Don't rely on your energy provider's estimate, these are often way out. If they're underbilling, you'll have a big whack to pay when your supplier receives your actual meter reading. If they're overbilling, then they've unfairly got your cash.
Use our Energy direct debit calculator to see if you're on track. If your direct debit is way off kilter, call up and ask for it to be changed and ask for any excess credit to be refunded to you.
Smart meters can help stop this as they send meter readings automatically to your supplier, so you only pay for what you use. See the Smart meters guide for more.