Solar Battery Backup Design for Homes: Sizing and Permit Basics

A homeowner calls and asks for “enough battery to keep the lights on.” That is not a design spec, it is a wish. Before a designer can pull a single line diagram or size a busbar, someone has to answer three questions: what loads actually need power, for how long, and where the battery physically sits in or next to the house. Get those three answers wrong and the customer either overpays for capacity they never use, or discovers during the next outage that the battery ran dry at hour six instead of running the fridge for two days. This guide walks through the sizing methodology we use before any permit paperwork starts, so you know how many kilowatt-hours to spec, which loads to back up, and where the unit belongs on the property.

Direct answer. Home battery backup sizing starts with a load audit, not a battery catalog. Add up the watt-hours of the loads you plan to back up, decide between a critical-loads subpanel or whole-home backup, pick an autonomy target in days, then divide total daily watt-hours by usable battery capacity to get the number of units. Most single-battery installs (10 to 16 kWh usable) cover a critical-loads subpanel for one day. Whole-home backup for a typical US house usually needs 2 or more battery units plus load management software.

This is written for Mike, the US residential installer who now gets a battery attached to nearly every proposal, and for the homeowner-facing EPC teams answering “how big a battery do I need” every week. It covers system-level sizing and design decisions. For the plan-set paperwork that turns a sized system into a permit-ready drawing package, see our companion piece on solar-plus-storage plan set requirements.

How to size a home battery: start with a load audit

A load audit lists every circuit a homeowner wants powered during an outage, in watts, with an estimate of how many hours per day each one actually runs. Skip this step and every later number is a guess.

Break loads into three tiers:

1. Must-run, continuous. Refrigerator, freezer, well pump, sump pump, medical equipment, internet router and modem, a few lighting circuits.
2. Must-run, intermittent. Furnace blower or heat pump air handler, garage door opener, kitchen outlets, TV and electronics.
3. Nice-to-have. Electric range, dryer, whole-house air conditioning, EV charging.

A typical must-run tier for a US home runs 800 to 1,500 watts continuous, with intermittent surges from the furnace blower or well pump pushing peak demand to 2,500 to 4,000 watts for a few minutes at a time. That peak, not the average, sets the inverter’s power rating. A battery with plenty of stored energy still fails the customer if its inverter cannot handle the well pump’s locked-rotor surge.

For whole-home backup, the audit gets longer and includes central HVAC, electric water heating, and cooking loads. Whole-home backup on an all-electric house easily exceeds 40 kWh of daily energy use in winter, which is why most whole-home installs pair multiple battery units with load-shedding software that automatically drops non-critical circuits when the battery runs low.

Critical loads panel vs whole-home backup

This is the single biggest design fork in a residential battery project, and it drives everything downstream: battery count, panel work, and price.

A critical-loads subpanel is a separate small panel wired to carry only the backed-up circuits. The main panel stays as-is; only the circuits moved to the subpanel get battery power during a grid outage. This is the default for single-battery installs because it fits inside the capacity of one unit.

Whole-home backup keeps the existing main panel intact and backs up everything behind it, sized against the panel’s full busbar rating rather than a curated circuit list. It costs more in equipment and often requires either multiple battery units or a load-management device that throttles high-draw circuits (like AC compressors) so the battery is not asked to power everything simultaneously.

Factor	Critical-loads subpanel	Whole-home backup
Typical usable capacity needed	8 to 12 kWh for 1 day	20 kWh or more, often 2+ units
Panel work	New subpanel, moved circuits	Existing panel stays, may need load management
Best fit	Budget-conscious, storm-prone areas, outages under 24 hours	All-electric homes, medical needs, long grid outages
Typical installed cost driver	1 battery unit + subpanel labor	2+ battery units + possible service upgrade

Neither option is universally right. A homeowner in a wildfire-shutoff zone with short, frequent outages usually gets more value from a well-sized critical-loads system than from an expensive whole-home build they will rarely stress-test. A homeowner with a medically necessary device or an all-electric house in a hurricane corridor is a better fit for whole-home capacity, even at higher cost. The job of the designer is to say this plainly instead of defaulting to whatever the manufacturer’s brochure pushes.

Autonomy days: how long does the battery need to last

Autonomy is the number of days the battery must carry the backed-up loads without any solar recharge, driven by the region’s outage pattern and the loads chosen in the audit.

Most residential designs target 1 day (24 hours) of autonomy for a critical-loads panel, since a paired PV system typically recharges the battery the next sunny day. Homes without solar, or homes in regions with multi-day cloud cover during outage season (Pacific Northwest winter storms, for example), often push the target to 2 or 3 days.

The math is simple once the load audit exists:

Required usable capacity (kWh) = Daily watt-hours of backed-up loads ÷ 1,000 × autonomy days ÷ system efficiency

A round-trip efficiency of 85 to 90 percent is a reasonable planning number for most modern lithium battery systems; use the manufacturer’s published figure when available rather than assuming.

Worked example. A critical-loads audit totals 9,600 watt-hours per day (fridge, well pump, router, a few lighting and outlet circuits). At 1 day of autonomy and 88 percent round-trip efficiency:

9,600 ÷ 1,000 × 1 ÷ 0.88 = 10.9 kWh usable capacity required.

That fits inside a single battery unit in the common 10 to 16 kWh usable range that dominates the US residential market today. Push the same load list to 2 days of autonomy and the requirement roughly doubles to about 22 kWh, which crosses into 2-unit territory for most product lines.

Average US household electricity use runs about 29 kWh per day according to the U.S. Energy Information Administration’s 2024 residential consumption data, which is why whole-home autonomy targets climb fast and why load-shedding software, not just bigger batteries, is often the more cost-effective answer for whole-home jobs.

Battery chemistry basics for residential design

Lithium iron phosphate (LFP or LiFePO4) chemistry has become the default for new residential storage products because of its thermal stability, long cycle life (often rated for 4,000 to 6,000 cycles to 80 percent capacity), and lower fire risk relative to older nickel manganese cobalt (NMC) lithium chemistries. Most major residential battery brands sold in the US today ship LFP cells.

Chemistry matters for design in three practical ways:

• Depth of discharge (DoD). LFP systems typically support 90 to 100 percent usable DoD without aggressive degradation, so the “usable capacity” figure on a spec sheet is close to the total energy content. Older chemistries sometimes reserve more headroom, so always design from usable capacity, never nameplate capacity.
• Temperature range. Cold garages and unconditioned outdoor enclosures can derate both charge and discharge performance. Check the manufacturer’s operating temperature range against the installation location before finalizing placement.
• UL 9540 listing. Chemistry alone does not certify safety. UL 9540 is the product safety standard for energy storage systems, and UL 9540A is the test method used to characterize thermal runaway fire propagation. A specific product’s UL 9540 listing, not its chemistry family, is what the AHJ and insurer actually check. Our BESS vocabulary glossary covers C-rate, depth of discharge, and state of charge definitions in more depth.

Never treat chemistry as a stand-in for a spec sheet. Verify usable capacity, continuous power rating, surge rating, and UL 9540 listing status directly from the manufacturer’s current documentation for the exact model being installed.

Where the battery goes: placement and clearance basics

Placement decisions get made too late on too many residential jobs, usually after the battery is already on the truck. Three things drive where a home battery can legally and practically sit: fire code setbacks, electrical code disconnect access, and simple practicality (garage wall, exterior wall, or basement).

NFPA 855, the installation standard for stationary energy storage systems, sets the baseline residential rules most AHJs enforce:

• Individual residential ESS units are generally capped at 20 kWh of energy content each before additional fire separation requirements kick in. Multiple units on one wall each need to meet this per-unit threshold, and AHJs often cap total energy content per room or enclosure as well.
• A minimum 3-foot clearance typically applies between the battery and doors, windows, and other means of egress, and often between adjacent battery units.
• Placement inside habitable space (bedrooms, living rooms) is commonly prohibited outright; garages, attached exterior walls, and dedicated utility rooms are the usual accepted locations, subject to local fire marshal review.
• Batteries are commonly restricted from installation directly below or above sleeping areas in some jurisdictions, and some AHJs add extra ventilation or smoke-detection requirements for indoor installs.

These are baseline figures from NFPA 855 and commonly enforced by AHJs. Always confirm the exact clearance table and any local amendments with the specific jurisdiction before finalizing a site plan, since fire departments frequently layer stricter local rules on top of the national standard.

On the electrical side, NEC Article 706 (2023 NEC) governs the battery’s disconnecting means, overcurrent protection, and required labeling, separate from NEC Article 690, which governs the PV array. Our glossary entry on NEC 706 covers the article’s disconnect and labeling requirements in more detail, and our NEC 706 battery storage permit guide walks through the full permitting process for the electrical side.

Interconnecting the battery with an existing PV system

Most residential battery projects attach to an array that is already installed and interconnected, which raises a design question competitors skip: does the battery connect on the DC side or the AC side of the existing inverter?

AC-coupled systems add a separate battery inverter that connects to the home’s AC wiring alongside the existing PV inverter. This is the more common retrofit path because it does not require touching the existing PV inverter’s DC wiring or warranty. DC-coupled systems tie the battery into the DC side before the existing inverter, which can be more efficient but usually requires inverter compatibility planned from the start, making it more common on new installs than retrofits. We cover the tradeoffs of each architecture in detail in DC vs AC coupling for solar battery systems.

Either path changes the busbar math. NEC 705.12 requires the interconnection calculation to count every power source on the busbar, including an AC-coupled battery inverter. A busbar sized correctly for PV alone commonly fails the 120 percent rule once a battery inverter is added, which is one of the most common rejection triggers we see on retrofit storage permits. Our glossary entry on NEC 705 explains the busbar rule in plain terms.

What most installers get wrong on residential battery sizing

The most common mistake is sizing off the battery brand’s marketed capacity instead of the customer’s actual load audit. A “13.5 kWh” battery sounds like a round number for whole-home backup until you run a real load list and realize the well pump and central air alone can exceed the inverter’s continuous rating, long before the stored energy runs out.

The second common mistake is ignoring surge current. A battery with ample stored energy still trips offline if its inverter cannot handle the momentary surge from a well pump or air handler starting up. Continuous power rating and surge (peak) power rating are two different numbers on the spec sheet, and the surge rating is what determines whether a critical-loads panel can actually start a well pump without nuisance tripping.

The third mistake is treating placement as an afterthought. We routinely see designs where the battery location on the site plan gets picked after the electrical design is finished, then has to be moved once the fire clearance table gets checked, which forces a rework of conduit runs and disconnect locations. Placement should be locked in the same design pass as sizing, not after.

A simple sizing checklist

1. Build the load audit: list every circuit to back up, in watts, with realistic run hours.
2. Decide critical-loads subpanel vs whole-home, and be honest about budget and outage pattern.
3. Set the autonomy target in days based on the region’s typical outage length.
4. Calculate required usable capacity using the formula above, then round up to the nearest full battery unit.
5. Check the inverter’s continuous and surge power ratings against the highest-draw loads (well pump, HVAC compressor).
6. Confirm the exact product’s UL 9540 listing and usable capacity from current manufacturer documentation.
7. Pick a placement location that clears NFPA 855 setbacks before finalizing the electrical design.
8. Hand the sized system to a permit designer to build the NEC 706 electrical sheets and NFPA 855 site plan.

That last step is where sizing turns into a permit-ready plan set. Our team at Heaven Designs builds the electrical sheets, busbar calculations, and NFPA 855 site plans for solar-plus-storage jobs across the US, working from exactly the load audit and sizing decisions this guide walks through. If you want a second set of eyes on a battery sizing calculation before it goes to permit, reach out to our design team or see sample battery storage plan sets from recent jobs. For the full permit-side plan set, our permit design service covers both the PV and storage drawing package.

FAQ

How many kWh of battery do I need for a home? It depends on the load audit, not a rule of thumb. A critical-loads panel (fridge, well pump, router, a few circuits) commonly needs 10 to 12 kWh usable for one day of autonomy. Whole-home backup on an average US house, at roughly 29 kWh of daily use according to EIA data, often needs 20 kWh or more, plus load-shedding software.

Can one battery power a whole house? Rarely on its own. Most single residential battery units (10 to 16 kWh usable, with continuous power ratings around 5 to 7.5 kW) cover a critical-loads subpanel well but fall short of whole-home demand, especially with central air conditioning or electric heat running. Whole-home backup usually needs 2 or more units.

What is the difference between critical-loads and whole-home battery backup? A critical-loads subpanel backs up a curated list of circuits (fridge, well pump, some outlets) through a dedicated small panel. Whole-home backup keeps the main panel intact and backs up everything behind it, which needs more battery capacity or load-management hardware to avoid overloading the system.

Where can a home battery legally be installed? NFPA 855 sets baseline clearances, generally 3 feet from doors, windows, and adjacent units, with most units capped at 20 kWh of energy content before extra fire separation applies. Habitable rooms are commonly excluded. Garages, exterior walls, and utility rooms are the typical accepted locations, subject to the local AHJ’s exact rules.

Does NEC 706 apply to residential battery systems? Yes. NEC Article 706 (2023 NEC) governs energy storage system disconnects, overcurrent protection, and labeling for residential and commercial installs alike, separate from NEC 690 for the PV array itself.

Should I choose AC-coupled or DC-coupled for a battery retrofit? AC-coupled is the more common retrofit choice because it does not require modifying the existing PV inverter’s DC wiring. DC-coupled can be more efficient but is usually planned into a new install rather than added later. See our DC vs AC coupling guide for the full comparison.

How many days of autonomy should a home battery provide? Most designs target 1 day for a solar-paired critical-loads system, since the array recharges the battery the next sunny day. Homes without solar, or in regions with multi-day storm outages, often target 2 to 3 days.

Does adding a battery affect my existing solar interconnection? Yes. NEC 705.12 requires the busbar calculation to count every power source, including an AC-coupled battery inverter, which frequently changes the 120 percent rule math from what was approved for PV alone. This is one of the most common reasons retrofit storage permits get rejected on first submittal.

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Sources: U.S. Energy Information Administration, residential electricity consumption data (2024), NFPA 855, Standard for the Installation of Stationary Energy Storage Systems, UL 9540, Standard for Energy Storage Systems and Equipment, NFPA, National Electrical Code Article 706, SEIA/Wood Mackenzie U.S. Solar Market Insight, storage attach-rate data.