It mentions, don’t use electricity. In my case I have solar power and I have multiple airco units that can heat up just fine too. Simple things like, during day open window for fresh air but close them before it becomes late because it will become too cold inside too.
So now i only heat up the living room for a small part with Airco unit, once the kids are to school (or in bed at night) i can turn it off and i don’t heat up the rest of the house (where you sleep it is fine if it is colder) i just need to wait and see how cold it needs to get untill i have to turn on the gas heating.