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. You could contact your energy supplier who may be able to give you an idea of likely consumption, based on the area of your premises, the number of computers and other pieces of hardware you will have running, your opening hours and the services you provide. 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.