Recent years have seen rapid growth in the number of installations of PV on to buildings that are connected to the electricity grid. This area of demand has been stimulated in part by government subsidy programmes (especially Japan and Germany) and by green pricing policies of utilities or electricity service providers (e.g. in Switzerland and the USA). The central driving force though comes from the desire of individuals or companies to obtain their electricity from a clean, non-polluting, renewable source for which they are prepared to pay a small premium.
Solar PV modules can be retrofitted on to a pitched roof above the existing roof-tiles, or the tiles replaced by specially designed PV roof-tiles or roof-tiling systems. If you are planning to put a PV system on to a building and have it connected to the grid supply there are likely to be local regulations that need to be met, and permission required from your utility or electricity service provider. The level of credit for any exported electricity will vary depending on local schemes in place.