Battery Size Calculator

Work out the battery capacity (in amp-hours) and number of batteries you need to power a daily energy load for a set number of backup days, based on system voltage, depth of discharge, and efficiency losses.

Quick Facts

Formula
Ah = (Wh/day × days) ÷ (V × DoD × efficiency)
Depth of discharge and efficiency losses both raise the required capacity above the raw energy demand.
Typical DoD
Lithium ~80-100%, lead-acid ~50%
Use the depth of discharge your battery manufacturer specifies for its rated cycle life.

Your Results

Calculated
Required battery capacity
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Amp-hours at your system voltage
Required energy storage
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Total watt-hours the bank must hold
Batteries needed
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Rounded up to whole units
Installed capacity
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Actual Ah once rounded up, with margin

Ready

Enter your daily energy use and backup requirements, then press Calculate.

Understanding Battery Sizing

This tool sizes a battery bank using the standard energy-based method used for off-grid solar, RV/marine, and backup-power systems: figure out how much energy you need to store, then account for the fact that you can't use 100% of a battery's rated capacity and that some energy is lost along the way.

The formula

Required battery capacity, in amp-hours, is:

Ah = (Daily energy use in Wh × Days of autonomy) ÷ (System voltage × Depth of discharge × System efficiency)

  • Daily energy use (Wh/day): the total watt-hours your loads draw in a day. Add up each device's watts times hours used, or read it off a utility bill or energy monitor.
  • Days of autonomy: how many days the bank must run the load with no recharge — 1 day is typical for a grid-tied backup, more for off-grid systems that must ride through cloudy weather.
  • System voltage: the nominal voltage of the battery bank — commonly 12V, 24V, or 48V.
  • Depth of discharge (DoD): the fraction of rated capacity you plan to use before recharging. Lithium (LiFePO4) batteries handle roughly 80-100% DoD; lead-acid batteries are usually limited to around 50% to preserve cycle life.
  • System efficiency: combined losses from the inverter, charge controller, and wiring — typically 80-90%.

Once the required amp-hours are known, the calculator divides by the capacity of a single battery and rounds up to a whole number of batteries, since a bank has to be built from complete units. The installed capacity — number of batteries times per-unit capacity — is always at or above the calculated requirement, and the gap between the two is your safety margin.

Worked example

A 12V system with 2000 Wh/day of load, 1 day of autonomy, 80% depth of discharge, and 85% efficiency needs: Ah = (2000 × 1) ÷ (12 × 0.80 × 0.85) ≈ 245 Ah. With 100 Ah batteries, that rounds up to 3 batteries (300 Ah installed), leaving roughly a 22% reserve margin above the calculated requirement.

Practical context

This formula assumes a resistive-style energy draw and does not model temperature effects, battery aging, or peak-current (surge) demands — all of which can reduce a real battery's usable capacity below its rated number. For safety-critical or off-grid installations, add margin beyond the calculated minimum and consult the battery manufacturer's datasheet.

Frequently Asked Questions

How is required battery capacity calculated?
Required capacity in amp-hours equals your daily energy use in watt-hours times days of autonomy, divided by system voltage times depth of discharge times system efficiency: Ah = (Wh per day × days) ÷ (V × DoD × efficiency). This accounts for the fact that you cannot draw a battery down to 0% and that inverters and wiring lose some energy along the way.
What depth of discharge (DoD) should I use?
Depth of discharge is how much of a battery's rated capacity you plan to use before recharging. Lithium (LiFePO4) batteries typically tolerate 80-100% DoD without significant lifespan loss, while flooded or AGM lead-acid batteries are usually limited to 50% DoD to avoid shortening their cycle life. Use the figure your battery manufacturer specifies.
Why does the calculator round up to whole batteries?
Batteries are sold in fixed capacities, so the calculator divides your required amp-hours by the capacity of a single battery and rounds up to the next whole unit. Rounding up guarantees the installed bank meets or exceeds the calculated requirement rather than falling just short of it.
What system efficiency should I assume?
System efficiency captures losses from the inverter, charge controller, and wiring between the battery and your load. A well-sized system typically runs 80-90% efficient, so 85% is a reasonable default; poorly matched or long-cable-run systems can lose more.

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