Solar Battery Capacity Calculator
Affiliate disclosure: SolOffGrid may earn a commission from qualifying purchases made through links on this site, at no additional cost to you. This solar battery calculator provides an educational estimate, not an electrical design.
Use this solar battery calculator to estimate nominal battery storage for an off-grid solar system. Enter daily energy use, days of autonomy, an assumed usable-capacity fraction, system efficiency, and a reserve factor. The calculation runs in your browser; the values are not sent to SolOffGrid.
Solar Battery Capacity Calculator
Estimate nominal battery storage from your daily energy use and assumptions. This is an educational estimate, not an electrical design or installation plan.
Enter your assumptions and select Calculate to see an estimate.
The result depends on the assumptions entered. Actual usable energy, voltage range, temperature limits, inverter losses, battery-management limits, and manufacturer guidance vary by system. Confirm product specifications and consult a qualified professional for system design.
How the estimate works
The calculator uses: daily energy (Wh) × autonomy days × reserve factor ÷ (usable fraction × system efficiency). It converts the result to nominal kWh and estimates amp-hours using the selected nominal voltage. For example, 2.5 kWh per day, one day of autonomy, an 80% usable fraction, 90% system efficiency, and a 15% reserve produce about 3.99 kWh of nominal storage.
What to enter
- Daily energy: Use the energy your loads consume in a typical day. If your figure is in kWh, multiply by 1,000 to enter Wh.
- Autonomy: Choose how many days you want the battery to support the assumed loads without meaningful solar input. This is a planning assumption, not a weather forecast.
- Usable fraction: Enter a decimal such as 0.8 for 80%. Check the battery maker’s stated usable energy and operating limits; do not assume every battery supports the same depth of discharge.
- Efficiency and reserve: These are planning allowances for conversion losses and extra margin. Actual values vary by equipment and conditions, so use manufacturer or designer guidance when available.
- Nominal voltage: This affects the approximate amp-hour conversion only. It does not confirm that a battery, inverter, or system voltage is compatible.
How to interpret the result
The kWh figure is an estimated nominal battery capacity under the assumptions you entered. Amp-hours are an approximate conversion: nominal Wh divided by nominal volts. Battery listings can describe nominal, usable, or rated capacity differently, so compare the manufacturer’s specifications on a like-for-like basis and account for reserve, temperature, discharge limits, and system losses.
Important limits
This tool does not verify product compatibility, battery communications, charge limits, inverter settings, wiring, overcurrent protection, or code compliance. It also cannot predict solar production or account for every load’s starting surge. Review current product manuals and consult a qualified professional for system design and installation.