Green Energy

A negative price is almost always walled on both ends by solid positive prices. And a negative price does not mean a negative transmission fee. The price of producing electricity and the price of delivering it are separate things, at least in Europe. You'd still pay a little to the grid operator (who is typically forbidden from being an energy producer, except in emergencies) to receive the surplus energy.

Over here, negative prices typically happen when the weekend (industry is resting) is both sunny and windy.

During weekdays and nights, prices quickly turn positive again.

I would describe it as an incentive to store energy. If a storage plant has capacity, it will be pumping water uphill, charging batteries, heating a thermal store or making hydrogen / methane / whatever it makes (alas, there are very few such plants here). It will continue until there are many, so they can absorb the surplus.

Until then, consumers charge their cars (alas, only 1% of cars) and heat their sauna when it happens.