Half the time it would be "off" (during summer), so that means I will pay for 100% of the actually used energy? In that case I'll stick with natural gas for my heating. Also this arrangement should not be called "free".
"Can I really heat my home for free using your heater?"
"Amazingly, yes! The Heater measures how much energy it uses to heat your home and you’ll get fully reimbursed for the cost of this energy. This means that with our Heater you’re heating your home for free!"
I can't find anything about a cost sharing agreement, the only cost I can see is the upfront purchase price in the BBC article (although nothing on the nerdalize site itself).