The ISOs and RTOs

What ISOs and RTOs actually do

An Independent System Operator (ISO) or Regional Transmission Organization (RTO) has two core jobs: operate the high-voltage transmission system in real time to keep the grid balanced and reliable, and run the wholesale electricity markets where generators and buyers transact. They're non-profit entities regulated by FERC, governed by stakeholder boards, and funded through transmission access fees.

The distinction between ISO and RTO is largely administrative. All RTOs are ISOs; RTOs meet additional FERC requirements around transmission access and governance. In practice, the terms are used interchangeably — PJM is technically an RTO, CAISO is technically an ISO, but both run markets and operate grids in essentially the same way.

The major ISOs at a glance

PJM Interconnection is the largest competitive electricity market in the world, serving 65 million people across 13 states and DC. It operates robust day-ahead and real-time energy markets, a capacity market (the RPM/BRA process), and well-developed ancillary service markets. The PJM capacity market has historically been one of the most important revenue streams for generators in the region.

CAISO serves most of California and operates the Western Energy Imbalance Market (EIM), which coordinates real-time balancing across much of the western US without full RTO formation. CAISO is characterized by high renewable penetration, the famous duck curve, significant curtailment challenges, and active development of new market products for distributed energy resources under FERC Order 2222.

ERCOT is unique: it operates almost entirely within Texas, is not interconnected with the rest of the continental grid, and is therefore outside FERC jurisdiction. ERCOT is an energy-only market — no capacity market — relying on scarcity pricing via the Operating Reserve Demand Curve (ORDC) to incentivize investment. The February 2021 winter storm exposed significant vulnerabilities in this design.

MISO spans the Midwest and parts of the South, operating both energy and capacity markets across a geographically large and diverse footprint. ISO-NE serves the six New England states with notably high electricity prices driven by constrained natural gas supply. NYISO serves New York state with distinct pricing zones reflecting transmission constraints between upstate and downstate. SPP covers the central plains from North Dakota to Oklahoma.

What non-ISO regions look like

In non-ISO regions — the Southeast, Pacific Northwest, and parts of the Southwest — vertically integrated utilities own generation, transmission, and distribution and serve customers at regulated rates set by state utility commissions. There are no competitive wholesale markets in the same sense; power is traded bilaterally between utilities at negotiated prices. For large C&I customers in these regions, procurement options look very different: fewer transparent market signals, more reliance on utility green tariffs, and limited demand response programs.

Why your ISO matters for procurement

Your ISO determines which market products are available to you — capacity market participation for demand response, which ancillary services you can provide, how your renewable energy certificates are structured, and what load management programs exist. A C&I customer in PJM has access to one of the deepest demand response markets in the world; the same customer in a Southeast non-ISO territory has far fewer options. Understanding your ISO is the starting point for any serious energy strategy.