What Grid Batteries Do and Who Pays Them

A battery energy storage system, or BESS, makes no electricity. It buys power at one moment and sells it at another, or it stands ready and gets paid for standing ready. Every grid battery is built around one or more of those jobs, and the job decides how big it is, how many hours it holds, where it sits and how much energy is packed onto the site.

The batteries and storage lesson already covers the two ratings (MW and MWh), round-trip efficiency and a full day of arbitrage arithmetic. This course picks up from there. Start with the menu of jobs.

The services a battery sells

Frequency regulation is the one most people have never heard of and the one batteries are best at. Power plants and load never match exactly, so frequency drifts. A battery's inverter can swing from charging to discharging in a fraction of a second, which a gas turbine cannot. The balancing lesson explains why the grid has to match supply and demand every moment.

What U.S. batteries actually do

Every large battery in the U.S. reports a primary application to EIA. The 2023 final data, published in the figure workbook for EIA's March 2026 market trends report, looks like this for power capacity:

These are primary uses only, and most batteries do more than one thing. And EIA's capacity numbers are a snapshot of 2023; the fleet has more than doubled since (the storage lesson has EIA's 2025 and 2026 additions).

The same workbook shows how the job shapes the hardware. ERCOT batteries averaged about 1.3 hours of duration and California's about 3.1 hours. U.S. batteries installed in 2015 averaged 0.6 hours; those installed in 2023 averaged 2.8. Early batteries chased fast services that need power, not energy. As solar grew and capacity rules rewarded four hours, the industry built longer.

Pairing tells a similar story. In ERCOT about 3,111 of 3,217 MW of battery capacity was independent, meaning not paired with any solar or wind plant. In CAISO about 4,328 MW was independent and about 3,209 MW was AC-coupled to a solar or wind plant. A battery next to a solar farm can still charge from the grid, and a standalone battery can still shift solar. Whether a given project is limited to charging from its paired plant depends on its contracts and, in some cases, tax credit rules, so ask.

FERC opened the door

FERC Order No. 841, issued February 15, 2018, required the regional grid operators that run organized markets to let storage take part in them. That is a big reason batteries piled into California and Texas wholesale markets first. Where there is no organized market, as across most of the inland West, batteries are usually bought by a vertically integrated utility through its resource plan and a utility commission case.

Idaho Power is that kind of utility. Its first large batteries, an 80 MW system at the Hemingway substation and a 40 MW / 160 MWh system next to the Black Mesa solar project, were chosen to cover a capacity shortfall the company projected for summer 2023, with 60 MW more planned at the Franklin Solar site, according to Utility Dive's March 2023 report. That is the capacity job: be there on the hottest evening.

Revenue stacking, and what it tells you

A merchant battery owner tries to stack revenues: capacity payments for being available at peak, regulation payments for fast response, and arbitrage on the days the price spread is wide. The stack decides the design. Regulation pays for megawatts and speed, so a regulation battery can be short. Capacity and arbitrage pay for energy delivered across the peak, so they need hours.

That is why the purpose a developer states at a public hearing is worth testing against the hardware in the application. Residents opposing large projects make a pointed version of this argument: grid support services such as regulation and voltage control can be met with smaller systems near substations, so a project with hundreds of MWh is mainly a trading and capacity asset, and its owner should explain why it needs to sit on a particular parcel. Developers answer that shifting and capacity are exactly what a grid adding solar needs, that four-hour batteries replace peaking plants, and that sites are chosen by available interconnection capacity at a substation or line, which is scarce and not movable. Both claims can be checked against the interconnection request, the power purchase agreement (if one exists and is public) and the utility's resource plan.

Proposal A: 150 MW / 150 MWh (1-hour)
  Covers 150 MW for 150 / 150 = 1 hour.
  Leaves 2 of the 3 peak hours uncovered.
  Good for regulation and fast reserves.

Proposal B: 150 MW / 600 MWh (4-hour)
  Covers 150 MW for 600 / 150 = 4 hours.
  Energy needed for the 3-hour peak: 150 x 3 = 450 MWh.
  Spare: 150 MWh, usable for arbitrage on other days.
  Grid energy to recharge 450 MWh at 85% round trip:
    450 / 0.85 = about 529 MWh.

Batteries at data centers

Data centers have long used batteries in their uninterruptible power supplies to carry servers for minutes until generators start; the data center power systems lesson covers that. Newer proposals pair large data center loads with grid-scale BESS for capacity or to smooth demand. Battery rooms inside buildings carry their own fire record: EPRI's database lists a May 22, 2025 fire in the battery room of a data center in Hillsboro, Oregon, extinguished after five hours.