> Edit: also why don’t they just mine some coins?

Nobody is going to pay for mining rigs that are idle 99% of the time.

If it happened enough you'd have people set up cheap resistor banks to eat all the excess, so that provides a natural cap on how much negative pricing there can be.

While subsidies are one cause, another cause is demand response problems:

Imagine you have a greenhouse factory producing strawberries. The strawberries come out of the factory on a conveyor belt. Now, you can't just turn off this factory easily since it might take a week to get it up running again and you don't have any storage, so what happens when nobody wants your strawberries for a day?

You pay someone to take them.

And yes, it happens to physical products too, e.g. household electronics - you need to pay someone to scrap them.

Because the German government is paying for renewables, via a tax on electricity, even when there is no additional demand.

They can't turn off power plants, because the renewables are ephemeral.

This happens when the electricity supply is higher than the demand. It only affects the wholesale rate, not consumer rates (at least in Uk and US)

Normally supply and demand need to be matched to obtain a balance. If the supply is not enough for demand then the frequency drops below 50Hz, if it drops too low bad things happen.

The renewable generators get a subsidy for each kWh that they pump on to the grid. The logical thing to do if electricity were oversupplied, i.e. you have to pay people to take it, would be to temporarily shut down some renewable generation, but then the renewable providers would lose out on the subsidy for those kWh.

The electricity rate only for short-term traded electricity goes negative. Much of the volume is on long-term contracts.

You have too much energy and you need to get rid off it.