If you have any business premises you will be billed every quarter for the electricity - and possibly gas - that has been used there.
If you run your eBay business from home you may be able to agree with HM Revenue & Customs (HMRC) that a certain proportion of your energy costs is treated as a business expense and set against your earnings. Talk to your accountant, who will be able to advise you whether or not you are able to claim a deduction and the percentage of the total monthly cost of your energy bills that would be reasonable to treat as a business expense.
If you enter the proportion of your energy bills here in your cash flow forecast remember to include only the balance of your domestic energy costs when calculating how much you will have to take in drawings to cover your living expenses.
Until you have been trading for a few months you will not know exactly how much energy the business will use. You can adjust the figures once you have been trading for a few months.
If you just intend to use a computer (or even just a smartphone), some peripherals and of course some lights for your business then you may find that your electricity bill barely increases when you start trading. However, if you need a workshop for work like restoration and repairs then you may use quite a lot of extra power. Remember too that your heating bills are likely to increase if the house was previously empty during the day.
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.
If you run your business from home, you can either agree a proportion of your heat and light bills that relates to your business or you can claim a flat rate monthly allowance which varies depending on how many hours business use of your home there is in each month. Note that the flat rate allowance covers all the property related expenses of running your business from home, including rent/mortgage interest, insurance, rates, water rates, light and heat.