Developers and buyers often treat four hours as the default battery configuration. In practice, the right duration is market-specific. The same 100 MW project can be worth more as a 2-hour system in one ISO and as a 4-hour system in another, and the difference shows up directly in valuation.
Each market answers that question differently, depending on how it pays for storage, how its price peaks are shaped, how capacity is accredited, and what interconnection costs. Five drivers decide the outcome.
Price shape sets the arbitrage case
Arbitrage value depends on how wide the peak is, not just how high. Where scarcity arrives in short spikes, the first one or two hours of discharge capture most of the spread. Where solar creates a long evening ramp, hours three and four still discharge into high prices.
Short, spiky scarcity
Hours 3–4 discharge into prices barely above the daily floor.
Solar-driven evening peak
A wide net-load peak keeps hours 3–4 well in the money.
The five drivers of optimal duration
| Driver | Favors 2-hour when… | Favors 4-hour when… |
|---|---|---|
| Revenue stack | Ancillary services (regulation, fast frequency, reserves) dominate. These pay for MW, not MWh, so added duration earns little. | Energy arbitrage and capacity dominate, and ancillary markets are saturated with prices compressed. |
| Capacity accreditation | There is no capacity market, or accreditation gives little extra credit for duration. | ELCC or duration-based rules accredit longer systems at a materially higher share of nameplate. |
| Price shape | Scarcity is short and spiky. | Solar stretches the evening net-load peak across several hours. |
| Cost & interconnection | Power-related costs are low relative to cell costs, and interconnection is readily available. | Cell costs are low and the point of interconnection is scarce, so more MWh behind a fixed MW right is valuable. |
| Offtake & operations | The strategy is merchant ancillary services, and heavier cycling is acceptable. | Tolls and resource adequacy contracts are written for a 4-hour product, which makes the asset contractible and financeable. |
How the major U.S. markets lean
| Market | Typical lean | Primary reason |
|---|---|---|
| ERCOT | 1–2 hr, trending longer | An energy-only market with no capacity payment. Early returns were driven by ancillary services. As those prices compressed, value shifted to arbitrage, which rewards more duration. |
| CAISO | 4 hr+ | The resource adequacy framework is built around a 4-hour standard, reinforced by a long solar-driven evening ramp. |
| PJM | 4 hr | ELCC accreditation favors longer duration, and a constrained interconnection queue rewards maximizing MWh per point of interconnection. |
| NYISO | 4 hr+ | Duration-based capacity derating gives shorter systems only partial credit. |
| MISO | 4 hr | Accreditation reform increasingly rewards duration and availability during tight hours. |
The direction of travel is the same everywhere. As storage penetration rises, batteries shave the tallest peaks and leave flatter, wider ones, and short-duration accreditation tends to decline. Both effects push the value-maximizing duration longer over time.
What it means for valuation and M&A
Buyers underwrite duration as closely as they underwrite interconnection. In PJM or NYISO, a 4-hour asset lets a seller lead with capacity revenue and contractability. A 2-hour asset in those markets needs a clear revenue story built on ancillary services and arbitrage, and a credible path to augmentation.
In the model
- Split capex into power ($/kW) and energy ($/kWh) components so the cost of moving from 2 to 4 hours is explicit.
- Build revenue by stream and by hour: ancillary, arbitrage, accredited capacity and contracted.
- Tie degradation and augmentation to the actual cycling profile.
- Measure the marginal return on the added hours, not only each case’s standalone IRR.
In diligence
- What accreditation class does the asset fall in, and where is that value trending?
- What share of forecast revenue is ancillary, arbitrage and capacity?
- Can added MWh be installed later without a new interconnection study?
- Is the duration what target offtakers and buyers actually contract for?
Bottom line. 2-hour storage wins where revenue is ancillary-driven, peaks are short and capacity rules pay no duration premium. 4-hour storage wins where accreditation rewards duration, solar stretches the evening peak, interconnection is the scarce resource and offtakers contract for a 4-hour product.
Market characterizations are directional as of October 2026. Accreditation values, ancillary service pricing and market rules change with each ISO rule cycle. This piece is for informational purposes only and does not constitute investment advice.