How Bulb knows the amount of energy

Good day
Every time I receive my account update the email reads that I have used certain amount of energy since the past month, my question is: how bulb knows how much I used if I don’t have a smart meter?
Thank you

Via the meter readings you take manually and submit each month.

If you haven’t submitted any meter readings, then the energy supplier takes a guess, they call it an “estimate”, and it could vary between mostly correct and wildly inaccurate.

If you’ve never submitted a meter reading it would be advisable to do so now.

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I send my meter readings every three months and the last was in December that’s why I was wondering why the email stating that was from January.
So in my case on the last 3 months my bill went up ± £30 so this can be just a guess

Your December bill was accurate.
Your January bill is a guess.

I’d strongly suggest submitting meter readings every month, despite what the email says about “another reading in 3 months will be fine to keep things shipshape”. It will NOT be fine.

Absolutely agree. Things could slip too far with once a quarter, especially the Winter one. R-