Solar Battery Sizing Calculator
Calculate the battery bank capacity needed for your solar system.
Enter daily energy usage, days of autonomy, and battery voltage for accurate Ah sizing.
How Solar Battery Sizing Works
A solar battery system must store enough energy to power your loads through nights and cloudy days. Undersizing means blackouts; oversizing wastes money. The calculation has four key steps.
Step 1 — Calculate daily energy consumption:
Daily Load (Wh) = Σ (Device Watts × Hours used per day)
Example household loads:
- Refrigerator: 150W × 8h = 1,200 Wh
- Lights: 60W × 5h = 300 Wh
- Phone charging: 10W × 2h = 20 Wh
- Total: 1,520 Wh/day
Step 2 — Determine days of autonomy:
How many days without sun must the system handle?
- Grid-tied backup: 1–2 days
- Off-grid: 3–5 days (more in cloudy regions)
Required storage = Daily Load × Autonomy Days = 1,520 × 2 = 3,040 Wh
Step 3 — Account for depth of discharge (DoD):
Lithium (LiFePO4) batteries: 80–90% DoD Lead-acid batteries: 50% DoD maximum
Usable capacity needed = Required storage ÷ DoD For lithium at 80%: 3,040 ÷ 0.80 = 3,800 Wh = 3.8 kWh
Step 4 — Account for system efficiency losses:
Inverter, wiring, and charge controller losses: approximately 15–20%
Battery bank size = Usable capacity ÷ 0.85 = 3,800 ÷ 0.85 ≈ 4.5 kWh
Common battery sizing:
A 48V system: 4,500 Wh ÷ 48V = 93.75 Ah → choose 100 Ah battery bank at 48V
For two days of autonomy with a modest household, a 10 kWh LiFePO4 battery (e.g., two 5 kWh units) is a typical residential choice.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.