When R328 applies
Section R328 of the International Residential Code covers energy storage systems in one- and two-family dwellings and townhouses. It first appeared in the 2021 IRC and was revised in the 2024 edition. Many states amend it — California’s Residential and Fire Codes, for instance, differ in places — so check which edition and amendments your jurisdiction has adopted. The summary below follows the 2021 IRC.
The 2021 edition exempts systems marked “For use in residential dwelling units” that are installed per the manufacturer’s instructions and NFPA 70. Many jurisdictions still check the location and limits below on every battery, so plan for them.
Listing and spacing
- Listing (R328.2): the ESS must be listed and labeled to UL 9540.
- Spacing (R328.3.1): individual units at least 3 ft apart, unless the manufacturer’s large-scale fire testing (UL 9540A) supports a smaller separation.
- Inverters (R328.6): listed to UL 1741, or part of the UL 9540 listing.
Where a battery can go
R328.4 allows four kinds of location:
- Detached garages and detached accessory structures
- Attached garages separated from the dwelling as R302.6 requires
- Outdoors or on an exterior wall, at least 3 ft from doors and windows that open directly into the dwelling
- Enclosed utility closets, basements, storage or utility spaces with finished or noncombustible walls and ceilings; unfinished wood framing needs at least 5/8 in. Type X gypsum board
Not allowed: sleeping rooms, and closets or spaces that open directly into sleeping rooms.
How much storage is allowed
| Location | Aggregate limit |
|---|---|
| Utility closets, basements, storage or utility spaces | 40 kWh |
| Attached or detached garages, detached accessory structures | 80 kWh |
| Outdoors on exterior walls | 80 kWh |
| Outdoors on the ground | 80 kWh |
No individual unit may exceed 20 kWh (R328.5). Above these limits the installation falls under the International Fire Code’s energy storage provisions instead.
A practical example: three 13.5 kWh units (40.5 kWh) are over the 40 kWh limit for a utility closet, but fit on an exterior wall or in a garage.
Detection, impact and ventilation
- Fire detection (R328.7): smoke alarms per R314 in rooms with an ESS; where a smoke alarm can’t be installed, such as a garage, a listed heat detector interconnected to the smoke alarms.
- Vehicle impact (R328.8): a battery where a car could hit it needs approved barriers, such as bollards or wheel stops.
- Ventilation (R328.9): required for batteries that can produce hydrogen or other flammable gases during charging.
What to show on the plan set
- The battery’s location on the site plan and an equipment elevation with the 3 ft clearance from doors and windows dimensioned
- Aggregate kWh by location against the table above, and the unit size
- The UL 9540 listing on the spec sheet, and the UL 9540A report where you rely on reduced spacing
- Wall and ceiling protection for indoor locations, vehicle barriers in garages, and the heat detector
- The electrical side under NEC 706: the ESS disconnect, labels and placards, and for backup, the transfer equipment under 710.15
How Ballast Solar checks storage placement
Ballast Solar checks battery placement against R328 before the set is released and flags anything over a limit, so the reviewer doesn’t have to. Pair this with the plan set checklist and, for backfed batteries, the 120% rule.
Questions
How many kWh of battery storage can go in a house?
Under the 2021 IRC R328.5, up to 40 kWh in a utility closet, basement or storage space, and up to 80 kWh in a garage, on an exterior wall or outdoors on the ground, with no single unit over 20 kWh. Local amendments can differ.
Can a home battery go in a garage?
Yes. Attached garages must be separated from the dwelling per R302.6, batteries where a car could hit them need barriers, and a heat detector interconnected to the smoke alarms is required where a smoke alarm can’t be installed.
How far must a battery be from doors and windows?
At least 3 feet from doors and windows that open directly into the dwelling when it is installed outdoors or on an exterior wall.
Can a battery go in a bedroom closet?
No. R328 prohibits energy storage systems in sleeping rooms and in closets or spaces that open directly into sleeping rooms.
This guide explains the code for planning purposes. The adopted edition, local amendments and the AHJ’s interpretation govern; the contractor of record is responsible for the design.