Whole-Home vs Critical-Loads Battery Backup: Which Do You Need?

The National Electrical Code’s rule for optional standby systems with automatic load connection gives two options: the standby source must be “capable of supplying the full load that is automatically connected,” figured by the Article 220 load calculation or another approved method; or, where an energy management system under section 750.30 automatically manages the connected load, the source must cover the maximum load that system will connect (2023 NEC 702.4(A)(2)). North Carolina’s state fire marshal has formally applied this rule to battery systems that back up a home through a transfer switch, concluding that the system may be smaller than the house’s full load only if a properly installed energy management system sheds load (NC OSFM interpretation). California’s 2025 electrical code, in effect since January 1, 2026, is based on the 2023 NEC (California Building Standards Commission); how an inspector applies the rule to a particular design is settled in plan review.

That leaves three practical designs: a backup panel that connects only essential circuits; a battery system whose output covers the whole house’s calculated load; or whole-home transfer with load management. Which one fits depends on the loads you need.

The installer moves the circuits you need in an outage — refrigerator, some lights, internet, furnace blower, a medical device, a few outlets — from the main panel into a backup subpanel fed through the battery’s gateway. Everything else stays on the main panel and goes dark with the grid. Because the connected load is small, the battery’s power rating is rarely the limit and its energy stretches: a 13.5 kWh battery holding a 10% reserve covers 7.2 kWh a day of essentials for about 1.7 days with no sun. With solar, recharge during an outage happens only if the system is designed to run off-grid; see why solar shuts off in an outage and batteries for PSPS shutoffs.

Whole-home backup keeps the main panel live, so the battery system must deliver everything that can run at once and start the largest motor. Three published numbers decide it: usable kWh (how long), continuous kW (how much at once) and the motor-start LRA rating (whether the compressor or pump starts). On the datasheets, one Tesla Powerwall 3 delivers up to 11.5 kW and starts motors up to 185 LRA; one FranklinWH aPower 2 delivers up to 10 kW, with 185 A LRA at its 11.5 kVA setting; Enphase’s IQ Battery 5P and 10C deliver 3.84 and 7.08 kVA per unit, with 48 A and 90 A LRA (Tesla; FranklinWH; Enphase 5P; Enphase 10C). The motor’s own figure is the LRA printed on the nameplate of the air conditioner’s outdoor unit or the pump; if the battery’s rating is lower, the motor will not start however many kWh are stored.

Energy runs out faster, too. Central air with the essentials at a steady 4.5 kW empties a 13.5 kWh battery, less a 10% reserve, in about 2.7 hours; compressors cycle, so real time runs longer, but cooling through a long outage takes more storage, more solar or a generator. The same example house run through four battery quotes is worked in best home battery for backup, and panel capacity questions are in do I need a main-panel upgrade.

Load management lets a battery back up the whole panel while switching off big, lower-priority loads when it cannot carry everything — the energy management option the code allows. Two examples from manufacturer datasheets: Enphase’s IQ Load Controller, used with the IQ System Controller, controls up to two 240-volt loads or four 120-volt loads during off-grid operation, and a site can use two of them (Enphase). FranklinWH’s aGate lists automatic load shedding and an optional Smart Circuits module with one 80 A and one 50 A 240-volt circuit, or one 80 A circuit and two 50 A 120-volt circuits (FranklinWH). Typical candidates for shedding are an EV charger, a dryer, a pool pump or a second air conditioner. The trade-off: those loads stop during an outage unless the battery has headroom, so decide in advance which ones you can live without.

If the essentials are what you need, a critical-loads panel keeps them running longest on the least battery. If central air, an electric kitchen or an EV charger must run, whole-home backup needs a battery system rated for those loads and motor starts, or load management that sheds them when needed. Start from the loads, then size with the battery sizing calculator and compare equipment in Powerwall vs Enphase vs FranklinWH.

Figures on this page come from the primary sources below and, where noted, from Cali Energy calculations using the stated assumptions. Rates, incentives, and program terms change; each was verified September 29, 2026.

1. North Carolina OSFM — Formal interpretation of 2023 NEC 702.4(A)(2): full load or energy management system (December 1, 2024)
2. California Building Standards Commission — 2025 Title 24 code changes (California Electrical Code based on the 2023 NEC; effective January 1, 2026)
3. Tesla — Powerwall 3 datasheet (13.5 kWh, up to 11.5 kW, 185 LRA; whole-home and partial-home configurations)
4. FranklinWH — aPower 2 datasheet (15 kWh, 10 kW / 11.5 kVA, 185 A LRA at 11.5 kVA)
5. FranklinWH — aGate X 1.1 datasheet (automatic load shedding, Smart Circuits, 200 A backup load port)
6. Enphase — IQ Battery 5P datasheet (5.0 kWh, 3.84 kVA, up to 48 A LRA)
7. Enphase — IQ Battery 10C datasheet (10.0 kWh, 7.08 kVA, up to 90 A LRA)
8. Enphase — IQ Load Controller datasheet (IQLC-DSH-00022, May 2024)
9. U.S. Department of Energy — Estimating appliance and home electronic energy use (nameplate wattage)