Solar Battery Bank Calculator
Battery bank size from daily use, autonomy days and depth of discharge, plus how long it keeps essentials running in a power cut.
By Syed Mubashir Ali · Last updated
How to Use Solar Battery Bank Calculator
- 1Choose the battery chemistry: Lead-Acid, AGM, Gel, LiFePO₄, NMC Li-ion, or Custom. Each carries its usable depth of discharge: 50% for flooded lead-acid up to 85% for NMC.
- 2Give your daily energy use one of three ways: type it in kWh a day (or a month, from a bill), build it from a list of appliances with their watts and hours, or enter one load in watts and the hours a day it runs.
- 3Set the autonomy you want in days: how long the bank must run with no charging.
- 4Choose the system voltage (12, 24, 36 or 48 V), the battery unit voltage (2 V cell, 6 V or 12 V) and the capacity per unit in amp-hours.
- 5Leave the system efficiency at 85% unless you know your inverter's figure, and optionally enter the essential load you'd run in a power cut (fridge, lights, Wi-Fi, a boiler or furnace fan) to see how many hours a full bank lasts.
- 6Optionally enter your panel array in kW and your peak sun hours to see whether the panels keep up with the load, how long they take to refill the bank, and the current the charge controller must handle.
- 7The results update as you type: the required bank capacity, the series and parallel arrangement, usable energy, charge current and time, the inverter size for an appliance list, and the backup time. A shared link keeps every input.
Frequently Asked Questions
What is depth of discharge and why does it change the answer so much?
It is how much of the nominal capacity you can actually use without damaging the battery. Flooded lead-acid is limited to about 50%, LiFePO₄ to around 80%. That means a lead-acid bank needs roughly 60% more nominal capacity than a lithium one for the same usable energy, which is most of why the two technologies cost less differently than the sticker price suggests.
How many days of autonomy should I design for?
Two to three days is typical for an off-grid system in a reasonable climate, more where winter cloud cover is prolonged. Each extra day multiplies the bank size and cost, so this is the input with the biggest effect on the budget.
Does it account for temperature?
No, and cold significantly reduces usable capacity: a lead-acid bank can lose a third of its capacity near freezing. It also matters for charging: most lithium chemistries must not be charged below 0°C without a heater or a BMS that prevents it. Size up for cold locations and check the chemistry's limits.
Does it consider inverter and round-trip losses?
Partly. The bank is sized with a system efficiency for inverter and wiring losses, 85% unless you change it, because your daily use is measured at the outlet but the battery supplies DC. The charge time allows for charging losses: 85% efficiency for lead-acid, AGM and gel, 95% for lithium. Cold weather and ageing are not included, so add a margin for those.
Can my solar panels recharge the battery?
Multiply the array's kW by your peak sun hours and about 80% for heat, dirt, wiring and controller losses: 3 kW at 4 sun hours puts roughly 9.6 kWh a day into the battery, or about 9.1 kWh stored after lithium's charging losses. If your loads use 5 kWh a day at the outlet, about 5.9 kWh from the battery with an 85% efficient inverter, that leaves about 3.2 kWh a day to refill the bank after cloudy days. Enter your array and sun hours to see the figures for your bank. Off grid, use your worst month's sun hours, not the yearly average.
How long will a home battery last in a power cut?
Usable energy divided by the load. A 13.5 kWh battery running 500 W of essentials (fridge, lights, Wi-Fi and a furnace fan) lasts about 23 hours after inverter losses; running a 3.5 kW central air conditioner too, about 3 hours. Enter your essential load to see the figure for your bank.
Is a bigger bank always better?
No. An oversized lead-acid bank that never reaches full charge sulphates and dies early, and every battery has a calendar life regardless of use. Size it to the demand rather than buying the biggest affordable bank.
About Solar Battery Bank Calculator
The Solar Battery Bank Calculator works out the capacity you need and how to arrange the units in series and parallel, from your daily consumption, the autonomy you want, the chemistry and the unit specification. If you don't know your daily use, build it from the appliances the bank has to run, and the calculator also gives the continuous rating the inverter needs. Add your panel array and sun hours, and it checks whether the panels can keep the bank charged and how many days they take to refill it. It runs entirely in your browser.
Chemistry does more to the answer than anything else, through depth of discharge. Flooded lead-acid gives you about half its nominal capacity; LiFePO₄ about 80%. A bank sized on nominal capacity without accounting for that is roughly 60% too small in lead-acid, which is a classic and expensive off-grid mistake.
Autonomy days is the input that drives cost. Each additional day of no-charge operation multiplies the bank, so it is worth being deliberate: two to three days suits most temperate off-grid sites, and more only where prolonged winter cloud genuinely occurs.
Inverter and wiring losses are allowed for through the system efficiency, and charging losses through the battery type. What it does not model is temperature: cold can cost a lead-acid bank a third of its capacity, and most lithium chemistries must not be charged below freezing without protection, so size up for a cold site. Battery installations also carry real fire and electrical risk and should be installed to local requirements by someone competent.