@ Christian,
In order for you to determine the size of your client’s pv requirements there are two ways in which you could go about it.
The first is to request for the latest electricity bill from the client and analyse the average load from the total kwh consumed. This is not a recommended method because it is very inaccurate and does not give the worst case design parameters as required.
The second method is to carry out an on site load inspection of the electrical services installed in your clients facility and note all the load details. From here you can determine the total connected and diversified load which will help you to determine the total output from the intended pv system. It is also worth noting the current overall power factor of the existing electrical installation.
Please note that the size of the pv arrays needed will determine the mounting area required (either the roof or the ground). therefore it will prudent for you to inspect and determine the available roof and the free ground spaces areas. otherwise you may come up with a pv array for the required pv power supply needs with no roof or ground space to mount them!
again the surface area available will also determine the type of pv technology that may be used as the pvs are of different types...
NEVER TALK OF A RHINO IF THERE IS NO TREE NEAREBY - ZULU PROVERB
...besides, the presence of a safe alone does not signify that there is money inside...