Solar Battery Size Calculator

Size a solar battery in kilowatt-hours for the evening load you want to cover.

About Solar Battery Size Calculator

Nominal capacity must exceed the energy actually needed by both the efficiency and depth-of-discharge factors. Lithium batteries allow 90 per cent depth of discharge against about 50 for lead-acid. Oversizing a battery beyond one day of autonomy rarely pays in a grid-connected home, because the extra capacity cycles too rarely to earn its cost.

Formula

Solar Battery Size

Capacity = load ÷ round-trip efficiency × autonomy ÷ depth of discharge

depth of discharge: Share of nominal capacity usable without shortening life
round-trip efficiency: Losses charging and discharging combined

Worked example

With the default values below, Solar Battery Size Calculator returns Battery Capacity Required of 9.877. Change any input above and every figure here is recalculated in your browser.

Inputs used

Energy needed outside generation hoursminimum 0 kWh: 8 kWh
Usable depth of dischargeaccepts 0–100 %: 90 %
Round-trip efficiencyaccepts 0–100 %: 90 %
Days of autonomy wantedaccepts 0.5–10 days: 1 days

Results

Battery Capacity Required: 9.877 kWh
Usable Capacity: 8.889 kWh
Energy Drawn From the Battery: 8.889 kWh
Equivalent Capacity at 48 V: 205.8 Ah
Cycles per Day at This Duty: 0.900

Step by step

1Values used
2Solar Battery Size
3Battery Capacity Required= 9.877 kWh
4Usable Capacity= 8.889 kWh
5Energy Drawn From the Battery= 8.889 kWh
6Equivalent Capacity at 48 V= 205.8 Ah
7Cycles per Day at This Duty= 0.900

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Solar Battery Size calculated?

Capacity = load ÷ round-trip efficiency × autonomy ÷ depth of discharge Nominal capacity must exceed the energy actually needed by both the efficiency and depth-of-discharge factors. Lithium batteries allow 90 per cent depth of discharge against about 50 for lead-acid.

Why does Solar Battery Size matter?

Oversizing a battery beyond one day of autonomy rarely pays in a grid-connected home, because the extra capacity cycles too rarely to earn its cost.

What values do I need to enter?

This calculator takes 4 inputs: Energy needed outside generation hours, Usable depth of discharge, Round-trip efficiency, Days of autonomy wanted. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.