It is called marginal pricing the wholesale electricity market uses which determines the spot price of electricity. It is based on the most expensive form of generation, typically gas, needed to meet demand regardless of how little is required. So here in the UK if 99% is generated by low cost renewable and only 1% is generated by expensive gas it all gets priced at the rate for the gas. Absolute madness when you live in a nation like the UK where all electricity retailers are getting their power from one national network operator.