Hmmm... Yes, well, it's a difficult problem, and most guests don't monitor their own consumption, and wouldn't expect to have it monitored in their STR.
We also have a large space, heated with gas, which here in Spain, is very expensive (as is electricity, but we have solar power.. Different set of issues).
The main living area of 80 m2 also has ceiling of 4-5m. That's over 350 cubic meters of airspace that needs to be heated (not including bedrooms) Granted, it doesn't get that cold here, but it can get near freezing on some nights. We need heat in winter.
Many guests live in somewhat average apartments. 2 bed, 2 bath, 2.5m high ceilings, maybe 70 cubic meters of airspace in the main living area. Comparatively easy to heat to 25°c, and that's what they expect to get. They simply don't understand that our cost of heating is 5-10x what they're used to. The costs can approach what they pay for their entire stay.
So, one solution is simply to jack the prices up and deliver their 25°c, if that's what they must have. But then, it makes the price appear abnormally high.
Basing it on consumption makes sense because guests are inclined to be careful about consumption, but that adds the burden of measurement, and we don't really have that facility, and getting a meter installed turns out to be a huge regulatory ordeal and ridiculously expensive (we have a large propane tank, not measured city gas).
So, we've simply created a flat rate energy surcharge of 15€ per day. It sometimes covers it, often not, but it helps a lot. I think 15€ is probably the limit we can charge without guests getting too testy about it. But so far, it's been without much resistance. A few question it, but when we point out the size of the living space, they seem to understand. Reluctantly.
I don't think there's an ideal way to cover this, but this is the best we've been able to come up with so far.
Really, it's all in the psychology of the guest. Some get it, others don't. Something we just have to navigate on a per case basis.