For stored power systems ~ and dammed hydro is stored power, its stored before conversion to electric power ~ paying for dispatchability is not precisely "idle capacity" ~ its capacity has both a total power stored dimension as well as a throughput dimension.

Quick dispatch from thermal power inside the power up cycle is spinning reserve, which loses ground to stored power with either increases in the cost of fuel or decreases in the cost per kWh storage capacity of stored power.

I've been accused of being a Marxist, yet while Harpo's my favourite, it's Groucho I'm always quoting. Odd, that.