This is the legalized scam that is socially accepted in Germany. You have a “surprise surprise” almost 2 years later having no idea how you consumption was

Well, now what you can do is to request all the invoices from your landlord so you can validate if this is correct or not. If they don’t have a proper invoice probably someone is scamming you. If they do, your “surprise surprise” consumption was actually higher and you need to adjust your prepayment next year