What to Expect! This is as bad as the most time absorbing hobbies! You look at the sky and scream at the clouds - give me sunshine PLEASE. My very modest system was installed in August 2011 and I admit to inspecting and recording the daily output as displayed on the inverter display - this is possible as the inverter is located in our utility  room.         PVGIS The system came with its anticipated annual generation total estimate so you soon start comparing the estimate with the actual and initially was unaware of systems that can 'model' my location - there are a number of modelling but I use PVGIS -hit this link to visit the site. You can play with this to 'experiment' on annual, monthly and daily basis.         Location In this model you give the very basic information about your location, the total installed capacity (mine is 1.4 kw - not a lot!), the orientation (azimuth) of your system - that is degrees to the side of due south - minus for East and positive for West (mine is +7) and the angle of your system with horizontal being 'zero' - (mine is 15 degrees)         Shading If you have a skyline that offers shading at any time of the year (for example trees in winter or a neighbour's chimney) you can represent this in a simple data file. The results page shows my shading profile.         Results Click this link to open the results  - two pages to review PVGISResults.pdf You will see that the computation has estimated various loss information all of which has a statistical basis.         Weather You soon realise that the best generation is achieved on cool days with an absolutely clear sky for a couple of hours either side of noon at your location - as I reported a while ago air traffic vapour trails reduce my generation by 30 % until the sun is fully open again so the systems are extremely sensitive to the solar irradiation level.         Sheffield As you will have spotted Aldous and his team 'treat' individual subscribers data and create a normalised  reference so that we all can compare apples with apples! If you track the bottom few rows of the Micro-Gen data base spread sheet you will see the Max, Min, Mean and Standard Deviation for each month - that is a very useful set of data. Hopefully your system generation will sit within monthly mean plus/minus the Standard deviation. If you consistently fall outside this range then it is likely you have a problem.  Mine did and I did have a problem which was resolved on the evidence of the Micro-Gen data base           Monitoring I have found the best monitoring period is a month - within a month daily variations can be massive so you need to look at longer timescales and the monthly period works well. The Eco-Eye.com has been fitted (Retro fit) and have found to be very good as it holds 64 days of data in memory. There is an option to store the raw data directly to your PC via the USB link at four second intervals which you can manipulate in spread sheets etc.          Hourly Example         Month Example         Data The data to produce these graphs is downloaded from a special SD card whenever you wish and as you an see, assuming the correct values are input, you get estimates for the associated tariffs.         Disclaimer The above is my take on Solar PV - there is a lot of professional advice around which will point up the finer detail and benefits of considering other engineering tweaks but with a system of my size there will be a 'cost'/'benefit' implications.           Micr0-Gen Data Base URN139