Payment for solar energy

You pay for your NET usage of electricity (plus, of course, the fairly small fixed charges).

So if you use 4.100 kWh of electricity in a year, and the panels generate 4.000 kWh in the same period, you would pay the consumption and transmission charges on just 100 kWh.

The original installation was probably designed to generate roughly the same amount of electricity as was typically used in a year, although that, of course, is a balance which it is impossible to get precisely right since both usage and generation fluctuate. If it generates considerably less, then either installation space was constrained (most likely) or the people who commissioned the original installation deliberately chose a smaller installation than the optimum (unlikely) or there is a fault.

I've just looked at the Infrax website and there are plenty of contact details including the location and opening hours of all their offices.
If that doesn't work for you then the notaris who acted for you should be able to advise - and should probably have given you the details before the sale was completed. There is a fixed price for handling the sale so you should not have to pay for any aditional questions which you have.

I don't know what Infrax is, but I do know that for every 1000 kwh produced, you get one green certificate, and the government guarantees that you'll get something like 65 euros for each certificate.

Thing is, the people who installed the panels likely received 40 green certificates as an advance, so you wouldn't receive anything until the installation produces over 40,000 kwh.

Besides the savings on your electricity bill as described above, you won't get anything else as far as I know.

I see that Infrax is a flemish organisation, given that the upfront benefits granted for installing solar panels have changed almost yearly (unsurprisingly becoming less generous each time), and that the process for claiming them (at least in Wallonie, and I can't imagine that Flanders is any better) is complex, I think that asking the notaris who handled your house purchase to explain it to you is the best route.

Claire I'm not clear what the rules are they change all the time and some one told me that eventually solar panels would be taxed in some way.
We have one for our water heating but getting s grant for that was problematic but we don't put anything into grid so we don't get anything out. I have a friend who said the meters on her house run backwards when she has sun on the panels. So I guess that at least is a bonus for you.
I think the others are right pop in or ring the office and ask them to send you information on the agreement at your home,