You will be billed every quarter for all the gas and electricity you have used in the business.
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 some guidance, based on the size of your premises and the nature of any equipment (for example power tools for cutting timber to customers' requirements) that you will be using. You can adjust the figures once you have been in the premises for a few months to make your financial forecast more accurate.
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.