Solar Sizing
Battery Bank Capacity Calculator
Size a battery bank in kWh, Ah and unit count
Updated 12 July 2026 · Live
What this tool does
Sizes a battery bank from your daily energy use, the days of autonomy you want and the usable depth of discharge, returning total kWh, amp-hours and battery count.
Enter daily energy use, days of autonomy, depth of discharge, system voltage and battery amp-hours; the calculator returns the total bank size and how many batteries it needs.
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Formula Used
How the battery bank capacity works
The usable energy target is your daily use times the days of autonomy. Dividing that by the depth of discharge grosses it up to the total capacity the bank must hold, because you never fully flatten a battery. Converting kWh to amp-hours at the system voltage, then dividing by one battery’s amp-hour rating and rounding up, gives the number of batteries.
What you enter
Enter daily energy use, days of autonomy, depth of discharge, system voltage and battery amp-hours; the calculator returns the total bank size and how many batteries it needs. Every field is editable, and the result recalculates the moment you change a value.
Reading the result
The total kWh is the headline; the amp-hour figure and unit count tell you the physical bank. Deeper discharge shrinks the bank but shortens battery life, so the depth-of-discharge input is the main lever.
Safe by design
This calculator does the arithmetic of sizing — quantities, geometry and cost. It deliberately stops at the line a certified installer must sign off. It will never tell you a cable or fuse size, an earthing arrangement, a battery charge voltage, or a roof structural load: those are regulated calculations where a wrong number causes fire, electrocution or collapse. All electrical work must be carried out by a qualified, certified installer, and this page is not an installation guide.
Assumptions and limits
This sizes capacity only. It does not set charge voltages, balance currents or BMS parameters — charging lithium outside its specification is a fire risk and belongs to the battery manufacturer’s data sheet and a certified installer. Cold temperatures also cut usable capacity below the rating.
Using it with the rest of BuildMetricLab
This calculator is one step in planning a system. Pair it with the other solar sizing, geometry and cost tools to go from an energy figure to a panel count, a layout and a costed bill of materials. Every BuildMetricLab tool runs entirely in your browser — no sign-up, nothing sent to a server, and the formula is shown on the page so the maths can be audited.
Sources & methodology
Sizes a battery bank from your daily energy use, the days of autonomy you want and the usable depth of discharge, returning total kWh, amp-hours and battery count. Every result is calculated from the values you enter, all inputs are editable, and no time-sensitive prices or tariffs are embedded.
Frequently asked questions
What depth of discharge should I use?
Lithium (LFP) banks are commonly rated to 80–90% usable; lead-acid is usually held to about 50% to protect its life. Use the figure from your battery’s data sheet.
How many days of autonomy do I need?
Two to three days covers a typical cloudy spell for a grid-backed system; remote off-grid sites often plan for more. Every extra day multiplies the bank size.
Why divide by depth of discharge?
Because only part of a battery’s nameplate capacity is usable. Dividing the energy you need by the usable fraction grosses it up to the nominal capacity to buy.
Does this tell me the charge voltage?
No, and deliberately so. Charge voltages and BMS settings come from the manufacturer and a certified installer — getting them wrong is a fire hazard.
Spotted something off?
Calculations or display — let us know.