Whole-Home Battery Backup System Design
Design a whole-home battery backup system — load prioritization, battery sizing, inverter selection, and NEC 706/710 compliance.
Scenario Overview
Design a whole-home battery backup system (e.g., Tesla Powerwall, Enphase IQ Battery) for a 200A residential service. The system will provide backup during grid outages and pair with an existing 10 kW solar PV system.
Given Information
- •Existing 200A, 120/240V residential service
- •Existing 10 kW solar PV system with string inverter
- •Critical loads: refrigerator (600W), lights (500W), internet/router (100W), sump pump (800W)
- •Desired backup: 8 hours for critical loads, 4 hours for full home (~5 kW average)
- •Battery chemistry: lithium iron phosphate (LFP)
- •Utility: net metering available
Calculation Steps
Calculate Critical Load Energy
Critical loads: 600 + 500 + 100 + 800 = 2,000W (2 kW).
8-hour backup: 2 kW × 8 h = 16 kWh.
Add 10% system losses: 16 × 1.10 = 17.6 kWh.
Result: 17.6 kWh for critical loads
Calculate Whole-Home Energy
Average whole-home load: 5 kW.
4-hour full backup: 5 kW × 4 h = 20 kWh.
Add 10% losses: 20 × 1.10 = 22 kWh.
Total battery: 17.6 + 22 = 39.6 kWh (or use 22 kWh since loads overlap).
Result: ~40 kWh battery capacity needed
Select Battery System
Tesla Powerwall 3: 13.5 kWh each, 11.5 kW continuous output.
Need: 40 kWh / 13.5 = 2.96 → 3 Powerwalls.
3 × 13.5 = 40.5 kWh total capacity.
Continuous output: 3 × 11.5 kW = 34.5 kW (sufficient for whole-home including motor starts).
Result: 3× Tesla Powerwall 3 (40.5 kWh, 34.5 kW)
Inverter/Gateway Configuration
Tesla Gateway provides automatic transfer between grid and battery.
Gateway acts as the critical loads panel (200A rated).
Solar inverter remains grid-tied but inverter must support islanding with Powerwall.
System can charge batteries from solar during outages.
Result: Tesla Gateway for grid/battery transfer
NEC Compliance
NEC 706 — Energy Storage Systems: requires disconnects, signage, and fire code compliance.
NEC 710 — Stand-Alone Systems: applies during islanded mode.
Rapid shutdown: NEC 690.12 still applies for solar component.
Labeling: all interconnection points and battery disconnects must be clearly marked.
Working space: NEC 110.26 clearances for all battery equipment.
Result: NEC 706/710 compliant with required signage and disconnects
Final Answer
Install 3× Tesla Powerwall 3 (40.5 kWh total, 34.5 kW continuous) with Tesla Gateway for automatic grid/battery transfer. System provides 8+ hours of critical load backup or 4+ hours whole-home backup. Integrates with existing 10 kW solar PV for charging during outages.
Key Takeaways
- •Sizing batteries by energy (kWh) for duration AND power (kW) for peak demand is essential
- •Motor starting loads (HVAC, sump pumps) require high instantaneous power — size inverter accordingly
- •Battery systems paired with solar can recharge during outages, extending backup indefinitely for critical loads
- •NEC 706 is relatively new — ensure AHJ is familiar with energy storage requirements
Calculators Used
NEC References
- NEC 706 — Energy Storage Systems
- NEC 710 — Stand-Alone Systems
- NEC 690 — Solar PV (for hybrid systems)
- UL 9540 — Energy Storage Safety