[Drivingforfun]

I notice solar panels are almost always laid out without making full use of the roof space - i.e. large gaps, not tessellated well etc.

If laying out the - fixed - costs for installation, why wouldn't people take the opportunity to have as many as possible fitted?

I'm sure there's a valid reason - regulations or something - but just wondering

[olduser]

The short answer is cost, the panels and control system are expensive, though cost has fallen and the output has risen over the last few years.
Early solar panels needed strong sunlight, todays panels just need daylight.

Then there are two strategies, first the user can be looking to minimize their electricity bill.
Or they can be seeking to make money by generating electricity to sell to the supplier.

The real money spinner would be to charge batteries, and sell the electricity at peek times when the price is high.
Actually, it would be profitable to do this even if charging the batteries from the mains at off peek times.

In most areas planning permission is not required, listed buildings have to get permission.
The only other regulation I know of is the panels must not be more than 200 mm above the roof, all electrical regulations must be complied with, and the structural integrity of the roof must not be compromised.

[NMNeil]

@Drivingforfun It may depend on the rest of the system.
I recently updated my system with another 12 panels but they kept overloading my charge controller. I had to disconnect two of the panels to reduce the voltage.
This may be one of the reasons that they don't cover the roof with panels.

[olduser]

Yes, all parts of the system must be sized to match maximum output.

[Rolebama]

Maybe weight could be a factor? I have an old one which I use occasionally to charge stuff in my shed and garage, size about 3ft x 1ft, and it is a noticeable weight to lug around.

[olduser]

I think the latest ones weigh about 10kG /square meter.

Installers should check the roof structure before installing panels.
Most roofs that comply with the building regulations will have sufficient spare capacity to take the extra load, if the roof is sagging, or looks untrue would be warnings there is something structurally wrong.

[Argonexio]

I’ve seen gaps happen because installers avoid shaded spots, awkward roof angles, or vents. My installer said fewer well‑placed panels can give better output than cramming every space.