Budgeting for water/gas/power etc

So I've been working on an all purpose business plan/feasibility spreadsheet recently and I cant figure out the best way to budget for power/water/gas usage.

On one hand, I'm used to keeping them within a proportion of revenue (say under 10%) but that lead me to a problem when trying to project. If theres a price war and I have to drop my prices, theres no reason to assume the cost of energy will drop. I would of course endeavor to keep these costs as low as possible, but surely one would do this anyway? So if my revenue drops, energy costs wont. Conversely, should a hostel get the chance to raise prices why would they budget for increased costs when in all honesty, they shouldnt rise? sure, they can afford a greater cost but they shouldnt plan on it.
So it cant be a function of revenue.

Perhaps a function of bed nights/occupancy then? More people taking a shower = higher water bill. More people using computers = higher power bill. But the bulk of these costs are fixed - a light on in a room costs as much whether theres one person in the room or 6. Same deal with heating (though that will be influenced by the number of room used, hopefully).
So it cant be a function of bed nights.

So if so much of these costs are fixed, is it then related to the sheer size/scale of the hostel - in SqM or SqFT? A building of 200m2 costs $X to heat whether its got 10 people or 40 in it, whether you're charging $10 a head or $50. But this seems a little too counterintuitive, obviously not all buildings are the same and surely there are a number of other factors that could have significant influence?

Obviously its a function of all 3 (and more) elements. I was just wondering how do people here budget for these kinds of costs? Or do you simply use your own historical data?