Even with smart meters, energy companies are totally shady when it comes to working out what price you pay. I switched tariffs which completed on a specific date, so I made sure I took a reading as late as I could on the day before my new contract started. The energy company refused to use this as my actual reading but used their smart meter derived reading which was taken two days before the new contract started. Out of principle, I queried it, but having to explain it to them was like banging my head against a brick wall and not worth the few pounds it would have saved me! But I imagine they do this to a lot of people and make a lot of money by stealth.