If you like a spreadsheet and want to know what you are actually going to pay for your energy on a monthly basis, rather than what the average bill will be, then you can download this Excel document, put your monthly usage in, add the unit costs and get a monthly cost and annual total for your usage.
It can be difficult to find actual unit rates, as all the announcements are for total average usage rather than the kWh unit cost and daily standing charge, but Money Saving Expert has them here. You can get past values from your old bills.
If your billing periods don’t equate to calendar months you can change the months and days columns in the document to suit.
This will show your actual costs (assuming you put the right data in) rather than the average bill cost which the energy price cap is usually quoted in, which is a bit more useful when it come to having an idea of what you’ll actually pay. Note that this isn’t what a monthly direct debit would be as those usually spread the annual cost equally across 12 months.