Battery Bank Sizing Calculator — Capacity, Configuration & Autonomy

How It Works

1. Enter Daily Energy Consumption

   Input your daily load in kWh or amp-hours at the battery bank voltage. For off-grid systems, use the highest-consumption day of the year. For backup systems, enter the critical-load consumption during an outage. Include inverter inefficiency (typically 5-10% overhead) in the load number because the battery must supply more energy than the AC loads consume.

2. Select Battery Chemistry

   Choose flooded lead-acid (FLA), sealed AGM, sealed gel, or lithium iron phosphate (LiFePO4). Each chemistry has different DoD limits, cycle life curves, temperature sensitivity, and cost per kWh. FLA is cheapest upfront but requires maintenance and tolerates only 50% DoD. LiFePO4 costs 2-3x more upfront but handles 80-90% DoD and lasts 3-5x longer.

3. Set Depth of Discharge and Autonomy Days

   Depth of discharge is the percentage of total capacity you plan to use daily. Lead-acid banks are typically limited to 50% DoD for acceptable cycle life. LiFePO4 banks routinely operate at 80% DoD. Autonomy days is the number of consecutive cloudy days the bank must sustain loads without solar recharging. Off-grid systems in cloudy climates typically require 3-5 days of autonomy.

4. Configure System Voltage

   Select the battery bank voltage: 12V, 24V, or 48V. Higher voltage means lower current for the same power, which reduces wire size and losses. Systems over 2 kW typically use 24V or 48V. Large off-grid systems (5 kW+) almost always use 48V. The voltage must match your charge controller input and inverter DC input specifications.

5. Review Bank Configuration

   The calculator outputs total bank capacity in Ah and kWh, number of batteries needed, series and parallel configuration, estimated cycle life at your operating DoD, temperature-derated capacity for your climate, and cost per kWh stored over the bank's projected lifetime. Use these numbers to compare chemistries and optimize the balance between upfront cost and long-term value.

Built For

• Off-grid solar designers sizing battery banks to match array production and daily load for year-round self-sufficiency
• Solar installers adding battery backup to grid-tied systems for essential-loads panels during utility outages
• Electricians configuring series/parallel battery strings to achieve the correct voltage and capacity for inverter specifications
• RV and marine electricians sizing house battery banks for boondocking with solar charging
• Telecom engineers sizing backup battery banks for cell tower sites with 72-hour autonomy requirements
• Homeowners comparing lead-acid versus lithium battery costs on a lifetime cost-per-kWh basis for off-grid cabins

Features & Capabilities

Multi-Chemistry Support

Covers flooded lead-acid, AGM, gel, and LiFePO4 with chemistry-specific defaults for maximum DoD, charge voltage setpoints, temperature compensation coefficients, self-discharge rates, and cycle life curves. Each chemistry behaves differently and the calculator applies the correct parameters automatically when you select the type.

Depth of Discharge Optimization

Shows cycle life projections at different DoD levels so you can find the economic sweet spot. A lead-acid battery cycled to 50% DoD may last 1,200 cycles, but the same battery at 30% DoD may last 2,500 cycles. The calculator computes cost-per-kWh-delivered over the bank's lifetime at your chosen DoD to identify the most economical operating point.

Temperature Derating

Battery capacity decreases in cold temperatures. A lead-acid battery at 32F (0C) delivers only about 80% of its rated capacity at 77F (25C). At 0F (-18C), capacity drops to roughly 60%. The calculator applies temperature correction factors based on your minimum expected operating temperature so the bank is sized for real-world conditions, not laboratory ratings.

Series/Parallel Configuration

Determines the exact number of batteries in series to achieve system voltage and the number of parallel strings to achieve total capacity. Displays the wiring configuration diagram notation (e.g., 4S2P = four in series, two parallel strings). Flags configurations that exceed recommended parallel string limits for the selected chemistry.

Autonomy Day Modeling

Calculates the total bank capacity needed to sustain loads through consecutive days without solar recharging. Accounts for the compounding effect of daily discharge without full recharge. For off-grid systems in the Pacific Northwest or northern latitudes, 3-5 autonomy days prevent generator dependence during extended overcast periods.

Lifetime Cost Analysis

Compares total cost of ownership across chemistries by factoring in cycle life, replacement frequency, and maintenance costs. LiFePO4 at $400/kWh with 5,000 cycles often costs less per kWh delivered than FLA at $150/kWh with 1,200 cycles. The calculator makes this comparison transparent so purchasing decisions are based on lifetime economics, not sticker price.

Frequently Asked Questions