Critical-load backup is not whole-home kWh—it is usable energy after DoD against the circuits that must stay on. This guide shows how to calculate usable battery capacity for critical loads from bank nominal kWh, depth of discharge, and your essential load tally.
Benefits
- Usable kWh = nominal × (DoD ÷ 100) for outage runtime checks.
- Compare usable Wh to Critical Load Analysis totals.
- Reserve kWh stays out of the critical-load budget.
How it works
- Sum critical load energy in Wh or kWh (fridge, furnace, networking, etc.).
- Enter installed or planned bank nominal kWh and operating DoD %.
- Verify usable kWh ≥ critical load × desired outage hours (before inverter loss).
FAQ
How do I size usable capacity for critical loads?
Usable kWh = nominal × (DoD ÷ 100). Example: critical loads need 4 kWh/day, target 24 h backup → need ≥4 kWh usable. A 5 kWh nominal bank at 80% DoD → 4 kWh usable—meets the floor with no margin; add headroom or lower DoD reserve.
Should I use whole-home or critical-load kWh?
Use critical-load kWh only for essential-circuit backup quotes. Whole-house daily kWh from energy budgets overstates what a sub-panel backup inverter must feed. Match usable bank energy to the load list you actually transfer.
What margin should I add after usable kWh matches load?
Plan 10–20% above calculated critical Wh for inverter efficiency, battery aging, and cold-weather capacity loss. Also keep BMS reserve outside DoD—usable kWh in this tool is already net of your DoD cap, not infinite depth.
Technical specifications
- Usable kWh = nominal_kWh × (DoD% ÷ 100).
- Required nominal ≥ critical_kWh ÷ (DoD% ÷ 100).
- Runtime h ≈ usable_Wh ÷ critical_load_W (DC/AC efficiency extra).
- Related: critical-load-analysis, home-backup-sizing, battery-dod-to-usable-energy-calculator.
Critical loads are a subset—size usable kWh to match
A 30 kWh whole-home figure does not belong in a critical-panel backup sheet. List transferred circuits—refrigeration, heat, water pump, communications—and total their Wh per outage day. Convert bank nominal kWh with your DoD % to usable kWh and check coverage. Oversizing nominal without adjusting DoD still leaves less energy than the sticker suggests.
Work backward from hours of autonomy
If critical loads draw 1.6 kWh over 12 hours, you need at least 1.6 kWh usable for that window—before inverter conversion loss. Scale to 24 or 48 hours by multiplying load energy. Then solve for nominal: nominal = required_usable ÷ (DoD ÷ 100). That is the bank nameplate to quote when vendors sell by kWh modules.
Document usable vs. reserve on the BOM
Project snapshots should show usable kWh, reserved kWh, and critical-load Wh in one line item so reviewers see the margin. Pair this calculator with Critical Load Analysis for circuit-level watts and with Home Backup Sizing when generator plus battery hybrid paths are compared. Usable capacity is the hinge between load study and battery SKU count.