Off-Grid Battery Bank Calculator — Ah Capacity for Cabin & RV | SolarRatio
Size your off-grid battery bank in seconds. Enter daily consumption, autonomy days, voltage, and depth of discharge to find the exact Ah capacity you need.
Battery bank sizing determines the amp-hour (Ah) storage capacity required to power loads through nights, storms, and consecutive cloudy days without solar input. The calculation hinges on daily energy demand, desired days of autonomy, allowable depth of discharge (DoD), system voltage, and round-trip efficiency. Get any of these wrong and the bank either fails to deliver during the storm you bought it for, or cycles into early failure from chronic over-discharge. This tool gives an honest Ah target for lead-acid, AGM, and LiFePO4 chemistries. In the US, stationary battery installations are governed by NEC Article 480 and must meet UL 9540 ESS listing requirements for code-compliant indoor or garage siting — a step many DIY builders overlook until the AHJ inspection.
How it Works
Multiply daily consumption (Wh/day) by autonomy days, divide by allowable DoD (lead-acid 0.5, AGM 0.5, LiFePO4 0.8), then divide by system nominal voltage (12V, 24V, 48V). Add a 10–20% safety margin for temperature derate (cold reduces lead-acid capacity by up to 50% at -20°C; LiFePO4 stops charging below 0°C). For 48V systems, current draw is 4x lower than 12V at the same power — thinner wire, lower losses, easier inverter sizing. The tool reports both raw Ah and recommended series/parallel battery count for common 100–300 Ah modules. Round-trip efficiency (lead-acid ~80%, LiFePO4 ~95%) should be folded into the panel-sizing calculation, not the bank capacity.
Usage Scenarios
Off-grid homeowners size a 48V 400 Ah LiFePO4 bank to deliver 3 days autonomy at 5 kWh/day, providing a buffer through multi-day Pacific Northwest rain events — Seattle's 3.7 h/day average PSH makes that 3-day buffer non-negotiable in winter. A Colorado mountain cabin owner specifies a UL 9540-listed 10 kWh LiFePO4 ESS with a UL 1741-SB certified inverter to qualify for the 30% federal ITC and satisfy county AHJ inspection requirements. RV and van builders convert house loads into a 12V 200–300 Ah LiFePO4 bank that survives boondocking weekends with reserve for the next leg of travel. Marine installations specify deep-cycle AGM with 50% DoD for safer sealed operation in below-deck compartments. Telecom backup designers spec a 48V VRLA string sized for 8 hours runtime at full critical load with N+1 redundancy. Emergency residential backup uses a hybrid inverter with a 10 kWh LiFePO4 bank tuned for whole-home or critical-loads-only operation during grid outages.
Frequently Asked Questions
What is depth of discharge (DoD) and why does it matter?
DoD is the percentage of battery capacity you can safely use. Lead-acid batteries should not exceed 50% DoD to preserve lifespan. LiFePO4 batteries can safely discharge to 80% DoD, making them more efficient per Ah.
How many days of autonomy should I plan for?
Off-grid systems typically plan for 2–3 days of autonomy (days without solar charging). In cloudy climates, 3–5 days is recommended. More autonomy means a larger, more expensive battery bank.
LiFePO4 vs lead-acid: which is better for off-grid?
LiFePO4 batteries cost more upfront but last 3–5× longer (3,000–5,000 cycles vs 500–1,000 for lead-acid), have higher DoD (80% vs 50%), and are lighter. Over 10 years, LiFePO4 is usually more cost-effective.
How do I wire batteries for the right voltage and capacity?
Series wiring increases voltage (two 12V batteries in series = 24V). Parallel wiring increases capacity (two 100Ah batteries in parallel = 200Ah). Combine both for higher voltage and capacity.
What battery voltage should I choose for my system?
12V systems suit small loads under 1,000W. 24V systems handle 1,000–3,000W efficiently. 48V systems are best for larger homes (3,000W+). Higher voltage means lower current and thinner wires.
How to Use the Battery Bank Sizing Calculator
Enter daily consumption, battery voltage, days of autonomy (days without sun), and depth of discharge (DoD). Formula: Required Ah = (daily Wh × autonomy days) / (voltage × DoD).
Depth of discharge significantly affects battery lifespan. Lead-acid batteries should not be discharged below 50% (DoD = 50%). LiFePO4 batteries can safely discharge to 80% DoD.
Days of autonomy is your buffer for cloudy days. Off-grid systems typically use 2-3 days. More autonomy means larger battery bank but better resilience.