During solar system installation, that panel orientation be vertical or horizontal is all acceptable.
Vertical direction could save more solar mounting rail, and the installation process would be easier and faster.
Some people said horizontal panel could bring higher power generation efficiency, it is not absolute. If you just put the panels on pitched roof, or with small angle, there is no difference on power generation efficiency. If installation site is open ground, and solar mount comes with big angle, as long as the space is large enough, the shadow of front array does not affect the rear one, power generation efficiency will also be the same.