Bonjour Benoir, welcome to the OEM forum.
If you are connected to the grid, why do you not send energy back to the grid? Here in the UK, everyone is encouraged to export their spare energy. For you, is it because of the way that your inverters are configured? Or is it a law of France or your energy supplier who says that you must not send energy into the grid?
If your inverters are not permitted to export energy, I do not think it is possible to calculate what your production might be, because you need to know how much current your panels could produce given the amount of light falling on them. The inverters might know that, but they might not make that information available to you.
The only alternative that I can think of is to install a small panel, whose only purpose is to measure the amount of light available and therefore estimate what your 18 panels might generate.