Solar Battery Size Calculator

Size a solar battery bank in kWh and amp-hours from daily energy use, days of autonomy, depth of discharge, inverter efficiency and battery temperature.

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Results will appear here once you enter your values.

The battery sizing formula

Nominal capacity (kWh) = daily AC energy × days of autonomy ÷ inverter efficiency ÷ depth of discharge × temperature factor.

Amp-hours = kWh × 1,000 ÷ system voltage.

Example: backing up 10 kWh a day for one day with a 95% efficient inverter and lithium iron phosphate cycled to 90% depth of discharge needs 10 ÷ 0.95 ÷ 0.90 = 11.7 kWh nominal, or 244 Ah at 48 V, which is three 5 kWh batteries.

This is the standard stand-alone PV design method taught in Sandia National Laboratories' recommended design practices and used by most off-grid system designers.

Depth of discharge and battery life

Depth of discharge (DoD) is how much of the rated capacity you draw before recharging.

Lithium iron phosphate tolerates 80-100% DoD for thousands of cycles, so 90% is a common design value.

Lead-acid batteries last far longer when kept shallow: a flooded or AGM bank cycled to 50% DoD may deliver two to three times the cycles of one routinely drained to 80%.

That is why the same daily load needs roughly twice the nominal lead-acid capacity.

Cold weather and lead-acid

Lead-acid capacity falls as electrolyte temperature drops.

The design multipliers used here come from the stand-alone PV battery temperature correction table: 1.04 at 70°F, 1.19 at 50°F, 1.30 at 40°F and 1.59 at 20°F.

A lead-acid bank in an unheated shed in a northern winter therefore needs about 30-60% more capacity.

Lithium batteries hold capacity better in the cold but most must not be charged below 32°F (0°C) unless they have built-in heaters, so plan an insulated or conditioned location either way.

How many days of autonomy?

For grid-tied homes that only need outage backup, one day of the essential loads is usually enough, because solar recharges the bank the next sunny day and most outages are short.

Off-grid cabins and homes typically design for 2-3 days in sunny climates and 3-5 days where winter brings long cloudy spells, often with a generator to cover longer gaps.

Every extra day adds a full day's worth of batteries, so trimming the daily load is usually cheaper than adding autonomy.

Energy is only half the sizing

A battery's kWh rating tells you how long it lasts; its continuous and peak kW ratings tell you what it can run.

A well pump or central air conditioner can require 5-10 kW for a few seconds at start-up, and some battery systems trip on that surge even when they hold plenty of energy.

Check the battery's continuous discharge rating and the inverter's surge rating against your largest loads, and pick a higher system voltage (48 V) for banks above a few kWh to keep currents and cable sizes reasonable.

Frequently Asked Questions

Common questions about the Solar Battery Size Calculator

For one day of backup with lithium iron phosphate at 90% depth of discharge and a 95% inverter, about 11.7 kWh of nominal capacity, such as three 5 kWh batteries or one 13.5 kWh unit. With lead-acid at 50% DoD you would need about 21 kWh nominal.

Sources & References

Off-Grid or Stand-Alone Renewable Energy Systems

Battery storage, days of autonomy and system design for stand-alone solar.
→Source: U.S. Department of Energy, Energy Saver

Solar Integration: Solar Energy and Storage Basics

Battery capacity, power rating, depth of discharge and round-trip efficiency explained.
→Source: U.S. Department of Energy, Solar Energy Technologies Office

Stand-Alone Photovoltaic Systems: A Handbook of Recommended Design Practices (SAND87-7023)

Origin of the autonomy / depth-of-discharge sizing method and the lead-acid battery temperature correction multipliers.
→Source: Sandia National Laboratories

PVWatts Version 5 Manual (NREL/TP-6A20-62641)

Documents the loss categories (soiling, shading, mismatch, wiring, connections, light-induced degradation, nameplate, availability) behind the 14% default.
→Source: NREL

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