There are various packages that you can use to estimate the amount of electricity that you will get out of your system, based on the area of panels you install, the orientation of your roof and the angle of inclination of your panels.
For example, here’s one on the website of the Energy Saving Trust:
http://www.energysavingtrust.org.uk/Generating-energy/Getting-money-back/Solar-Energy-Calculator
Just enter your roof area, orientation and inclination, plus your geographical location and it will generate figures for the amount of electricity you can expect to generate.
Your analysis will look something like this (but obviously, the figures will apply to your property – plus this one was generated for our system when the higher rate of feed-in tariff applied):
For a more detailed view, see here.
It contains estimates for the amount of electricity you can expect to generate each year, taking into consideration the slight annual decrease of efficiency; the rate of feed-in-tariff you can expect to get; how much you can expect to recieve for exporting electricity to the grid (see below); how much you will save by using the electricity generated; and from these figures, the expected payback time for your initial outlay (in the right-hand column, in this case you can see the colour changes from red to green after 9 years).
Thanks to Norman Phipps for the information.