Bills may vary slightly depending on the meter readings you provide to us.
We calculate monthly rates based on estimated usage, which are then compared with meter readings and adjusted accordingly. Our estimates are generally quite accurate with our customers, though naturally with student houses they fail to take into account periods when you're not in the house. Once we have accurate meter readings we are able to adjust your account, and reduce the following months' bills to compensate. It is unlikely that monthly bill payments will vary significantly though, so no need to worry!
Also if you find you have paid for more than you've used, this is easily rectified with a final meter reading at the end of your tenancy.