Advocates of intermittent power such as wind or solar generally suppress discussion of the costs such plants impose on the rest of the grid. Thanks to Peter Lang and John Morgan we have a very up to date analysis of the cycling costs imposed on a typical 500 MW coal plant: Make Your Plant Ready for Cycling Operations.
Aside from cost, there is another important impact of intermittent wind/solar – the inefficient operation of the backup plants increases their GHG emissions. We don’t yet have definitive studies to confirm where the wind power cross-over is – from my reading I’m guessing that marginal new wind power creates more (not less) GHG emissions when penetration gets above 20 to 30%.
UPDATE: please see also the related APTECH study “Integrating Wind: Cost of Cycling Analysis For Xcel Energy’s Harrington Station Unit 3 (phase 1 of 3 part report)”. Unit 3 is a pulverized coal plant.