How to Size a Battery Bank
Sizing a battery bank means determining the total capacity required to meet your energy needs over a specified period without recharging. This guide covers the standard engineering method.
The Sizing Method
The process starts with your total daily energy consumption in watt-hours. From there, you account for how many days the system must run without charging (autonomy), how deep you are willing to discharge the batteries (DoD), how much energy is lost in the system (efficiency), and how cold the environment gets (temperature derating).
The result is a required capacity in watt-hours, which you then convert to amp-hours at your system voltage.
Battery Sizing Formula
The temperature factor is typically 0.85-0.90 for cold climates (0°C) and 1.0 for room temperature.
Worked Example
Given:
- Daily energy consumption: 1,200 Wh (1.2 kWh)
- Autonomy required: 3 days
- DoD limit: 80%
- System efficiency: 90%
- Temperature factor: 0.90 (cold climate, 0°C average)
- System voltage: 48V
Step 1: Calculate required capacity in Wh:
Step 2: Convert to Ah at system voltage:
Step 3: Round up to nearest practical capacity. A 120 Ah or 200 Ah bank at 48V would be appropriate, with 200 Ah providing margin for future load growth and capacity fade.
Temperature Derating Factors
| Temperature | Lithium Factor | Lead-Acid Factor |
|---|---|---|
| 25°C (77°F) | 1.00 | 1.00 |
| 10°C (50°F) | 0.95 | 0.90 |
| 0°C (32°F) | 0.90 | 0.80 |
| -10°C (14°F) | 0.80 | 0.65 |
| -20°C (-4°F) | 0.70 | 0.50 |
Key Design Considerations
Chemistry Selection
LFP (LiFePO4) offers the longest cycle life (4,000-6,000 cycles) and best thermal stability. NMC provides higher energy density but shorter cycle life. Lead-acid is lowest cost per kWh but heaviest and shortest-lived.
Series/Parallel Configuration
Once you have the total Ah requirement, determine how many cells or modules are needed in series (for voltage) and parallel (for capacity). Use the Parallel String Calculator for this step.
Try It
Use the Battery Sizing Calculator to determine the required pack capacity for your system.
Open Sizing CalculatorNext Step
Once sized, configure the series and parallel cell arrangement with the Parallel String Calculator.
Open String CalculatorFrequently Asked Questions
What is days of autonomy?
Autonomy is the number of days the battery bank can supply loads without any external charge input. For off-grid solar, 2-3 days is typical to account for consecutive cloudy days. Critical systems may require 5-7 days.
Why do I need to derate for temperature?
Chemical reaction rates slow at low temperatures, reducing available capacity. A battery rated at 100 Ah at 25°C may only deliver 85 Ah at 0°C. Failing to account for this undersizes the bank for cold-climate installations.
How does depth of discharge affect sizing?
A lower DoD limit means you need a larger battery to access the same usable energy. If you limit discharge to 50% DoD to extend cycle life, you need twice the capacity compared to a system that discharges to 100%.
Should I oversize the battery bank?
Slight oversizing (10-20%) is recommended to account for capacity fade over time, temperature variation, and load growth. A bank that is exactly sized at installation will become undersized within 2-3 years as capacity degrades.