The only reason power costs go less than zero is the contracts that the green (primarily wind) generators have that guarantee a minimum price for their power re

The only reason power costs go less than zero is the contracts that the green (primarily wind) generators have that guarantee a minimum price for their power regardless of time of supply. This creates an incentive for them to keep producing electricity beyond what people are currently willing to pay for it. These minimums have been required to fund the capital expenditures to build the plants and have generally been funded by government subsidies. Coal plants have a different incentive structure - they are very hard to bring online/offline quickly ("base load"). Natural gas plants are _relatively_ easy to spin up/down. I expect this will resolve itself in changes to energy storage and contracts as the technologies involved become more pervasive.