You will be billed every quarter for all electricity and any gas 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. 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.
Energy prices are high and seem very likely to keep on rising in future, so it makes good sense to include energy efficiency measures when you fit out and equip your premises. Even things like low-energy lighting can make a big difference to your costs over time.