This is another very commonly asked question.Here are some guidelines to determine the size of install in India.
Money as a constraint : If Rooftop schemes with reverse or net metering is available in your locality ( not unless you are in Gandhinagar for example http://mnre.gov.in/file-manager/UserFiles/presentations-pwc-workshop-06092012/GEDA.pdf) install as much as you have cash to spare (at today’s prices its about Rs 120 Per Wp , landed cost inclusive of the electronics , structures etc) – till you hit the next constraint which is space.
Shade free space as a constraint : You will need about 100 SqFt per kW installed with area free of shade , south facing.
However -rooftop net metering is going to take some time to become a reality. If you still want to add Solar PV (while you keep following up with the local utility about reverse metering )  – its usually because a) You think you want to do some good to the environment  b) you are in the unfortunate town or village where power cuts are the norm.
A). For someone who is looking to do ‘good’ – and cannot export the power generated to the Grid – you look at the two constraints : Money and Space to determine what is the size of install. You may have to rewire the connections – and add storage because of the nature of Solar PV power production. Accept the fact that not all extractable energy can be used all the time.
B) Solar PV as a back up power supplement : Unless you are prepared to ’ waste’ otherwise extractable Solar energy – the size of the install is much lesser than the total connected loads. Sizing the Solar PV is related to the hours of power cut, the size of the batteries and the average power consumption in your home. Sadly there are no electronics that manage the usage of Solar PV with Grid, loads and back up in a way that is seamless to the end user- yet.  Till such solutions are available – be clear on whether you want assured back up ( even in the face of ’ wasting extractable solar energy) Or Maximal usage of Solar.
We will look at some simple math in a later post for the sizing.
Recently Bridge to India Folks put up a Solar Calculator – Check this out : http://indiasolarhomes.com/calculator-2/