About the Off-Grid Battery Sizing
This calculator sizes the battery bank needed for an off-grid solar system, based on daily energy use, how many days of backup power (autonomy) you want, and the usable depth of discharge for your battery chemistry. It's designed for early-stage off-grid system planning, translating a household's energy needs directly into a battery capacity and amp-hour figure.
How It Works
You enter your average daily energy use in kilowatt-hours, how many days you want the system to run without sun (days of autonomy), your battery's usable depth of discharge percentage, and the system's DC voltage. The calculator multiplies daily use by autonomy days to get total energy needed, divides by depth of discharge to find the actual battery capacity required since batteries shouldn't be fully drained, then converts that capacity to amp-hours at your chosen system voltage.
Formula & Methodology
With the defaults of 8 kWh daily use, 2 days of autonomy, an 80% usable depth of discharge, and a 48V system, required energy is 8 x 2 = 16 kWh. Dividing by 0.80 gives a battery bank sized at 20 kWh, since only 80% of a lithium battery's rated capacity is meant to be used regularly. At 48V, that's (20 x 1000) / 48, or about 417 amp-hours.
Examples
Lithium System, Default Household Load
An 8 kWh/day load with 2 days of autonomy, an 80% depth of discharge, and a 48V system needs a 20 kWh usable battery bank, about 417 Ah.
Lead-Acid System, More Autonomy
A 5 kWh/day load with 3 days of autonomy, a 50% depth of discharge for lead-acid, and a 24V system needs a 30 kWh usable battery bank, about 1,250 Ah, showing how a lower depth of discharge and lower voltage both push amp-hour requirements up.
Advantages
- Converts everyday energy use directly into the two figures battery suppliers actually quote: usable kWh capacity and amp-hours at a given voltage.
- Builds the depth-of-discharge derating into the required capacity automatically, rather than leaving it as a manual adjustment the user has to remember.
- Lets you compare how battery chemistry choice and system voltage both change the physical size and amp-hour rating of the bank needed for the same load.
Common Mistakes
- Sizing a battery bank to exactly match daily energy use without adding any days of autonomy, leaving no buffer for cloudy stretches when solar production drops.
- Applying a lithium-level depth of discharge assumption of 80-90% to a lead-acid battery bank, which shortens the battery's usable lifespan since lead-acid typically shouldn't be routinely drained past about 50%.
- Sizing only the battery bank while forgetting that the solar array and charge controller also need to be sized to fully recharge that capacity within the available daily sunlight.
Edge Cases to Watch For
- Depth of discharge varies substantially by battery chemistry: lithium batteries typically allow 80-90% while lead-acid is usually limited to around 50% to preserve lifespan, so the same daily load and autonomy target produces a much larger required battery bank with lead-acid.
- If depth of discharge or system voltage is entered as zero, the calculator returns zero for usable capacity or amp-hours rather than dividing by zero, since both values sit in the denominator of the formula.
- This sizes the battery bank only; it does not size the solar array or charge controller needed to actually refill that capacity within your local sunlight hours.
Common Use Cases
- Off-grid homeowners or cabin owners planning a solar-plus-battery system from scratch.
- RV or van-life setups sizing a battery bank against daily appliance and device loads.
- Solar installers or DIYers comparing how battery chemistry and target autonomy affect required bank size before requesting equipment quotes.