Solar Battery Storage Sizing Calculator

Solar battery sizing formula and worked example

Formula

required battery kWh = (Σ appliance watts × hours ÷ 1000) ÷ inverter efficiency ÷ depth of discharge × days of autonomy

where depth of discharge is 0.8 for lithium and 0.5 for lead-acid; inverter efficiency applies only when loads run through an inverter (85% typical)

Worked example

A homeowner wants one day of backup for a refrigerator (200 W for 8 h), LED lights (50 W for 5 h), and a Wi-Fi router (6 W for 24 h), using a lithium battery bank at 48 V through an 85%-efficient inverter.

1. Daily energy per appliance: refrigerator 1.60 kWh + lights 0.25 kWh + router 0.14 kWh = 1.99 kWh
2. Inverter losses: 1.99 kWh ÷ 0.85 = 2.35 kWh drawn from the battery
3. Depth-of-discharge reserve: 2.35 kWh ÷ 0.8 lithium DoD = 2.9 kWh of installed capacity
4. Battery bank size at 48 V: 2.9 kWh × 1000 ÷ 48 V = 61 Ah
5. Inverter rating: (200 W + 50 W + 6 W) × 1.25 surge safety factor = 320 W

Result

One day of backup for these three loads needs about 2.9 kWh (61 Ah at 48 V) of lithium capacity and a 320 W inverter.