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. However you could base your estimate for the cash flow on the amount you pay every quarter at home, perhaps increased by, say, 25%. Your machinery suppliers may also be able to give you an indication of energy consumption. 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.