You make no money when your system fails because it's overloaded. Also, most utilities need to purchase energy from other areas, at a significant premium, during peak usage days.

Most customers pay a fixed rate per kWh, but the cost to the utility is dynamic. When the grid is near capacity, electricity may be sold at an order of magnitude below its marginal cost to the utility.

Here in northern California, our utility (PG&E) is experimenting with "peak day" pricing. During peak hours of certain days, electricity costs increase by ~10x. In return we enjoy a lower baseline rate the rest of the year, encouraging us to shift consumption to off-peak hours. It's a great idea for everyone involved.

This and Uber's "surge pricing" are perfect examples of how technology enables more efficient markets.