Solar Battery Bank Sizing Calculator
Size your battery bank for off-grid or backup solar systems. Enter your daily energy usage, autonomy requirements, and battery specs to get the exact capacity, battery count, and wiring configuration - with depth of discharge and round-trip efficiency losses accounted for.
⚡ Energy & Battery Parameters
🔋 Battery Bank Results
How Battery Bank Sizing Works
Sizing a battery bank for solar requires balancing several factors: how much energy you use daily, how many days of backup you want, battery chemistry limitations, and system voltage.
The Sizing Formula
Start with your daily energy consumption (kWh), multiply by days of autonomy, then account for depth of discharge and round-trip efficiency losses. This gives you the total battery capacity needed.
Usable Capacity = Total Energy ÷ (Round-Trip Efficiency ÷ 100)
Total Capacity (kWh) = Usable Capacity ÷ (Depth of Discharge ÷ 100)
Total Ah = Total Capacity × 1000 ÷ System Voltage
Batteries in Series = ⌈System Voltage ÷ Battery Voltage⌉
Strings in Parallel = ⌈Total Ah ÷ Battery Capacity (Ah)⌉
Total Batteries = Series × Parallel
Depth of Discharge (DoD)
No battery should be fully drained. LiFePO4 batteries handle 80-90% DoD across 5,000+ cycles. Lead-acid batteries degrade rapidly below 50% DoD. A lower DoD means more batteries but longer battery life.
Round-Trip Efficiency
Energy is lost during charge and discharge. LiFePO4 loses only 2-5% (95-98% efficient). Lead-acid loses 15-20% (80-85% efficient). You need to oversize your bank to compensate for these losses.
Series vs Parallel Wiring
Batteries in series add voltage (four 12.8V batteries in series = 51.2V system). Batteries in parallel add capacity (four 100Ah strings in parallel = 400Ah total). A 48V system with 400Ah uses a 4S4P configuration = 16 batteries.
Step-by-Step Example
Scenario: 10 kWh/day usage, 2 days autonomy, LiFePO4 (80% DoD, 95% efficiency), 48V system, 100Ah 12.8V batteries, indoor installation (temp factor 1.0).
Step 1: Total Energy = 10 × 2 × 1.0 = 20 kWh
Step 2: Usable Capacity = 20 ÷ 0.95 = 21.05 kWh
Step 3: Total Capacity = 21.05 ÷ 0.80 = 26.32 kWh
Step 4: Total Ah at 48V = 26,316 ÷ 48 = 548.2 Ah
Step 5: Batteries in series = ⌈48 ÷ 12.8⌉ = 4
Step 6: Strings in parallel = ⌈548.2 ÷ 100⌉ = 6
Step 7: Total batteries = 4 × 6 = 24 batteries in a 4S6P configuration
Actual capacity: 6 × 100Ah × 48V ÷ 1000 = 28.8 kWh nameplate (23.04 kWh usable at 80% DoD)
Temperature Considerations
Battery performance degrades in extreme temperatures. In cold climates (below 32°F / 0°C), battery capacity can drop 20-40%. Apply a temperature derate factor of 1.1-1.2 to oversize the bank and ensure adequate capacity in winter.
Frequently Asked Questions
It depends on your daily usage, desired days of autonomy, battery chemistry, and system voltage. For a typical home using 30 kWh/day wanting 2 days of backup with LiFePO4 batteries at 80% DoD and 95% efficiency: you need about 78.9 kWh of total capacity (1,645 Ah at 48V), which requires 68 batteries (100Ah, 12.8V each) wired as 4 in series × 17 in parallel for a 48V system. Using higher-capacity batteries (e.g., 200Ah or 300Ah) significantly reduces the count.
LiFePO4 batteries can safely discharge to 80-90% DoD with minimal cycle life impact. Lead-acid batteries should only be discharged to 50% to preserve lifespan. Using a deeper DoD means fewer batteries but shorter battery life for lead-acid.
Round-trip efficiency measures how much energy you get back versus what you put in. LiFePO4 batteries achieve 92-98% round-trip efficiency. Lead-acid is typically 80-85%. Lower efficiency means you need more battery capacity to store the same usable energy.
48V is standard for systems over 3kW because it reduces current (and wire size) by 4× compared to 12V. 24V works for 1-3kW systems. 12V is only practical for very small systems (<1kW) like RVs or small cabins.
LiFePO4 (lithium iron phosphate) is the modern standard: 80-90% DoD, 95%+ efficiency, 5,000+ cycles, and lighter weight. Lead-acid costs less upfront but has lower DoD (50%), lower efficiency (80-85%), and only 500-1,000 cycles. LiFePO4 typically costs less over its lifetime.