SolarAdvanced
Plan a Battery Backup / Critical-Load Panel
Time
240–480 min
Steps
8
Pre-check
4 items
Skill
Advanced
Scope
Plan a home battery and critical-load (essential-loads) subpanel so your home keeps the important circuits running during a grid outage. Covers building a load list, sizing battery energy and power, deciding whole-home vs. critical-load backup, how islanding and anti-islanding work, battery siting and clearances, and the permitting/interconnection path. This is a PLANNING and design guide — the physical installation of a battery and subpanel is licensed electrical work.
Safety
Read before starting
Home batteries store large amounts of energy at dangerous voltage and current, and tying one into your service involves the main panel and utility interconnection. In nearly every jurisdiction this is permitted, inspected, licensed work — it is NOT a homeowner DIY install. This guide helps you design the system and have an informed conversation with your installer and AHJ; it deliberately stops short of wiring instructions.
Pre-Check
4 items · complete before you start0 / 36 complete
Steps
Build a critical-loads list
- List the circuits that genuinely must run in an outage and the wattage of each (nameplate or measured).
- Separate continuous loads (fridge, internet, lights) from intermittent ones (well pump, microwave) and from large motor starts (AC, sump) that spike on startup.
- Total the running watts and note the single largest starting surge — both drive the inverter power rating.
- Be ruthless: the shorter the list, the smaller and cheaper the battery and the longer it lasts.
Tips
- A clamp meter or a plug-in energy monitor gives real watts instead of inflated nameplate numbers.
Size battery energy (kWh) for runtime
- Add up the daily kWh of your critical loads for the outage duration you want to cover (e.g., one evening, or multiple days if paired with solar recharge).
- Apply usable capacity, not nameplate: lithium iron phosphate (LFP) batteries allow deep depth-of-discharge, but plan around the rated usable kWh and round-trip efficiency (~90%).
- If solar can recharge the battery each day, a smaller battery can cover a multi-day outage; without recharge, size for the full outage.
- Example: 8 kWh of critical loads per night ÷ ~0.9 efficiency ≈ a 9–10 kWh usable battery for one night with margin.
Code notes
- Most modern home batteries are LFP chemistry for safety and cycle life; "usable kWh," depth-of-discharge, and round-trip efficiency are the spec numbers that determine real runtime.
Size battery power (kW) for the load
- Energy (kWh) sets how long; power (kW) sets how much at once. The battery/inverter continuous kW must exceed your simultaneous running load.
- Check the surge/peak kW rating against your largest motor start (well pump, AC, sump) — surge is brief but must be met or the load won’t start.
- If one battery can’t cover the surge, either stack units, shed the big motor from the backup, or add a soft-starter to the motor.
- Confirm both continuous and surge ratings on the spec sheet, not just capacity.
⚠ Warnings
- A battery sized for energy but not power will trip on a large motor start even with plenty of charge left. Size both.
Decide whole-home vs. critical-load backup
- Critical-load (essential-loads) subpanel: move only the backed-up circuits to a separate subpanel the battery feeds during an outage — cheaper, smaller battery, predictable.
- Whole-home backup: the battery/inverter backs the entire main panel, usually with smart load management to avoid overloading — more capacity and cost.
- Critical-load is the common residential choice; whole-home suits larger batteries or homes that can’t tolerate triage.
- Sketch which circuits move to the critical-load panel and confirm the count fits a subpanel.
Plan the critical-load subpanel
- The critical-load subpanel sits between the battery/inverter and the loads it backs up; in an outage the inverter feeds it while disconnected from the grid.
- Relocating circuits into it is main-panel work — breakers, neutrals, and grounds must be handled by a licensed electrician.
- Keep the subpanel’s total backed load within the inverter’s continuous rating from Step 3.
- Leave space for future additions if you may back up more later.
Code notes
- NEC Article 710 (stand-alone/backup operation) and the energy-storage requirements of Article 706 govern how the storage system and backup panel must be built and labeled.
Understand islanding and anti-islanding
- During normal operation a grid-tied inverter must shut down in an outage (anti-islanding) so it can’t back-feed the grid and endanger line workers — NEC 690.12 / IEEE 1547.
- A backup-capable hybrid inverter uses an automatic transfer/islanding device that first physically disconnects from the grid, THEN energizes your critical-load panel — a safe "island."
- This is why you cannot just leave a normal grid-tie system running in an outage; backup requires the islanding hardware.
- Confirm your inverter/battery includes a listed automatic transfer/islanding function — don’t assume.
Code notes
- Anti-islanding protection (NEC 690.12, IEEE 1547) and a listed transfer means are mandatory — back-feeding the grid during an outage is illegal and lethal to line workers.
✅Continue Gate:Do you understand that backup requires a listed automatic islanding/transfer device that disconnects from the grid before energizing your loads, and that no part of this is DIY wiring?
Plan battery siting, ventilation, and clearances
- Pick a location within the battery’s rated temperature range — garages and shaded exterior walls are common; avoid living-space interiors unless the unit is listed for it.
- Respect manufacturer clearances and the NEC working-space rules around the equipment.
- Many jurisdictions cap energy per room/garage and require specific separations for ESS — confirm with your AHJ early.
- Plan the conduit run from battery to inverter to the critical-load panel so the electrician isn’t improvising on install day.
Code notes
- NEC 110.26 sets electrical working clearances; NEC 706 and NFPA 855 set energy-storage siting, separation, and ventilation limits the AHJ will enforce.
Plan permitting and interconnection
- Adding storage almost always requires a permit and an updated interconnection agreement with the utility — NEC 705 governs the interconnection.
- Your installer files the interconnection application; the AHJ inspects the install before it can be energized.
- Check for storage incentives (federal tax credit, state/utility rebates) and any required equipment listings before you buy.
- Assemble your load list, sizing, and equipment choices into a package for installer quotes and the permit application.
Code notes
- NEC Article 705 (interconnected power production sources) governs how the battery system ties to the grid and the main service; the utility’s interconnection rules apply on top of the NEC.