Solar battery planning
Solar Battery Bank Size Calculator
Estimate the stored-energy capacity your off-grid solar battery bank needs based on daily appliance consumption, cloudy-day autonomy targets, and chemistry DOD limits.
Size a solar battery bank
How to Size an Off-Grid Solar Battery Bank
- Determine Daily Appliance Load (kWh/day): Daily load energy is the energy delivered to the loads. Inverter losses are modeled separately when inverter efficiency is below 100%.
- Select Days of Autonomy: Choose how many consecutive sunless/cloudy days the battery must sustain without generator or solar recharge.
- Choose Battery Chemistry: Select modern LiFePO4 (80%–90% usable DOD) or Lead-Acid/AGM (50% usable DOD).
- Select System Voltage (12V / 24V / 48V): Review Amp-hour (Ah) requirements across voltage options to choose the right battery wiring layout.
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 (79.6 Ah @ 48V) | ~7.6 kWh (159.2 Ah @ 48V) | ~11.5 kWh (238.7 Ah @ 48V) |
| 5.0 kWh / day (Energy-Efficient Off-Grid Home) | ~7.6 kWh (159.2 Ah @ 48V) | ~15.3 kWh (318.3 Ah @ 48V) | ~22.9 kWh (477.5 Ah @ 48V) |
| 10.0 kWh / day (Standard Off-Grid Family Home) | ~15.3 kWh (318.3 Ah @ 48V) | ~30.6 kWh (636.7 Ah @ 48V) | ~45.8 kWh (955.0 Ah @ 48V) |
| 20.0 kWh / day (Large Home + Well Pump + Heat Pump) | ~30.6 kWh (636.7 Ah @ 48V) | ~61.1 kWh (1,273.3 Ah @ 48V) | ~91.7 kWh (1,910.0 Ah @ 48V) |
Calculation Formulas & Mathematical Methodology
Calculates nominal stored-energy capacity required for off-grid autonomy during sunless periods, accounting for Depth-of-Discharge (DOD) reserves, inverter efficiency, and available capacity factor.
Variable Definitions
Daily_Load_kWhLoad-Side Daily Energy(kWh/day)- Total energy delivered to connected AC/DC loads per day. Inverter losses are modeled separately.
Autonomy_DaysDays of Autonomy(days)- Continuous days of battery support required without meaningful solar or generator recharge.
MarginDesign Margin(fraction)- Planning safety buffer applied to total storage (e.g., 0.10 for 10%).
Usable_SOCUsable DOD Window(fraction)- Nominal minus minimum reserve SOC (e.g. 0.80 for LiFePO4, 0.50 for Lead-Acid).
Inverter_EffInverter Efficiency (η)(fraction)- AC inverter DC-to-AC conversion efficiency (typically 0.88–0.94).
Available_Capacity_FactorAvailable Capacity Factor(fraction)- Planning derating applied to nominal capacity. This is not a battery-aging prediction.
Calculation Notes
- Amp-Hour equivalent at nominal voltage V: Ah = (Bank_kWh × 1,000) / V.
- Higher-voltage battery systems (e.g., 48V vs 12V) reduce DC current by 75% for identical power, simplifying conductor sizing and reducing I²R resistive losses.
- Hardware configurations must arrange individual 12V or 24V battery units into balanced series strings matching nominal system voltage (e.g., 4S for 48V).
Technical References & Model Basis
PowerLab implements a deterministic, simplified sizing model designed for pre-engineering planning. The following technical references provide context for battery depth of discharge, string configuration, and safety criteria:
Recommended practice for sizing lead-acid batteries in stand-alone photovoltaic (PV) systems, establishing depth-of-discharge and temperature derating methodologies.
Recommended practice for sizing stationary battery installations, addressing duty cycles, design margins, and capacity rating conventions.
Safety and operational requirements for secondary lithium cells and batteries used in industrial and stationary energy storage systems.
National Electrical Code safety standards for Energy Storage Systems (ESS), including disconnecting means, overcurrent protection, and conductor sizing.