This solar battery calculator estimates nominal stored energy needed to support a chosen AC electricity load for a specified autonomy period. It accounts for an allowed depth of discharge and an aggregate inverter-plus-discharge efficiency. The main outputs are nameplate capacity in kilowatt-hours, an equivalent amp-hour capacity at a selected nominal voltage, and the energy delivered to AC loads after the modeled reductions.

Battery energy and output power answer different questions. Kilowatt-hours indicate how much energy can be supplied over time. Kilowatts indicate how quickly it can be supplied. A battery with ample stored energy may still be unable to run a large appliance because its inverter, battery management system, or discharge-current limit cannot support the required instantaneous power. Motor starting surges can exceed normal operating demand and need separate assessment.

The optional solar recharge mode adds a daily energy-balance calculation. It estimates an array that covers ongoing daily demand while replacing the usable reserve consumed during the autonomy period. This is useful for comparing backup recovery or preliminary off-grid scenarios, but it does not establish reliable off-grid operation. There is no hourly dispatch model, equipment compatibility check, or live weather data. All efficiency, sunlight, and operating-window values are assumptions you can edit rather than recommendations for a specific battery.

Depth of discharge describes the fraction of nameplate energy that is allowed to be withdrawn. An 80% setting means the model uses 0.8 of nominal energy and leaves the remainder outside the planned discharge window. It is not the same as an 80% loss. Increasing allowed depth reduces the calculated nameplate capacity, but whether that operating window is appropriate depends on the product, chemistry, temperature, warranty, and intended cycle life.

The discharge efficiency combines losses on the path from stored energy to the chosen AC load boundary. Use a documented or justified aggregate that fits your equipment and avoids double counting. If a specification provides round-trip efficiency, do not automatically enter it as both charging and discharging efficiency. Round-trip efficiency combines multiple stages; applying the same combined figure to each stage would exaggerate losses. Inverter standby consumption may also need a separate energy allowance rather than a constant efficiency.

Lithium-based products and lead-acid products do not share one universal discharge limit or charging behavior. Lead-acid capacity can vary with discharge rate, while lithium systems depend on their management and protection limits. Low temperatures, high temperatures, aging, and operating power can change usable energy. The example defaults are not chemistry recommendations. If a manufacturer already quotes usable AC-delivered capacity, first reconcile that rating with these inputs so that depth and conversion reductions are not applied twice.

For an ideal nominal-energy conversion, watt-hours equal nominal voltage multiplied by amp-hours. The calculator therefore converts nameplate kWh to Ah by multiplying by one thousand and dividing by nominal bank voltage. The 13.889 kWh example at 48 V gives approximately 289.352 Ah. At 24 V, the same nominal energy corresponds to approximately 578.704 Ah. The energy requirement has not changed; only its nominal voltage-based expression has.

Nominal voltage is a reference label, not a statement that battery voltage remains constant throughout discharge. Actual voltage depends on chemistry, state of charge, temperature, and operating conditions. Manufacturer energy ratings can therefore be more useful than reconstructing energy from a rounded nominal voltage and an amp-hour label. Compare ratings on the same basis, including the stated test conditions and whether capacity is nominal, usable, DC-side, or AC-delivered.

The amp-hour output does not specify a wiring arrangement. It cannot tell you how many modules may safely be placed in series or parallel, which batteries are compatible, or what protection and cable sizes are required. Nor does a higher selected nominal voltage automatically make a particular product suitable. This calculator intentionally provides no connection instructions. Treat the voltage input as a way to interpret capacity, then rely on approved equipment documentation and qualified design for actual system architecture.

Continue with a 5 kWh daily AC load, two autonomy days, 80% depth, and 90% discharge efficiency. Suppose the depleted usable reserve must be restored over two days, charging efficiency is 95%, peak sun hours are four per day, PV loss is 20%, and each panel is rated at 400 W. The normal daily demand remains 5 kWh. Restoring the 10 kWh delivered reserve over two days adds another 5 kWh per day on an AC-equivalent basis.

Each of those two daily components requires 5 ÷ 0.9 ÷ 0.95 = approximately 5.848 kWh of PV energy after PV losses. Their combined target is approximately 11.696 kWh per day. Dividing by four sun hours and by 0.8 gives a required array of approximately 3.655 kW DC. Dividing by 0.4 kW per panel and rounding upward produces ten panels, or 4 kW installed. Modeled daily PV output after PV losses is 4 × 4 × 0.8 = 12.8 kWh.

If the recharge period increases to four days, the ongoing-demand component stays unchanged while the reserve-recovery component halves. Reducing allowed depth of discharge instead increases required nameplate battery capacity, but does not increase the energy consumed by the same AC loads. That is why depth affects the battery size without independently multiplying recharge energy in this model. These distinctions help avoid oversizing through double counting or undersizing by forgetting continuing consumption.