I'm not sure about this logic though. I've been thinking about it recently as I'm on a similar renewable-only tarrif with a different energy company and was thinking of switching to an eletric car.

If the price is the same as the non-renewable suppliers and not all electricity is from renewable sources, then by switching to a renewable supplier all you're doing is bumping someone else onto a non-renewable supplier aren't you?

Presumably if everyone switched to a theoretical renewable-only supplier with no offset certificates now, they would have to stop making the renewable energy promise?