Solar battery planning
Solar Battery Bank Size Calculator
Estimate the stored-energy capacity a solar battery bank needs from your daily load, autonomy target and editable battery planning assumptions. The result shows kWh first, with voltage-dependent Ah equivalents.
Size a solar battery bank
How to size a solar battery bank
Start with the energy your loads use each day, then choose how many days the battery should cover without meaningful solar or other charging input. This calculator intentionally does not subtract daytime solar production; solar generation is a separate calculation.
Solar battery capacity formula
The model calculates load energy as daily load multiplied by autonomy days. It then accounts for inverter efficiency, one usable SOC window from a fully charged starting point to the minimum SOC, battery-health derating and a design margin:
Recommended kWh = daily load × autonomy ÷ (inverter efficiency × usable SOC window × battery health) × (1 + design margin)
Daily load and autonomy
Daily load energy means the energy delivered to your AC loads or appliances. Inverter losses are applied separately. Autonomy means stored-energy coverage without assuming meaningful recharge during that period, so the sizing model does not use peak sun hours, weather or PVWatts.
Battery kWh and Ah
Kilowatt-hours are the primary capacity result. Amp-hours depend on system voltage, using Ah = kWh × 1,000 ÷ voltage. The 12 V, 24 V and 48 V rows are reference equivalents for the same energy capacity, not recommended system voltages.
Battery chemistry and health assumptions
Chemistry initializes planning assumptions such as minimum SOC. If you edit or save that SOC assumption, changing chemistry does not silently overwrite it. Battery health is a planning derating for available capacity; this calculator does not predict battery aging, cycle life or degradation.
Worked solar battery sizing example
With 5 kWh/day, 1 day of autonomy, a 100% starting SOC, 20% minimum SOC, 90% inverter efficiency, 100% battery health and a 10% design margin, the minimum nominal capacity is about 6.94 kWh and the recommended capacity is about 7.64 kWh. At 48 V, that is approximately 159.1 Ah.
Capacity check only
This calculator estimates stored-energy capacity in kWh and Ah. It does not verify inverter power, surge demand, battery discharge-current limits, BMS limits, C-rate, wiring or installation compatibility. A 7.64 kWh result does not mean every 7.64 kWh battery bank can safely run your load.
Compare with solar production
Use the Solar Panel Output Calculator to model production from its own location, system size, geometry and PVWatts inputs. Daily load can provide usage context, but load, battery capacity and autonomy do not determine solar production.
Off-Grid Solar Battery Bank Sizing Guide
Recommended nominal battery bank capacity (kWh and 48V Ah) based on daily household electrical demand and days of autonomy without sun:
| Daily Household Energy | 1 Day Autonomy | 2 Days Autonomy | 3 Days Autonomy (Cloud Buffer) |
|---|---|---|---|
| 2.5 kWh / day (Small Off-Grid Cabin / RV) | ~3.8 kWh (79 Ah @ 48V) | ~7.6 kWh (158 Ah @ 48V) | ~11.5 kWh (239 Ah @ 48V) |
| 5.0 kWh / day (Energy-Efficient Off-Grid Home) | ~7.6 kWh (159 Ah @ 48V) | ~15.3 kWh (318 Ah @ 48V) | ~22.9 kWh (477 Ah @ 48V) |
| 10.0 kWh / day (Standard Off-Grid Family Home) | ~15.3 kWh (318 Ah @ 48V) | ~30.6 kWh (636 Ah @ 48V) | ~45.8 kWh (955 Ah @ 48V) |
| 20.0 kWh / day (Large Home + Well Pump + Heat Pump) | ~30.6 kWh (636 Ah @ 48V) | ~61.1 kWh (1,273 Ah @ 48V) | ~91.7 kWh (1,910 Ah @ 48V) |
Solar Battery Bank Sizing Formulas
Calculates nominal stored-energy capacity required for off-grid autonomy during sunless periods, accounting for Depth-of-Discharge (DOD) reserves and power conversion losses.
Bank_kWh = (Daily_Load_kWh × Autonomy_Days × (1 + Margin)) / (Usable_SOC × Inverter_Eff × Battery_Health)Variable Definitions
Daily_Load_kWhLoad-Side Daily Energy(kWh/day)- Total AC/DC electricity required by your household appliances per day.
Autonomy_DaysDays of Autonomy(days)- Continuous days of battery support required without meaningful solar recharge.
Usable_SOCUsable DOD Window(fraction)- Nominal minus minimum reserve SOC (e.g. 80% usable for LiFePO4, 50% for Lead-Acid).
Inverter_EffInverter Efficiency (η)(fraction)- AC inverter DC-to-AC conversion efficiency (typically 88%–93%).
MarginDesign Margin(fraction)- Planning safety buffer (typically 10%–15%).
Engineering Notes & Standards
- Amp-Hour equivalent at nominal voltage V: Ah = (Bank_kWh × 1,000) / V.
- 48V battery systems require 1/4 the current (amperage) of 12V systems for the same power, significantly reducing wire gauge and resistive heat losses.
Frequently Asked Questions (FAQ)
How many batteries do I need for an off-grid solar system?
Should I build a 12V, 24V, or 48V solar battery bank?
How many days of autonomy should I design for?
Why should you avoid mixing old and new batteries in a bank?
Related calculators
Build appliance usage with the Electricity Usage Calculator, then compare solar production using the Solar Panel Output Calculator. See the methodology and sources for broader guidance.