We have two roofs with 21 and 19 150W panels giving a total of 6000W. The two groups of panels are connected to two converters which log the actual output. It is possible to connect to the converters using Bluetooth to retrieve and then process the data. For the days when we are here (and my computer is running) the results are uploaded every five minutes to our web site.
We have 21 and 19 panels rather than two sets of 20 panels because of the position of a chimney stack on one roof. It is possible to drill down into all the data on the web site to see day-by-day output (and the sunny days!). The chimneys are also the reason why the set of 19 panels is more efficient; the set of 21 panels has more partial shade from one stack in the early part of the morning.
In France it is more usual to sell all the output to EDF rather than using some and just selling the surplus generated. There is only payment for the electricity actually sold (unlike the UK). We get paid at about three times the amount per unit we can buy at.