Most hydroelectric turbines in the US can go from cold start to full operations in less than 10 minutes, which is a faster start-up time than any other US electric generators, according to the US Energy Information Administration.

EIA said about 25% of US power plants can start up — going from being shut down to fully operating — within one hour, based on data collected in its annual survey of electric generators. The time it takes a power plant to reach full operations can affect the reliability and operations of the electric grid. Some power plants, especially those powered by coal and nuclear fuel, require more than half a day to reach full operations.

Generator startup time differs because of the differences in the complexity of electricity generating processes, especially when starting again after all processes have been stopped (cold shut down). A generator’s startup time is different from a generator’s ramp rate, which reflects how quickly that generator can modify its power output once it’s operating.

After hydroelectric turbines, combustion turbines (which use a combusted fuel-air mixture to spin a turbine) are also relatively fast to start up.

Steam turbines often require more time as the steam needs to reach certain temperature, pressure and moisture content thresholds before it can be directed to a turbine that can spin the electricity generator.

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Nuclear power plants use steam turbines, but these plants have additional time-intensive processes that involve managing their nuclear fuel. Almost all nuclear power plants require more than 12 hours to reach full operations. Power plants that require more than 12 hours to start up are increasingly rare. Only 4% of the generating capacity that came online from 2010 to 2019 requires more than half a day to reach full load.

Natural gas combined-cycle systems, which involve both a steam turbine and a combustion turbine, account for more capacity than any other generating technology in the US Most of those systems can reach full operations in one hour to 12 hours, although some can start up within an hour.

The percentage of the generator fleet that does not respond to this question in EIA’s survey has doubled — from 6% in 2013, when EIA first collected this data, to 12% in 2019 — as a result of the number of utility-scale solar and wind power plants added in recent years. This question is not relevant for these types of plants.

This article was originally published on our sister portal HYDRO REVIEW.