That's not exactly true because interconnection capacity is very limited. That's why quebec has rock bottom prices for electricity[1] but 250mi away in boston the electricity cost several times more.

[1] https://www.hydroquebec.com/data/documents-donnees/pdf/compa...

The way it works is that your electricity provider company buys the amount of electricity you consume from renewable electricity producers. It balances out and it's equivalent on average to you using only renewable electricity.

Multiple providers are working like this in Europe at least. They partner with many local solar/hydro/wind producers. It's approximate of course they can't predict exactly the amount required in advance but in the grand scheme of things it works by supply/demand mechanics..

You could trace btc to a reward allocated to a miner on a renewable electricity contract. Then you know an equivalent amount of energy used to mine these coins was produced sustainably.

There are times of day that solar and wind farms produce more energy than the grid can use. This excess energy has no marginal value to the grid. You can also have off-grid solar bitcoin farms that don't need interconnectivity.