Hi @Sonja559
Your cover picture is very inviting, I'm sure you have no shortage of guests wanting to book!
The snowy white cover picture also helps me to understand your problem. When I read your post, my first instinct was (like your guest!) "why don't you just increase the nightly rate and include the electricity for free?". However, there was a host from Finland a while ago, who explained the realities of heating charges in winter. It made me realise how expensive it can be in some parts of the world if a guest's behaviour doesn't take the cost of electricity into account.
I agree with you that, if you have to charge extra, it's difficult to put the extra electricity charge in your listing. The description is the correct place to put it, but you will also have to specifically mention this in the message thread as soon as you get an enquiry or booking. This is necessary because it's highly unusual in most parts of the world to pay for an Airbnb's electricity after you leave, so many guests may be surprised.
Here is the previous message thread that I think you will find very useful. It includes discussions about seasonal pricing and how one can avoid having to charge extra, and also deals with some technologies that may help you:
High electricity cost during Winter - what to do? - Airbnb Community