Sizing whole-home generators and backup batteries

What the code requires

Section 702.4 sets how large an optional standby source, such as a whole-home generator, must be. The answer depends on how the loads get connected to it.

If loads are connected manually or nonautomatically, meaning a person makes the transfer, the system must be able to supply all the loads intended to run at the same time.

If loads are connected automatically, 702.4(A)(2) gives two choices. The standby source can be sized to carry the full load that is automatically connected. Or, where an energy management system that meets 750.30 controls the connected load, the source can be sized for the maximum load that system will allow. Either way, the load is calculated using Article 220 Parts I–IV or another approved method.

On the job

• A whole-home automatic generator or backup battery has to carry the calculated full load, or sit behind a 750.30 energy management system that limits what it picks up.
• A grid-tied battery is not a stand-alone system, so its backup panel is not sized by the largest-single-load rule in 710.15(A).

What changed in 2023

The 2023 NEC now bases sizing on how loads are connected (manual or nonautomatic versus automatic) instead of on the type of transfer equipment, added a note that manual and nonautomatic equipment needs a person to operate it, and replaced the 2020 load management option with an energy management system meeting 750.30. Separately, the 2023 scope of Article 710 limits stand-alone systems to those not connected to the utility, so a grid-tied battery's backup is not sized by the 710.15(A) stand-alone rule.

Related sections

• NEC 702.5: Transfer switches and interlocks for standby power
• NEC 750.30
• NEC 710.15: Off-grid (stand-alone) power systems
• NEC 705.13: Energy management systems (formerly PCS) for solar

Where this comes from

• Mike Holt (EC&M): NEC Requirements for Emergency and Standby Systems, May 16 2025
• Georgia DCA: code amendment form (Tesla), 4/12/2023, quoting 2023 NEC 702.4(A)(2)
• NFPA: 2026 NEC (A2025) CMP-13 First Draft public input report, reprinting 2023 text
• Mike Holt: Summary of the 2023 Changes to the NEC
• Read the official NEC free at NFPA