You will be billed every quarter for all the gas and electricity you have used in the business. Alternatively, you might buy heating oil on a regular basis instead of gas, particularly if you're located in a rural or isolated area.
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 suppliers who may be able to give you an idea of likely consumption, based on the area of your premises and the services and facilities you will offer (catering, accommodation and so on). 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.