Solar Battery Size Calculator

A solar battery size calculation starts with the energy that must remain available during an outage, not with an inverter wattage label or a battery’s advertised amp-hour rating. The required result is the Required battery capacity: the nominal stored energy needed to support the selected daily load for the selected backup period after accounting for usable depth of discharge, round-trip efficiency, and reserve.

The calculator produces the battery-bank requirement in three equivalent forms:

• Required battery capacity in kWh for storage-system planning
• Required battery capacity in Wh for equipment and energy calculations
• Required battery capacity in Ah at the selected Battery system voltage

That capacity target is used during an early solar-plus-storage design to review candidate battery banks, inverter-charger capacity, DC feeder loading, disconnects, overcurrent protection, and the physical arrangement of battery equipment. It does not select a listed energy-storage system or establish conductor ampacity, fault-current ratings, or code compliance.

Backup Load Energy

Daily energy use is the amount of energy the battery must supply in one day, entered in kWh/day. It should represent the actual loads intended to remain energized during backup operation, rather than the total utility consumption of a building unless the entire building is intended to be backed up.

The calculator multiplies this value by the Autonomy period:

\(\text{Autonomy load energy (kWh)} = \text{Daily energy use (kWh/day)} \times \text{Autonomy period (days)}\)

A two-day autonomy period means the battery is sized to provide two days of the entered daily energy use before considering depth of discharge, conversion losses, or reserve.

Battery sizing is energy-based. A battery bank may have enough stored kWh for the expected outage duration but still be unsuitable if it cannot deliver the required instantaneous current to a large motor, resistance heater, pump, compressor, or inverter surge load. Those loads require separate review of inverter output and battery discharge capability.

Usable Capacity Adjustment

A battery’s nameplate energy is not generally treated as fully available usable energy. The calculator applies two inputs to determine the Usable factor:

• Usable depth of discharge is the percentage of nominal stored battery energy intended for use.
• Round-trip efficiency accounts for battery and conversion losses in the stored-energy estimate.

\(\text{Usable factor} = \left(\frac{\text{Usable depth of discharge}}{100}\right) \times \left(\frac{\text{Round-trip efficiency}}{100}\right)\)

The Base battery capacity is then calculated as:

\(\text{Base battery capacity (kWh)} = \frac{\text{Autonomy load energy (kWh)}}{\text{Usable factor}}\)

This converts the energy needed by the loads into the nominal battery capacity required before reserve is added.

For example, an 80% usable depth of discharge and 90% round-trip efficiency produce a usable factor of:

\(0.80 \times 0.90 = 0.72\)

In practical terms, every 1 kWh required by the backup load requires approximately (1 \div 0.72 = 1.3889) kWh of nominal battery capacity before the reserve adder.

Reserve Capacity

The Reserve adder is applied after the usable-capacity adjustment. It provides an additional percentage of nominal capacity above the calculated base requirement.

\(\text{Required battery capacity (kWh)} = \text{Base battery capacity (kWh)} \times \left(1+\frac{\text{Reserve adder}}{100}\right)\)

The same final capacity is expressed in watt-hours:

\(\text{Required battery capacity (Wh)} = \text{Required battery capacity (kWh)} \times 1000\)

The amp-hour result uses the entered Battery system voltage:

\(\text{Required battery capacity (Ah)} = \frac{\text{Required battery capacity (Wh)}}{\text{Battery system voltage (V)}}\)

Amp-hours are therefore voltage-dependent. A 48 V battery system requires fewer Ah than a lower-voltage battery system for the same stored energy. The kWh result remains the primary comparison value when evaluating storage capacity across different nominal system voltages.

Calculation Example

Using the entered values:

Input	Value
Daily energy use	12 kWh/day
Autonomy period	2 days
Battery system voltage	48 V
Usable depth of discharge	80%
Round-trip efficiency	90%
Reserve adder	10%

The calculation proceeds as follows:

\(\text{Autonomy load energy} = 12 \text{ kWh/day} \times 2 \text{ days} = 24 \text{ kWh}\)

\(\text{Usable factor} = 0.80 \times 0.90 = 0.72\)

\(\text{Base battery capacity} = \frac{24 \text{ kWh}}{0.72} = 33.3333 \text{ kWh}\)

\(\text{Required battery capacity} = 33.3333 \text{ kWh} \times 1.10 = 36.6667 \text{ kWh}\)

\(36.6667 \text{ kWh} = 36{,}666.6667 \text{ Wh}\)

\(\frac{36{,}666.6667 \text{ Wh}}{48 \text{ V}} = 763.8889 \text{ Ah}\)

The resulting planning target is 36.6667 kWh, or 36,666.6667 Wh, equivalent to 763.8889 Ah at 48 V.

DC Equipment and Field Verification

The Required battery capacity identifies stored energy; it does not determine all electrical ratings for the installation. Final equipment selection requires the actual listed battery system, inverter or inverter-charger, battery management system, and manufacturer installation instructions.

Separate field and design checks include:

• Maximum battery discharge and charge current at the actual system voltage
• DC feeder and branch-circuit conductor ampacity, AWG or kcmil selection, terminal ratings, insulation temperature rating, and applicable adjustment or correction factors
• DC voltage-drop performance between batteries, combiner equipment, inverter, and distribution equipment
• Overcurrent protection, disconnecting means, available fault current, and equipment short-circuit ratings
• Inverter continuous output, surge capability, and the starting characteristics of motor loads
• Battery cabinet, rack, raceway, and conductor routing clearances, including physical layout and access requirements
• AHJ requirements, adopted electrical code, and manufacturer limitations for the listed energy-storage system

A 48 V nominal value is used here only to convert watt-hours into amp-hours. Actual battery voltage can vary with state of charge, charging conditions, and operating load. Use the listed equipment’s electrical ratings—not the nominal Ah conversion alone—when sizing DC conductors, overcurrent devices, terminals, and related distribution equipment.