If HMRC have made an error in processing your registration, then you should contact the VAT registrations team and request they correct their error. Suggest emailing initially so that there is an audit trail/evidence of you making contact. Then I would suggest calling the registration helpline and persist (they can be engaged or not answer most of the day) and then keep calling until you speak with someone.
If the error was actually yours (typo), then you can also write to HMRC and seek a change of date, HMRC manual link below to the guidance, again, try the registrations helpline first on this and then they may ask you to write in formally.
VAT is nothing to do with financial years. If this is a voluntary registration (sales below £85k per rolling 12 months) then you have to specify what date you want to register from.
Rolling 12 months is where you look at your turnover at the end of each month and looking back 12 months, so you add up your sales from February 2023 back to February 2022 and you do this each month, if the turnover during each monthly review is over £85k then you have a compulsory VAT registration, again, nothing to do with financial years (April-Mar). A voluntary registration means you are not over the £85k turnover in any 12 month period but you want to register for VAT anyway.
I would suggest reading the basic VAT guide here https://www.gov.uk/guidance/vat-guide-notice-700 to get familiar with it before you register for VAT, VAT requires some precision and getting it wrong can lead to large penalties, so you do need to know the basics in order to avoid making mistakes (such as thinking you will be registered from the 6th April automatically).
Same link, read section 14 about tax points as these are vital to getting VAT right. If you are VAT registered from 01st April 2023 then any sales you make on or after that date are subject to VAT. If you register for VAT from 6th April then any sales before the 6th are not subject to VAT. You may need to speak with your clients about the fact you are VAT registered and your prices will be going up.
Please make sure you understand VAT enough to ensure you are compliant, do you have the accounting/bridging software to file returns electronically (MTD is now mandatory), have you crunched the numbers and satisfied that voluntary VAT registration will benefit your business?
You have a new VAT number, you need a new government gateway/login.
The "old" account is there for historical purposes and if you or an Accountant ever needs to review the old VAT number, etc.
I'm not aware of an upper limit on how much kw an install can be, but if the customer is having 150kw installed, then you need to be absolutely clear that this is indeed domestic/residential. Average households would have an install capacity, as you know, of around 4kw-15kw in most situations, so unless your customer is Buckingham Palace, that seems an awful lot of generation for an "average" household. You are looking at what, 300-455 solar panels depending on their individual generation capability, so that is a lot of roof area (if roof mounted), but if you are saying it is wholly domestic, the property isn't being used for a business purpose (ie, National Trust property charging an admission fee is in business), then in theory the project can be zero rated. Is this a farmer or someone with a large holding who are installing the panels on their household roof but also intend to power their stables and barns? If so, be careful that these outbuildings meet the definition of a "domestic/household" and are not deemed part of the business. A 150kw system will cost a significant amount of money so you need to be confident the works are compliant with the zero rating, you may even want to write to HMRC and seek a "non-statutory clearance" where HMRC will given you written opinion based on the facts you give them. Extract of the legislation for zero rating below :- https://www.legislation.gov.uk/en/uksi/2022/361/made Meaning of “residential accommodation” 2.—(1) For the purposes of this Group “residential accommodation” means — (a)a building, or part of a building, that consists of a dwelling or a number of dwellings; (b)a building, or part of a building, used for a relevant residential purpose; (c)a caravan used as a place of permanent habitation; or (d)a houseboat. "relevant residential purpose" usually refers to something like a care home or similar type establishment..
Your effective date of registration (EDR) was 01st October 2022, that is the date you are registered from. All your sales from that date are then subject to VAT.
Waiting for the letter from HMRC to be received in the post is irrelevant, your EDR is 01st October 2022. Link below, scroll down about half way and it explains that you should charge VAT whilst awaiting for your VAT number.
If your clients are other businesses, then you should be able to raise VAT only invoices to them, but if your customers are consumers/general public then you'll have to account for VAT from within the sales made from 01st October to 20th December.
Also suggest having some training or reading various guides to understand VAT, it can be quite a precise tax and is unlike direct taxes. You may also need to seek further advice to make sure you can meet the various obligations such as Making Tax Digital, etc.
Finally, I'm not sure I agree with your EDR, if you went over the £85k sometime in October (15th October) then your EDR should have been 01st December. The "next 30 days" rule is what is called the forward look test and is meant for a single sale/contract where that sale alone would take you over the £85k threshold. Not to be confused with the backward test where you just look at your historical turnover on a rolling 12 month basis.
Unless you knew in September that you had one sale that was coming in October that on its own was worth more than £85k
Suggest you call HMRC, advise you got the EDR wrong and see if you can move it to 01st December, you'll still have sales you made where you are liable for VAT but it'll only be for sales from 01st December to 20th December that you are on the hook for, thereafter you were charging VAT.
You need to call the VAT registration helpline on 03003227870 and you need to persist/keep calling and holding until you speak to someone - it seems that although the application can be made online, where there are queries then this can sometimes lead to a breakdown in communication and cannot be dealt with just by emails, so calling HMRC will be the quickest and simplest route.
The business will be registered from the date requested on the application form, so if you requested to be registered from 01 September 2022, then your sales should be including VAT from that date. See this link section 5.1 https://www.gov.uk/government/publications/vat-notice-7001-should-i-be-registered-for-vat/vat-notice-7001-should-i-be-registered-for-vat#Accounting-for-VAT
Seems correct to me. See link and particular attention to the last paragraph.
B2B place of supply of digital services is where the customer is, therefore the customer accounts for VAT at their end using reverse charge. Your supply is outside the scope of UK VAT (no VAT charged). https://www.gov.uk/guidance/vat-place-of-supply-of-services-notice-741a#sec6