The closest we have is buying green energy in blocks, which means you reserve that much generation capacity. In theory, they have to build more capacity if demand outstrips suooly, but if they produce more than is reserved, they just sell at the normal (lower) rate. If you use less than you reserve, you just pay more.

It's a wonky system and I'd prefer to choose by provider instead. At least our electricity provider has to ask the state legislature for permission to raise prices, so that's nice. Energy here isn't all that expensive (around the nationwide median) and moving toward green energy, but I think I'd prefer a more competitive system.