Battery Storage / ESS Sizing Calculator

Size residential and commercial battery storage systems (ESS), backup duration runtime, inverter kW, and 125% circuit OCPD per NEC Article 706.

NEC edition

Inputs

kWh
%
kW
kW
%

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⚡ Test your knowledge
NEC Article 220 was renumbered in the 2026 code cycle. What is the new article number?
  • A. Art. 100
  • B. Art. 120
  • C. Art. 200
  • D. Art. 230
💡 NEC 2026 renumbered Article 220 (Branch-Circuit, Feeder, and Service Calculations) to Article 120.
Energy Management Systems were moved from Article 750 to which article in NEC 2026?
  • A. Art. 100
  • B. Art. 125
  • C. Art. 130
  • D. Art. 150
💡 NEC 2026 moved Energy Management Systems from Article 750 to Article 130.
Which NEC standard size overcurrent device comes after 90A?
  • A. 95A
  • B. 100A
  • C. 110A
  • D. 105A
💡 Per NEC §240.6(A), the standard sizes are …90, 100, 110, 125… There is no 95A standard size.

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How it works & the NEC rules behind it

Energy Storage Systems (ESS) are governed by NEC Article 706. Sizing an ESS requires evaluating both energy capacity (total storage in kilowatt-hours, kWh) and power capacity (continuous output in kilowatts, kW), along with code-mandated electrical protection.

Key Sizing Rules under NEC Article 706:

  • 1. Continuous Duty Sizing (§706.30(A)): ESS output circuits are rated for continuous operation. Circuit conductors and overcurrent protection devices (OCPD) must be sized for at least 125% of the maximum continuous current.
  • 2. Usable Energy vs. Nameplate Capacity: Usable capacity equals Nameplate kWh × Depth of Discharge (DoD) (typically 90%–100% for modern Lithium Iron Phosphate LiFePO4 cells).
  • 3. Backup Autonomy Runtime: Calculated as (Usable kWh × Round-Trip Efficiency) ÷ Average Hourly Essential Load (kW).
  • 4. Disconnecting Means (§706.15): A readily accessible, lockable disconnecting means must be provided within sight of the ESS equipment.
  • 5. Interconnection Rules (§705.12 / §706.16): Point of connection to electrical panelboards must comply with the 120% busbar rule or supply-side tap rules.

Formulas & equations

Usable Battery Energy (kWh) = Nameplate Capacity (kWh) × Depth of Discharge (DoD) Backup Runtime (Hours) = (Usable kWh × System Efficiency) ÷ Average Load (kW) Inverter Continuous AC Current = (Continuous kW × 1000) ÷ AC Voltage Min Required ESS Breaker = Inverter Continuous AC Amps × 1.25 (§706.30)

NEC reference table

ESS Model / Capacity Usable kWh Continuous kW Peak Surge kW Min 240V Breaker (125%)
Tesla Powerwall 313.5 kWh11.5 kW18.5 kW60 A
Enphase IQ Battery 5P5.0 kWh3.84 kW7.68 kW20 A
FranklinWH aPower (13.6)13.6 kWh5.0 kW10.0 kW30 A
SolarEdge Home Battery9.7 kWh5.0 kW7.5 kW30 A
Dual Powerwall 3 (2 Units)27.0 kWh23.0 kW37.0 kW125 A

Worked example, step by step

Example: Sizing 13.5 kWh Lithium ESS for Residential Critical Backup

Given: 13.5 kWh Nameplate, 95% DoD, 92% round-trip efficiency, 7.6 kW Inverter @ 240V, average critical backup load = 1.5 kW.

  • 1. Usable Energy: 13.5 kWh × 0.95 = 12.825 kWh.
  • 2. Delivered Energy: 12.825 kWh × 0.92 = 11.80 kWh.
  • 3. Backup Duration: 11.80 kWh ÷ 1.5 kW load = 7.87 Hours continuous runtime.
  • 4. Inverter Output Current: 7,600W ÷ 240V = 31.67 Amps.
  • 5. Conductor / Breaker Sizing (125% §706.30): 31.67A × 1.25 = 39.58 Amps → Install standard 40A Breaker with #8 AWG Copper.

Frequently asked questions

Under NEC §706.30, all energy storage system conductors and overcurrent devices must be sized for 125% of the maximum continuous current output of the inverter or converter, because battery systems can discharge at full power for more than 3 continuous hours during grid outages.

In an AC-coupled system, the battery has its own dedicated bidirectional inverter that connects directly into the AC panelboard. In a DC-coupled system, solar panels and batteries connect directly to a single hybrid DC inverter, sharing one connection to the home AC service.

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