Your understanding is mostly wrong then. Modern combined-cycle turbines are more efficient but I would not call them cheap to stop and start. You are still looking at a 20-30min startup time and a similar cool-down period. So yes designed to be able to shutdown I think the general range is an on-time of 30-70% but I believe with those ranges you get different efficiency curves. You get increased wear, higher fuel cost and also need to predict that demand will not spike. When prices dip below zero it may be more cost effective to keep the plants online.