They did this to me back in may of 2019, wanting to increase my ,o they payment from £65 to £104. Was alarming and confusing so I did the math and wrote them an essay explaining why their proposition was absurd.
They had just raised their prices a few months prior, so my direct debit needed increasing by £4 to £69 in order to remain in credit.
The reason I suspected, is because I’m electricity only, and my costs in winter are substantially higher than in summer. Peak summer my bill is around £20, but in December/January would exceed £150. Bulb don’t seem to include your past data in the calculations to bear any weighting, and instead predicted my summer electricity usage on the basis of my winter electricity usage.
It’s annoying, and you don’t have to agree to the increase, but if you’re in debit, then you do owe them money, as per the terms, you must remain in credit, rather than be in debit through winter and break even in summer like with other providers.
I’ve never had an energy company not pull these shenanigans though, so what I did was cancel the direct debit, set up an easy access savings dedicated for electricity, and use my own calculated average to pay into that, then make ad-hoc payments for the amount due each month and withdrew it.
I think it’s just a crap algorithm. It makes sense when you reverse engineer how it works and calculate in accordance. In my case for instance, had my summer usage been the same as my usage over winter, their increase would have been spot on. It’s a best efforts thing, and it probably captures a large portion of dual fuel customers through automation which keeps the overheads very low.