For Phase II compliance the options were numerous, but for Phase I they were constrained by the time available to implement a decision. Because it takes 3–5 years to design and build a scrubber at an existing coal-fired unit, and longer to repower or build a new facility (e.g., 6–11 years for coal, 10–14 years for nuclear units),[15] electric utility decision options for Phase I plants were limited to scrubbing, switching fuels, purchasing or transferring emissions allowances to allow continued use of high-sulfur coal, retiring units, or trimming unit utilization and substituting capacity from another source.