Batteries and Storage: Power, Energy, Duration and Fire

Storage does not make electricity. It moves electricity from one hour to another. That job used to belong almost entirely to pumped hydro. Now it belongs to rows of shipping-container-sized lithium-ion battery cabinets. EIA reported a record 15 GW of utility-scale batteries added in 2025 and 24 GW planned for 2026, more than half of that in Texas. The California ISO's grid went from 8.0 GW of batteries in 2023 to 11.6 GW in 2024.

Two numbers, always

Every storage system has two ratings, and press releases and news coverage mix them up constantly.

• Power (MW): how fast it can push electricity out or take it in. Think of the width of a pipe.
• Energy (MWh): how much it holds. Think of the size of the tank.

Duration (hours) = energy (MWh) / power (MW)

200 MW / 800 MWh  = 4-hour battery
200 MW / 400 MWh  = 2-hour battery
200 MW / 20,000 MWh = 100-hour battery

An announcement that says "a 200-megawatt battery" has told you half of what you need. Ask for the MWh. Most new U.S. grid batteries are built around four hours, partly because several grid operators' rules for counting resources toward peak demand have rewarded four hours, and partly because four hours covers the steep evening ramp after sunset.

Lithium-ion chemistries

Inside a lithium-ion cell, lithium ions shuttle between a negative electrode (usually graphite) and a positive electrode through a liquid electrolyte. Charging pushes ions into the graphite; discharging lets them flow back, driving electrons through the outside circuit. The "chemistry" name usually refers to the positive electrode.

Grid batteries do not care much about weight, so the industry has shifted heavily to LFP. BloombergNEF's December 2025 survey put the average lithium-ion pack at $108/kWh across all uses and stationary storage packs at $70/kWh, 45 percent cheaper than a year earlier. Pack price is not project price: inverters, transformers, enclosures, fire systems, land, interconnection and installation add a lot. And prices can move the other way. Lazard's 2026 report found utility storage costs rose that year, reversing earlier declines, citing new tariffs on imported cells and federal "foreign entity of concern" rules pushing supply chains away from China.

Round-trip efficiency

Some energy turns to heat on the way in and on the way out. EIA measured an 82 percent average monthly round-trip efficiency for U.S. utility-scale batteries in 2019, against 79 percent for pumped storage.

Charge at midday:  400 MWh / 0.85 = about 471 MWh bought
Price at midday:   $15/MWh  x 471 = $7,065
Discharge evening: 400 MWh sold at $110/MWh = $44,000
Gross spread:      $44,000 - $7,065 = $36,935 for the day

What storage costs per MWh

EIA's 2026 levelized cost report estimates a four-hour battery entering service in 2031 at about $190 per MWh discharged before tax credits and about $153 with them, in 2025 dollars, assuming it cycles enough to reach a 10 percent capacity factor. That looks expensive next to a solar MWh, and it is the wrong comparison. A battery sells evening hours, which are often worth several times midday hours. The integration and cost lessons of this module return to this point.

Safety and fire

Lithium-ion cells can fail through manufacturing defects, physical damage, overcharging or overheating. A failing cell can enter thermal runaway: it heats, vents flammable and toxic gases (including hydrogen fluoride), and passes heat to its neighbors. Modern systems use cell-level monitoring, spacing between enclosures, gas detection, deflagration vents and fire barriers designed under the National Fire Protection Association's NFPA 855 standard, which many states and localities have adopted into code.

What the data says

The Electric Power Research Institute keeps a public database of grid battery failure events. Its report released in October 2026 found that annual failures per deployed GWh fell 98 percent worldwide from 2018 to 2025, even as new installations passed 300 GWh in 2025 alone, according to Energy-Storage.news coverage of the report as updated with EPRI's clarification. EPRI said battery system failure rates are now comparable to large oil-filled transformers. It found indoor facilities overrepresented in failures and newer systems failing less often, and it could not separate chemistry from design maturity as a cause.

The case everyone asks about

On January 16, 2025, fire broke out at Vistra's 300 MW / 1,200 MWh Moss Landing 300 system in California. EPRI's database notes evacuation orders, closed roads and a highway, and a one-day closure of local schools. Firefighters let it burn out. EPA says the building held about 100,000 lithium-ion modules and roughly 55 percent were damaged. Air monitoring by EPA, the local air district and Vistra found hydrogen fluoride below acute exposure thresholds, while Moss Landing Marine Laboratories reported nickel, cobalt and manganese in nearby soil at roughly 100 to 1,000 times normal levels. As of EPA's July 2026 fact sheet, recyclable batteries had been removed and crews were demolishing the building and removing the most damaged modules.

Both sides of a local battery fight cite Moss Landing. Opponents use it to show what can go wrong. Supporters point out that it was an older system packed inside a big building, a design modern outdoor containerized projects avoid, and that the same EPRI data shows failures falling sharply. A careful account at a hearing or online says both: the risk is real and has dropped, and the details of a specific project's design decide how much of that risk a community carries.

Other ways to store power