If it's using 1/5th the amount of electricity, surely you're seeing a decrease in hashing rates.
Strange enough: That's not the case. It hashes happily on all its ASICS with the normal hashrate.
I use solar as well and I can say that I think sending the overproduced energy to your electric company in order to receive a credit has to be better than just burning the electricity. You could probably pay to run your actual heater with the saved funds and be more comfortable and have more money. Not trying to discourage you, but for $0.04 a day you're probably better off solo mining with that thing and hoping to hit the lottery. That at least has the possibility of getting you a return. I think everyone else would agree though, you're better off just turning that thing off unless you need heat and your heater is broken or something.
I - partly - agree with you. My setting is, though, a bit different.
Overproduction from the roof goes to the electricity grid and I get paid for it. The mining thingies are decentralized. For example: My tractor's generator is defect. Instead of replacing it, I put a solar panel as roof on the tractor. It loads the battery. Once the battery is full, energy goes to a raspi with a SIM card. The raspi is connected to a little pod (50 GH/s).
Then I have a cave under my house. Two solar panels are producing electricity and first filling two batteries. Once full, the solar power is running a dehumidifier which collects the water to a barrel. Once the barrel is full, a pump - powered by solar energy - pumps the water to a cistern. Now all excess power goes into two old Bitmain S1 which hash with crappy 160 GH/s. Their heat is funneld into the cave for condensation. The water goes - you guess it - into the barrel, too. The excess heat is directed out of the cave again.
Then there is a solar loading station for my electric lawn mower. Once its batteries are full, the power goes - you guess it - into mining.
There are several spots on my land like these, they all run old devices. Does it pay off? No. Of course not. But if every solar powered device which is not connected to the grid would produce some GH insstead of heat when not being used, it would be awesome and a great step towards decentralisation of mining (again).