16 x 250KWh solar panels, in two separate strings to minimise loss of energy via afternoon shading. system supplemented by Solar iBoost mechanism to divert surplus energy to hot water heating [thus reducing gas c/h consumption].
Just installed [late October 2013] and surprising how much power has been generated despite heavy cloud and rain since then!
1. Research things thoroughly so you can ask 'hard' questions of potential installers - soon sorts out the sheep from the goats.
2. Ensure you have a shading analysis done unless you are confident there is no shade at all - that will enable you to configure a layout/system that minimises power generation loss.
3. Ensure your supplier will undertake a proper roof structural survey - unless your house is very new your roof will almost certainly need some strengthening to meet current building Regulation requirements.
4. Don't believe all the financial projections prospective suppliers will provide - test their assumptions and model your own to ensure you get 'like for like' comparisons.