Spark and dark spreads indicate improved profitability of natural gas, coal power plants
Data note: The specifics of the calculation methodology are detailed in a previous article with minor adjustments to heat rates used. The heat rate used for the dark spread was 10,500 British thermal units per kilowatthour (Btu/kWh), while the heat rate for the spark spread was 7,000 Btu/kWh.
Higher average daily wholesale electricity prices between January and November 2025 may be improving the operational competitiveness of some natural gas- and coal-fired generators in the PJM Interconnection compared with the same period in 2024. PJM is the largest wholesale electricity market in the United States. The spark and dark spreads, common metrics for estimating the profitability of natural gas- and coal-fired electric generators, have both increased over the past two years.
Since 2023, the dark spread for coal, the difference between the fuel costs for coal-fired generators and the wholesale electricity price, has increased. In 2023, the January through November dark spread averaged negative $14 per megawatthour (MWh), indicating the fuel cost was more than the wholesale price of electricity. The spread increased to nearly $21/MWh in 2025. In 2025, the dark spread increased nearly 250% in the first 11 months of the year compared with the same period during 2024, even though the average price of coal increased 5% over the same period.
The spark spread for natural gas, a similar measure of revenue relative to fuel costs for natural gas-fired generators, increased from a January through November average of $21/MWh in 2023 to $28/MWh in 2025.
Since 2023, the dark spread for coal has increased faster than the spark spread for natural gas. The narrowing of the two spreads indicates that the economics of coal plants have improved relative to that of natural gas plants since 2023. Between January and November 2025, the dark spread averaged $21/MWh, and the spark spread averaged $28/MWh.
Principal contributor: Augustine Kwon
Tags: generation, coal, electricity, natural gas, power plants