3 phase I assume?
I'd prefer to install to whereever the actual incoming suppliers meter is so that I can locate the solar generation meter in the same place to make it more obvious where it is for meter readers and store managers who'll no doubt change through the years - I forget the exact wording of the guidance on this, but t at least has to be in an accessible location.
Then the question becomes where you're going to locate the inverter, which is going to be around 800mm x 1000mm and need around 2-300mm clear air around it, and a reasonably well ventilated space, and needs to be accessible. This will determine if you're running cabling for AC or DC through the building from the roof. If it's DC then my preference would usually be SWA externally, probably 6mm dual core with strings parallelled on the roof, but that's probably not your concern as long as there's an external route. If there's no option but to run DC internally, and you're doing it at first fix stage now, then IMO you want to be running it completely separated from all AC circuits in 6mm2 single core double insulated cables that are properly mechanically protected (earthing the armour on SWA doesn't work as a protection mechanism as the DC is a separated circuit), but should be avoided as far as possible.
Ideal solution for me would be inverter on plant room on top floor external wall so cable to roof can go out and up, with 10mm (probably) 3 phase cable (depending on distance) run down to the main distribution board, with a spare way left for it to connect in. If it's a particularly long cable run then you might need 16mm2, but I'd suspect 10mm2 would do the job - you're aiming for below 1% volt drop along the cable, but a bit over's usually ok unless the grid voltage is particularly high.
oh... college project? ok, download sunny explorer from SMA, it'll take you half an hour to sus out, but it has cable sizing as part of it - just pick out a 250Wp panel, x 40 panels in 2 strings of 20 on a 10,000TL inverter then have a look at the cable sizing.
kit wise, personally on commercial jobs we'd be looking at a remote meter / smart meter that uploads the meter reading to a web portal for remote monitoring (Orsis do ours), 16amp 3 phase MCB in the main CU (assuming it'd not be on a shared RCD - if there's shared RCD's then pull it out onto a new circuit from the tails via henley blocks to avoid nuisance trips), then meter, then rotary isolator, then 6-10mm cable depending on length, then terminate in rotary isolator at the location for the inverter and test the circuit. Would be perfect for us.
TBH though, usually we'd just ask the other electrical contractor to run the cable for us, and we'd definitely expect the contractor to discuss it with us first if possible.
some starters for you anyway.
eta - actually I just realised you said retail store, not supermarket, so talking of 10 and 16mm cables is probably overkill for the distances, and you're more likely to be looking at 4-6mm2
