IL SB3606

Amends the Environmental Protection Act to define "heat rate" as the gross energy used by an electric generator or power plant, expressed in British thermal units (Btus), to generate one kilowatt hour (kWh) of electricity, measured using a 12-month average.

Requires electric generating units (EGUs) and large greenhouse gas-emitting units with a heat rate of 7,000 Btus/kWh or greater to reduce CO₂ equivalent emissions by at least 50% from existing emissions by January 1, 2035.

Specifies that the 50% emissions reduction requirement for high heat rate units must be measured using a 12-month gross average in 2034.

Maintains existing deadlines requiring all remaining EGUs and large GHG-emitting units with heat rates at or above 7,000 Btu/kWh to permanently reduce emissions to zero by January 1, 2040.