Sizes: From a Garage Wall to a 1,200 MWh Plant

The same chemistry sits in a garage-wall unit and in a 300 MW plant. What changes is how much energy is in one place. That number, in kilowatt-hours or megawatt-hours, is the best single measure of how much can burn, how long a fire can run and how far a county's emergency plan has to reach.

Quick refresher from the storage lesson: power (kW or MW) is how fast; energy (kWh or MWh) is how much; energy divided by power is duration in hours. One MWh is 1,000 kWh.

The size ladder

EIA's 2023 data puts the average U.S. large-scale battery at 27.9 MW and 76.2 MWh. Averages hide a spread. California's batteries averaged about 50 MW and 169 MWh; New England's averaged about 3 MW. Single sites can be far larger than the average: Moss Landing 300 alone held 1,200 MWh.

Where the fire code draws lines

Idaho enforces the 2018 International Fire Code statewide; the State Fire Marshal lists it as the adopted edition. Its Section 1206 sets the numbers worth memorizing.

• 20 kWh: the threshold quantity for lithium batteries of all types (Table 1206.2). Above it, the stationary storage battery rules apply: permits, construction documents, spacing, detection, suppression and more. Below it, those rules do not apply. Commentary on the 2026 NFPA 855 notes the same rough line, with lithium-ion systems under about 20 kWh possibly falling outside that standard.
• 600 kWh: the maximum allowable quantity of lithium batteries in one fire area of a building (Table 1206.2.9). Beyond it, the space becomes a high-hazard Group H-2 occupancy with much stricter construction rules.

Put those next to the plants in the table. Moss Landing 300 held 1,200 MWh, which is 1,200,000 kWh, inside one large building. That is 2,000 times the 600 kWh indoor limit for an ordinary fire area. Large indoor systems are built under the high-hazard rules or other approvals, and EPRI's October 2026 report found indoor facilities overrepresented in failures and recommended fire walls or other barriers to break up large indoor sites. The fire and siting lessons later in this course come back to what Idaho's 2018 code does and does not cover for outdoor utility plants.

Nameplate, usable and degraded

More than one number gets called "the MWh."

• Nameplate (rated) energy: what the cells are rated to hold.
• Usable energy: what the system lets you draw. The battery management system keeps a margin at the top and bottom of charge to protect the cells.
• Energy at the meter: usable energy minus losses in inverters and transformers, and lower still as cells age.

Developers often overbuild the DC battery so the AC output still meets the contract after years of fade, then add modules later. So the stored energy on the site can be larger than the contract number. For fire planning, the number that matters is the total energy in the cells, the largest of the three.

Duration:            800 MWh / 200 MW   = 4 hours
Containers:          800 MWh / 5 MWh    = 160 containers
Energy per acre:     800 MWh / 25 acres = 32 MWh per acre
In kWh:              800 MWh x 1,000    = 800,000 kWh

Compare with the 2018 IFC lines:
  800,000 kWh / 20 kWh  = 40,000 times the threshold
  Per container: 5,000 kWh / 600 kWh = about 8 times
  the indoor fire-area limit, in one walk-in box

Compare with a home unit of, say, 13.5 kWh:
  800,000 / 13.5 = about 59,000 home batteries

Scale and the size of an incident

Bigger sites are not automatically more likely to fail per MWh; EPRI's data points to design, age and enclosure. But scale sets the ceiling on a bad day. A one-container fire at a 160-container site may be handled by letting that box burn while cooling its neighbors. At Moss Landing, EPA estimates about 56,000 of roughly 100,000 modules burned in January 2025. That is the difference between hours of smoke and a cleanup EPA describes as the largest lithium-ion battery cleanup in its history.

Small systems are not trivial either. The 2 MW / 8 MWh battery near Melba burned for days, with roads closed and the sheriff's office saying the recommended response was to let it burn out, according to KIVI's October 2023 coverage. A 5 MW / 40 MWh system at East Hampton closed roads and stopped trains for about an hour, per EPRI's entry.