The only way to get an accurate estimate from a company is if you know roughly (or better still, accurately) how much energy you use in an average year. If, instead, you have to do a comparison based on the size of your house/number of people living there, it will only give a rough idea of how much a year you will pay a new company.
Some people don't realize that a fixed tariff means that the price per unit you pay for your energy is fixed, , not your total/monthly payment.
In other words, the more energy you use throughout the contract, the more you will have to pay (on the plus side, if you use less energy than expected, you will pay less than the estimate suggested)
That means that most companies will want to adjust your monthly payments if, after a few months, their computers start telling them that you might be using more/less energy over the year than the initial comparison suggested.
Companies are now discouraged by OFGEM from letting customers either build up a lot of credit or get badly into debt. That is why they might suddenly want to either refund credit if a lot has built up, or increase Direct Debit payments. However, there is a huge difference between companies as to how this works in reality.
Good ones allow some flexibility, whereas bad ones (as I said, I have had one of those!) suddenly impose increased Direct Debits in winter, even if a customer is in credit, sometimes refuse to refund even large amounts of credit, change terms and conditions without informing customers, and generally treat their customers like dirt, refusing even to respond to queries.
Sorry for the length of the post 