Size a battery bank for off-grid solar or backup systems based on daily load and autonomy requirements.
Total energy consumed per day in watt-hours. Sum all appliance wattages multiplied by their hours of use. Example: 100W light x 5 hrs = 500 Wh.
Wh/day
Number of days the battery bank should power loads without recharging. Typical: 2-3 days for solar, 1 day for backup. More days = larger bank.
days
DC bus voltage of the battery bank. 12V for small systems, 24V for medium, 48V for larger systems. Higher voltage = lower current = thinner cables.
Maximum percentage of capacity to use. Lead-acid: 50% for long life. Lithium LiFePO4: 80-90%. Deeper discharge shortens battery life.
%
Amp-hour rating of each battery at nominal voltage. Common: 100Ah, 200Ah lithium; 100Ah, 225Ah lead-acid. Use the 20-hour rate for lead-acid.
Ah
Results
Required Bank Capacity-Ah
Required Bank Energy-kWh
Batteries in Series (per string)-batteries
Parallel Strings Needed-strings
Total Batteries Required-batteries
References:
Bank capacity (Ah) = (Daily load Wh x Days) / (System voltage x DOD). Series connections increase voltage; parallel connections increase capacity. Based on IEEE 1013 and IEEE 1561 battery sizing standards for stand-alone PV systems.