[antnisp]

Power generators connected into a grid, pool their production and then consumers draw from that pool. A consumer can't claim "My electrons came only from "green" sources". Ones electrons came from the nearest sources. What a consumer can do, is pay a producer connected to the grid, to supply "green" energy matching the consumer's draw from the pool.

It's like balancing a checkbook. When you act as a producer, your windfarm puts 1MW into the pool. Logistically, you have to buy it back to claim you consume green power, otherwise you run on the grid's energy mix.

(This is broad strokes, and may vary depending on your location.)