Load Prioritization
Not all loads need backup. Prioritize: Tier 1 (essential) — refrigerator, lights, communications, medical equipment, sump pump. Tier 2 (comfort) — HVAC, cooking, hot water. Tier 3 (convenience) — entertainment, laundry, EV charging.
Typical critical load budgets: residential = 3-8 kW, small commercial = 10-25 kW. A well-designed system backs up Tier 1 loads for 8-24 hours, or Tier 1+2 loads for 4-8 hours.
Battery Capacity Calculation
Formula: kWh_battery = (Load_W × Hours) / (DoD × Inverter_η × 1000). Where DoD = depth of discharge (lithium-ion: 80-100%, lead-acid: 50%), Inverter_η = inverter efficiency (92-97%).
Example: 5 kW load for 10 hours: kWh = (5000 × 10) / (0.90 × 0.95 × 1000) = 58.5 kWh. Tesla Powerwall 3 = 13.5 kWh each → need 5 units (67.5 kWh).
Temperature derating: lithium batteries lose 10-20% capacity below 32°F (0°C). Lead-acid loses 30-40%. Indoor conditioned battery rooms maintain optimal performance.
Inverter and Power Rating
Battery inverter must handle both continuous load AND motor starting surge. A central AC unit may draw 30-40A surge (7-10 kW) for 1-2 seconds during start.
Continuous rating: must exceed total Tier 1 load. Surge rating: must exceed largest motor inrush + other running loads. Most residential battery inverters: 5-11.5 kW continuous, 2× surge for 10 seconds.
Grid-forming vs grid-following: grid-forming inverters can create their own AC reference (essential for backup without grid). Grid-following requires grid or generator reference signal. Most modern ESS inverters are grid-forming.
Integration Options
Solar + Battery: PV charges battery during day, battery powers loads at night and during outages. NEC 706 governs energy storage systems. The battery inverter must coordinate with the PV inverter for seamless transition to backup mode.
Generator + Battery: Generator charges battery and powers heavy loads. Battery handles rapid load changes and provides instant switchover (no 10-30 second generator start delay). Common in commercial applications.
Whole-home vs essential panel: whole-home backup requires large battery and inverter (15-30 kW). Essential panel (subpanel with critical loads only) reduces battery and inverter size by 50-70% — the most cost-effective approach for most residential installations.
NEC Requirements for ESS
NEC 706 — Energy Storage Systems: requires disconnecting means, overcurrent protection, and safety signing. Battery systems must be listed (UL 9540) and installed per manufacturer instructions.
Fire safety: NEC 706.4 requires ESS installations to comply with local fire codes. Indoor lithium battery installations may require fire-rated enclosures or dedicated battery rooms with ventilation. Outdoor installations simplify fire code compliance.
Rapid shutdown: If connected with PV, the combined system must comply with NEC 690.12 rapid shutdown requirements.