Hey,
Sounds good! If you're only going for hot water why not get thermal panels? [Edit: the plumbing is more hassle than wiring?] I looked at the ex solar farm panels and while they would be cheaper I'm going for new panels:
(a) I can only fit 8 panels (max 1.06m x 1.78m). Actually I could increase the size slightly but I don't want to go too near my neighbours roof (I've got an end terrace) or get too near the edge due to wind noise and because I'm scared of falling off
Also, being lazy, restricting to a single renusol 4.4m fixing rail would avoid any splicing. In any case, getting close to 3kW (or above) on my roof basically means using newer more efficient panels. All the ex farm panels I saw were older, max 275W.
(b) New panels have a warranty.
(c) New black panels look wayy better than scratched silver framed panels
Basically even doing it myself there's a lot of fixed costs, e.g. I still need to hire scaffolding and spend a couple of days on the roof. Electrical work is fixed (same 4mm2 DC, isolators, inverter AC side and NICEIC electrician to do final connections for Part P). BTW I think strictly speaking Part P applies to off grid systems too. Anyway, I don't think it's worth scrimping on panels. I think ex farm 250W panels are around £50? Well, you can get new black 330W panels for as low as £110. Most new panels I saw quoted 0.6% linear loss per year. So 5 year old "250W" panels could easily only be 240Wp. In £ per W adjusted terms that's only £330 extra for the warranty and smarter looking panels. Overall let's say max 15% extra. It's worth it for me. If you have more space or can get the ex farm panels cheaper I can see they make sense.
Inverter- I'm looking at the Sofar 2700TL-G3. It's definitely not the cheapest (around £400). I wouldn't mind grabbing a cheap 2nd hand inverter, as it's easy to replace. I already have a 2nd hand Sofar ME3000P battery storage inverter that has been fine.
My CU is in an integrated garage. It's a short and easy run for the DC cables directly down to the garage where I'll put the inverter. There's an old wire for a defunct burglar alarm I can (hopefully) use to pull the new cable through.
Scale mock-up (based on 340w 1m * 1.68m panels and renusol fastenings):
So far no structural engineer I've emailed has given a quote. I'll update here if they do.