A 3.7 year ROI would be incredible (and fairly uncommon AIUI)

South facing?

House is facing South, but I'm laying them flat on the floor for economic and aesthetic reasons, with some 3D printed brick-based ballast [1].

Buying supports for PV is actually less economical than buying additional PV panels.

--

[1] https://imgur.com/a/tDPevmM

Mine was around 4 years and its west south + a tree in the middle. So spring and autom the tree is no problem but in summer lunch hpeak there is shadow on it.