Government grants

Dear Sir,
I have a question regarding government grants:

An entity (producing green electricity) receives green certificates for the green electricity generated from the government in the preceeding year. These certifcates then are sold to other electricity produces to gain income. The electricity is produced and sold simultaneously, no inventory remains at any time.

In my opinion, they are government grants that are provided to reimburse the costs incurred in the past. So, when should the grants be recognised as a deduction from costs of sales of electricity? I meant when the grants are received from the goverment or when income is realised by selling the certificates to other producers?

Thank you
Brs,
Nhung Nguyen