative—
Enter valid inputs and calculate to see results.
These are estimates, not guaranteed performance.
Formula and method
Required Ah = daily Wh × autonomy days ÷ (voltage × DoD × efficiency).
Percent fields are entered as percentages; the calculation converts them to decimal fractions.
Displayed results are rounded to two decimal places.
What the inputs mean
- Daily energy use (Wh/day)
- Enter daily energy use in Wh/day.
- Autonomy (days)
- Enter autonomy in days.
- Battery voltage (V)
- Enter battery voltage in V.
- Depth of discharge (%)
- Enter depth of discharge in %.
- System efficiency (%)
- Enter system efficiency in %.
- Conservative retained fraction (%)
- Enter conservative retained fraction in %.
Worked example
For 1,200 Wh/day and two days at 24 V, 80% DoD, and 90% efficiency, the typical bank is about 138.89 Ah.
Why the real result may differ
- Temperature, battery age, wiring resistance, and changing loads can move actual results away from the estimate.
- Manufacturer ratings may be measured under different test conditions. Measure the load when practical.
Common mistakes
- Do not confuse watts (power) with watt-hours (energy).
- Do not enter a percentage as a whole number: 90% is entered as 90 in this form and converted internally.
Frequently asked questions
Is this result guaranteed?
No. It is an engineering estimate; equipment, temperature, age, and duty cycle affect real performance.
Why are three estimates shown?
The ideal case ignores practical losses, while typical and conservative cases apply the efficiency and margin entered above.
Related tools
References
Last reviewed: August 12, 2026