Payment system & monthly pricing

Thinking of joining but do not understand how the first payment (prior to supply) is calculated: I can only assume an annual costing is calculated using the price for the current month and then divided by 12 to form the first payment/ money in hand?
Meter readings are submitted I assume on the last day of the month and an actual calculation is made and charged against ‘my’ money in hand? Please advise.
If Bulb is reviewing prices monthly I assume they are themselves buying short term which can only mean their tariffs will rise sharply as the temperature goes down. The only way to resolve this is for them to buy longer term that the next 30 days. Their method for sourcing supplies is very unspecified and could well be no better than " trust me it will be OK". I appreciate you can leave immediately and probably actually switch in 3 weeks but in peak winter any available term tariff with another supplier would almost certainly not be overgenerous. Please advise

When you apply online - https://join.bulb.co.uk/join/quote - you’ll be able to enter consumption figures from the last 12 months with your previous supplier.
This will enable Bulb to estimate your monthly direct debit amount. Depending when you apply, the payment date could initially be any day of the month, but this can be changed online after the first payment. You can also amend the amount, or make ad hoc top ups if necessary - it’s all very flexible and under your control.
To get accurate bills rather than estimates, submit meter readings two days before the bill date.

Thank you: you have explained billing but if they are reviewing prices each month they are buying very short term which must produce a very reactive tariff: if you buy spot prices you pay spot prices? Any informed views would be appreciated:)

@marktarasek They buy energy 3 months in advance. Stated at the bottom of this article:
https://bulb.co.uk/blog/wholesale-energy-market-update-dec

I assume if my provided annual usage is costed at Bulb prices at £1200 they take £100 at the start of supply and if at the first bill was my actual usage was £120 they will take £120 by direct debit: and hence still maintain a £100 balance in ‘my’ account. I assume all monies are safely retained in an escrow account - at least one energy supplier has had to cease trading?

@marktarasek The monthly payments would stay the same, so if your annual cost was £1200, we would take £100 every month regardless of your energy usage. Over the course of your Bulb life, if it looks like you’re building up a lot of credit or debit, we would get in touch to amend the payment amounts.