How do you calculate what energy costs per booking?

Deduct the consumption of the empty weeks from your annual consumption and divide the remainder by the number of weeks rented. That gives you an average, but that average doesn't account for the differences between bookings: a weekend trip uses much less energy than a family with an electric car in the driveway. If you really want to know what a booking costs, you need a measurement per stay instead of an annual total, for example via a metering module in the fuse box. With one property and few bookings, reading the meter is usually sufficient.