You will be billed every quarter for all the gas and electricity you have used in the practice.
Until you have been trading for a few months you will not know exactly how much energy the business will use. Your energy supplier may be able to give you an estimate, based on the area of your outlet. You can adjust the figures once you have been in the premises for a few months.
It's worth shopping around the different energy companies to get the best deal for your business, particularly if you're going to be a heavy energy user.