Backup sizing starts with the loads you truly need during an outage. Separating essential loads from whole-home demand usually produces a safer and more affordable first estimate.
List essential energy
Estimate the average essential load and the number of backup hours. A 500 W average load for 12 hours uses 0.5 × 12 = 6 kWh before battery and inverter losses. Starting surge loads need separate inverter consideration.
Account for usable capacity
Nominal battery capacity is not always fully usable. If the design needs 6 kWh, depth of discharge is 90%, and round-trip or discharge efficiency is 90%, nominal capacity is approximately 6 ÷ 0.90 ÷ 0.90 = 7.41 kWh.
Separate energy from power
A battery can have enough kWh but still fail to start a pump or support several appliances at once. Check continuous power, surge power, transfer behavior, and load sequencing with the system documentation.
Verify the installation
Temperature, code requirements, fire clearances, warranty limits, solar integration, and islanding controls are site-specific. The calculator is a planning aid, not an electrical design or permit substitute.
Sources
The figures and rules in this guide are checked against:
- Solar Integration: Solar Energy and Storage Basics — U.S. Department of Energy
- Energy Storage Systems (safety) — National Fire Protection Association
- BU-808: How to Prolong Lithium-based Batteries — Battery University (Cadex Electronics)
- Powerwall — Tesla