A lot of grid is wasted during off peak times. Energy is stored or sold in inefficient manners. Instead, they should use any excess energy in mining bitcoins to sell. Rather than raising electric rates, they can increase their customer base. Their price discovery will be the electricity/btc pair. They can choose which will be most profitable. Storage can then be bought by customers for their needs based on rates. This just seems more efficient than being forced to sell to only one market.
You cannot really store energy, those methods popped up in the renewables sphere mostly, more often only there.
The issue regarding POW mining is that it can absorb any amount of power supply. On the long, we need something else besides that.
I'm not sure what you mean by "absorb any amount of power supply." Mining is voluntary. PoW proves the energy. It's a matter of physics what you do with the work. You can always turn it back into energy.
The problem with selling energy units is counterparty risk, especially with energy market manipulation. I'm not saying we burn all our produced energy, just the energy that the grid wastes over long distances. Energy is easy to store locally, or at least it should be if we want to get away from burning carbon.