Battery Sizing Calculator

How the Battery Sizing Calculator works

Battery sizing calculator: usable energy and depth of discharge

A battery's headline capacity is not the energy you can use. Two corrections stand between the number on the box and the number that matters.

Depth of discharge. Batteries last longer when they are not fully drained. Lithium iron phosphate, the chemistry in almost every current home battery, is typically rated for 90 to 100 percent usable capacity and manufacturers quote the usable figure directly. Lead-acid, still common in cheap off-grid installations, should not be taken below 50 percent routinely and wears out quickly if it is. The calculator divides by the usable fraction, so a 10 kWh requirement becomes 11.1 kWh at 90 percent depth of discharge and 20 kWh at 50 percent.

Round-trip efficiency. Energy is lost charging the battery and lost again discharging it. A good lithium system with its own inverter returns 85 to 92 percent of what goes in; lead-acid returns 75 to 85. Because you have to put in more than you get out, the array or grid charging the battery needs to supply the gross figure, and the calculator applies the efficiency to the capacity so that the usable energy after losses still meets your target.

Days of autonomy. A grid-connected home battery needs to cover one evening and night, so one day (or less: many people size for the 6 pm to 6 am window only) is right, and the grid fills in when the battery is flat. An off-grid system has no such backup and is usually sized for two to three days of cloudy weather, sometimes more in winter climates, with a generator for the rest. Each extra day multiplies the capacity, which is why off-grid batteries are large and why a small generator is often cheaper than the last day of autonomy.

Amp-hours and voltage. Battery banks assembled from individual cells or lead-acid blocks are specified in amp-hours at a nominal voltage: 12, 24 or 48 V for most solar systems, with 48 V standard for anything above a few kilowatt-hours because the current, and therefore the cable size and losses, is a quarter of what it would be at 12 V. Kilowatt-hours equal amp-hours times volts divided by 1000. Packaged home batteries are sold in kWh and the amp-hour figure is irrelevant to them.

Matching to solar. A battery only helps if there is surplus energy to fill it. Enter the average surplus power available in the middle of the day and the calculator shows roughly how long a full recharge takes; if that is longer than the hours of good sun you get, either the array is too small for the battery or the battery is bigger than it needs to be. The solar sizing calculator sizes the array, and the solar payback calculator checks whether the battery's cost is recovered by the grid electricity it displaces.

Battery size by daily use

One day of autonomy, 90% depth of discharge, 90% round-trip efficiency, a 48 V bank and 5 kWh modules.

Nominal capacity, amp-hours and modules by daily energy use

Battery size formula and worked examples

The formula: nominal capacity (kWh) = daily energy × days of autonomy ÷ (depth of discharge × round-trip efficiency). Amp-hours = kWh × 1,000 ÷ system voltage, and modules = capacity ÷ module size, rounded up.

First example. Covering 8 kWh of evening use for one day with a lithium battery at 90% depth of discharge and 95% efficiency needs 8 ÷ (0.9 × 0.95) = 9.4 kWh nominal.

Second example. An off-grid cabin using 5 kWh a day for 3 days on lead-acid at 50% depth and 85% efficiency needs 15 ÷ 0.425 = 35.3 kWh — 735 Ah at 48 V.

How many batteries do I need? Divide the nominal capacity by one module's capacity and round up: 9.4 kWh with 5 kWh modules is two.

Related tools