Salesperson A had no knowledge if the immersion could be bypassed to operate from the grid if necessary ( it can ), no brand name of panel or hybrid battery, no detail how app worked and did not forward specs as promised by email, had me take a picture of the standard quote from his blurb on my phone rather than send me the exact quote by email, said he would forward specs by email, never arrived.
Salesperson B claimed to be an engineer but when I asked what efficiency the panels were he quoted me wattage, the most efficient are around 23% with average 20%, I wanted close to 20% as they do degrade, did not call back when he said he would, rushed discussions with do you know how busy I am mentality, I can't get back to everyone when I said I would.
Salesperson C, I meet him this week, very responsive to initial enquiry, very clued in and knowledgable on the phone, forwarded all specs to me by email which allows me do my homework on the kit.
The battery has a ten year guarantee but that's reassuring only if the company you choose is around a long time, this is new technology and will be popular with electrical technicians, know who you buy from.
As for key questions, do your homework, it's a big purchase but one thing Don't allow them drill holes in your roof tiles as suggested to me, that is not a good idea.
Of course to be efficient some adjustment will be required in the home so that you use appliances during the time you generate your own electricity, eg dryers used during the day rather than at night.