Definately the wrong place, but a quick answer is to contact your electricity supplier and get an Export Meter fitted: It may take 6 months to come! (Also check to see the numbers go back, and not just the disc! - I was never sure, as I never watched it for that long!)
There are 2 tariff arrangements at present - in one you are paid for what you generate, and then also for what you export, and in the case of not having an export meter, some assumptions are made. You have 3.5kWp installed.
Of courser, if you had them installed as part of a deal where the panels are actually owned by someone else, you'll have to look at your agreement with them; it may be simply free solar electricity you use, that you get, and you pay for what you 'import'.