Coming from another country, I always wonder why the two utility companies I have here in the UK, Thames Water and Octopus Energy, would calculate an amount that they think I should pay monthly, instead of just charge whatever I used last month. To me, the latter way makes much more sense and is the standard practice in the countries I lived before.
The amount they calculated seems to generate either a huge credit balance, or a huge underestimation. Thames Water changed my monthly bill from £29 to £7, and then to £17 over the course of a year and a half. Octopus Energy built up more than £200 of a credit balance (not sure if it's a result of the UK government energy gift credit last winter), then set a minimal amount of £61 monthly. They say the purpose is to make sure that the credit balance would be always be more than £100. Okay...but why? If I want to save money, I'd go to a bank.
I could see that it might make sense if the measurement is not as easy or accurate, but come on, it's the 21st century and the meter shows me my energy usage by the hour, surely they can calculate the exact amount rather than pull a random number out of nowhere?
The way they do it here is to estimate your usage, and then come check the meters every 3 months and adjust your bill for actual usage. Although they just installed a new networked smart meter a couple months ago, so I guess they might be charging for our exact usage now. I do not really know, because I didn't ask them.