I think my mining is complete free
rigs are paid back themselves years ago and every W they generate will be part of winter heating

Yes, with Winter coming the effective rate for power is even cheaper if you can use mining heat to offset your normal heating bill. Unfortunately, I usually have a surplus of heat, even in the Winter, so electricity is still a "cost" for me. I need to figure out a way to harness all this heat for other useful purposes, like heating my water.