What is RTO/ISO?
Seven nonprofit organizations operate the transmission network and run the wholesale markets across about two thirds of US electricity demand. They own none of the equipment they control.
An operator that owns nothing
When federal rules required transmission owners to carry competitors’ power on equal terms, the obvious conflict was that the owner still decided who got access. The answer was to hand operational control to an independent body with no financial stake in any plant. An independent system operator dispatches the generators, runs the market and manages the flows; the utilities keep the wires, maintain them, and earn a regulated return on them.
A regional transmission organization is the same institution with a wider remit, taking on regional planning and cost allocation across several states. In practice the two terms are used interchangeably, and the differences between any two of them are larger than the difference between the labels.
They do
Dispatch and balance
Decide which plants run in each five-minute interval and keep the region’s supply and demand matched.
They do
Run the markets
Clear day-ahead and real-time energy, reserves, and in most regions a forward capacity auction.
They do
Study and plan
Process interconnection requests and produce the regional transmission plan that determines what gets built and who pays.
They do not
Own assets or build
No plants, no lines, no retail customers. They cannot order a plant built, only identify the need and run the process.
The seven US operators
| Operator | Footprint | Peak demand | Distinguishing feature |
|---|---|---|---|
| PJM | Thirteen states from Illinois to New Jersey, plus DC | ~150 GW | The largest market in North America, and the epicenter of the data center load surge |
| MISO | Fifteen states from Manitoba’s border to the Gulf | ~125 GW | North–south span of 1,700 miles; a large multi-value transmission portfolio |
| ERCOT | Most of Texas | ~85 GW | Energy-only market with no capacity auction; its own interconnection, outside most federal jurisdiction |
| SPP | Fourteen states across the central plains | ~55 GW | The highest wind share of any US market; now extending services into the West |
| CAISO | Most of California and part of Nevada | ~50 GW | Runs the Western Energy Imbalance Market for utilities across the wider West |
| NYISO | New York State | ~32 GW | A single state, sharply divided between upstate generation and downstate load |
| ISO-NE | The six New England states | ~25 GW | Winter reliability depends on gas pipeline capacity shared with home heating |
Where there is no operator
The Southeast and most of the West never joined one. There, vertically integrated utilities run their own control areas, plan their own resources through state processes, and trade with neighbors bilaterally. Utilities in those regions point to low rates and direct accountability to state commissions; critics point to the cost of every utility holding its own reserves rather than sharing them across a wider footprint.
The gap has been closing at the edges rather than through new memberships. Western utilities have joined real-time imbalance markets that let a shortfall in one state be covered by surplus in another without surrendering planning authority, and both CAISO and SPP now offer day-ahead versions of the same idea. Whether that becomes a full western market is one of the live questions in US grid policy.
The Southeast
Large integrated utilities and TVA, coordinating through a voluntary energy exchange rather than a market.
The Northwest and Mountain West
Dozens of separate balancing authorities, now increasingly linked by real-time and day-ahead imbalance markets.
Alaska and Hawaii
Island and isolated systems with no interstate connection, and no basis for a regional market.
Why this matters for new generation
For anyone trying to build a power plant, the regional operator is the gatekeeper. It runs the interconnection queue that determines how long a project waits and what network upgrades it must pay for, and its market design determines what the plant can earn once it is running. Queue backlogs measured in years, and cost allocations that can exceed the cost of the plant itself, are now a larger obstacle to new capacity than construction.
Federal reforms have pushed operators to study projects in clusters instead of one at a time and to plan transmission for the long term rather than project by project. The results are arriving slowly, and regional differences in how the rules are implemented are becoming a significant factor in where new generation, including new nuclear, actually gets built.