A green certificate are a tradable commodity proving that certain electricity is generated using renewable energy sources.
Several countries use green certificates as a means to make the support of green electricity generation closer to a market economy instead of more bureaucratic investment support and feed-in tariffs.
Such national trading schemes are in use in e.g. Poland, Sweden, the UK, Italy, Belgium (Wallonia and Flanders), and some US states.
This makes purchasing of a green certificate equal to purchasing a claim, that the certificate owner consumed energy from the renewable portion of the whole energy in the grid.
Therefore certificate purchase does not affect how much renewable energy was actually generated – only how it was distributed.