The Last Five Percent: Testing the Feasibility of a Fully Emissions-Free Grid
A recent analysis by NYISO’s independent Market Monitoring Unit shows that moving from a 95% emissions-free electric system to 100% would require a disproportionate increase in cost and construction, making the last five percentage points the most difficult part of the transition.
The report by Potomac Economics also shows that while reaching a 90% or 95% emissions-free system would still require a major buildout, it would require less new construction and would cost substantially less than eliminating the final share of fossil-fueled generation.
In compiling the report, Potomac reviewed the scenarios in NYISO’s 2025–2044 System & Resource Outlook, which examines how different combinations of demand, public policy, generation, and transmission could shape the grid in the future.
In the 95% scenario, existing or repowered fossil-fueled generators could operate on a limited basis during the relatively few hours when demand is highest all year and wind, solar, and batteries cannot fully meet the need.
Because these generators would run infrequently, the system could achieve deep emissions reductions while retaining dependable capacity for extended periods of system stress.
The 100% decarbonization scenario does not have that option. To keep the lights on and meet consumer demand without fossil generation, New York would need to build enough hydrogen-fueled generation or energy storage to provide the same dependable power those plants supply today. Furthermore, the future system requires resources capable of supplying electricity through longer periods of low renewable output. However, the hydrogen-fueled generation and long-duration energy storage needed to perform that role are not yet commercially available in New York at the required scale.
Potomac estimated that, by 2044, annualized investment, fixed, and production costs in the 100% case would be about $15 billion higher each year than in the comparable 95% case, measured in 2025 dollars. It calculated an incremental cost of about $1,500 for each additional ton of carbon dioxide avoided in moving from 95% to 100%. These are modeling results, not forecasts of customer bills, but they show how sharply costs rise when the system is required to eliminate all remaining emissions with the technologies assumed in the study. At a time when energy affordability is becoming as important as the pace of the transition, it cannot be assumed that technology, construction, and financing costs will decline quickly enough to offset an expansion of this scale.
The construction challenge is just as significant. NYISO’s Outlook finds that a highly decarbonized system will require major additions of wind, solar, storage, transmission, as well as firm capacity. In one illustrative high-demand scenario for 2035, meeting a 100% emissions-free requirement would involve approximately 30 gigawatts of front-of-the-meter solar, 14.9 gigawatts of land-based wind, 4.6 gigawatts of offshore wind, and 6.3 gigawatts of energy storage. While not a construction forecast, this demonstrates the scale and pace of development that a high-demand, zero-emissions system could require.
The NYISO Outlook analysis matters for current and future public policy. A 90% or 95% emissions-free system would still require a historic buildout of clean generation and transmission, but it would leave room for dispatchable generation to protect reliability when other resources are unavailable. Preserving greater operational flexibility reduces the amount of costly backup infrastructure that must be built and maintained for a small number of critical peak-demand hours. In an age of energy affordability, each additional investment must therefore be weighed against the reliability benefit and emissions reduction it delivers. A plan that is technically possible but cannot be paid for by customers is not a durable path for the state’s economy or its clean-energy goals.
A 90% or 95% target could provide a more achievable path to deep emissions reductions while allowing time for emerging technologies to develop, supply chains to expand, projects to be permitted, and operational experience with weather-dependent resources to grow.
The immediate priority is to make measurable progress with the resources New York can build now. That means accelerating renewable generation, storage, transmission, efficiency, and flexible demand while maintaining enough dependable capacity. In some locations, repowering aging plants with newer, more efficient equipment could reduce emissions and preserve essential reliability capability until non-emitting replacements are ready.
The practical objective should be to achieve the greatest feasible emissions reduction while maintaining reliability and affordability. The analysis suggests that reaching 90% or 95% could deliver substantial environmental benefits with fewer unproven technologies and less costly construction, while preserving a path to deeper carbon reductions as dependable emissions-free resources become commercially available.