You will be billed every quarter for all the electricity and any gas you have used in the business.
Until you have been trading for a few months you won't know exactly how much energy the business will use. Your energy supplier may be able to give you some guidance as to what to expect. You can adjust the figures once you have been in the premises for a few months to make your financial forecast more accurate.
Even if you're not going to be a heavy energy user, it's worth shopping around the different energy companies to get the best deal for your business.