You will be billed every quarter for all the electricity and any gas you have used in the business.
If you intend to use equipment like industrial heaters and dryers on a regular basis then your energy costs may be quite substantial.
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, based on the size of your premises and the nature of any equipment you will be using (for example, a pressure washer). 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.