If prices are negative, then all fossil fuel power generation that can be turned off has already been turned off. So you are not helping global warming by consuming the power.
If you were already going to use the heat later in the day when fossil fuels are burning again, then whatever you can do to reduce that future consumption, through storing some thermal heat produced now, can still reduce that fossil fuel consumption overall. Water heaters, warming any living spaces that might need to be heated at night, etc.
It doesn’t even have to be efficient when prices are literally negative. All it has to do is be somewhat effective at reducing later consumption.
If prices are negative, then all fossil fuel power generation that can be turned off has already been turned off. So you are not helping global warming by consuming the power.
If you were already going to use the heat later in the day when fossil fuels are burning again, then whatever you can do to reduce that future consumption, through storing some thermal heat produced now, can still reduce that fossil fuel consumption overall. Water heaters, warming any living spaces that might need to be heated at night, etc.
It doesn’t even have to be efficient when prices are literally negative. All it has to do is be somewhat effective at reducing later consumption.
If prices are going negative, then I would think that people are already doing this all they can.