Well, this is their series C, but yeah, I'm kind of amazed at the amount.
The credits are not bought by the state. They're bought by utilities, which have state-imposed quotas to meet, if their production isn't clean enough. This is sensible, since utilities will otherwise have a relative market advantage from externalities in the form of pollution.
Nearly 5000 kWh/yr from a 3 kW system would be very high specific production. I think you're overestimating a bit.
You might want to check out PVWATTS, which is an online estimator, or if you really want to get into it, PVsyst is an excellent modeling application that has a free demo.
To write good code is a worthy challenge, and a source of civilized delight. -- stolen and paraphrased from William Safire