Thanks to your solar panels, you can possibly reduce your advance amount with your energy supplier (provided that your consumption remains approximately the same). Attention: keep in mind that you still have to pay the prosumer rate if you have an analog meter. 

If you have a digital meter, you benefit from the injection rate. If you have more production than self-consumption, it is a safer option to reduce your advance amount.

In both cases, your advance amount will be automatically adjusted after your next annual statement. The annual consumption for electricity will of course decrease thanks to your solar panel installation, so you pay less for your purchase in total than for your purchase for the installation of your solar panels.