A 12kw array on a south facing roof at 15º Pitch in Dublin should produce roughly 11,250 KwHrs per year. If all of this power can be used on the site, this represents a saving of €1350 per annum at 12c per KwHr. (A roof pitch of 30º would produce about 7% more power, but this pitch would be unusual on an industrial site)
Such a system mounted on a trapezoidal steel roof can give a return on investment of 10% to 11% per annum. As energy costs ultimately rise, the savings would increase over time.