Pross
Guru
Surely brownfield can have any slope (facing any direction or flat), just as arable land?
For developers it's not an either/or when it comes to brownfield vs arable as plenty of farmers happy to sell arable for development (at above arable land prices) and just leave brownfield as unused. Planners seem happy to approve development on arable land as they can only respond to applications and can't say "why not build it on this brownfield site".
In general, if someone has previously developed a site they will have levelled it first so sloping sites are few and far between. Also, if competing for sites with housing developers I would guess that a company developing a solar farm will lose out as they won't be able to generate as much income from it as the house builder. In general I believe they lease the land they build on rather than purchasing.