Solar Battery Sizing Calculator

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 to 2 days
• Off-grid: 3 to 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 to 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 to 20%

Battery bank size = Usable capacity ÷ 0.85 = 3,800 ÷ 0.85 ≈ 4,471 Wh (4.5 kWh)

Common battery sizing:

A 48V system: 4,471 Wh ÷ 48V = 93.1 Ah → choose a 100 Ah battery bank at 48V, the next standard size up.

Notice how far the answer has drifted from where it started. You wanted to store 1,520 Wh a day for two days, which is 3,040 Wh, and you are buying 4,800 Wh of nameplate capacity to do it. Depth of discharge and system efficiency together cost you about 60% on top. That gap is the single most common reason a first off-grid build comes up short in its first cloudy week.

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.