Re: I've been pointing this out for years.
If you are on a 100% renewable scheme and use 1000 kWh, then it guarantees that 1000 kWh of green power is put into the system instead of 1000 kWh of non-green.
That's what the greenwash vendors would have you believe - but there is ZERO truth in that. That 1000kWh would be put into the grid regardless - the rules on renewables mean that pretty well all renewables get "first bite of the cheery" in supplying demand.
So wind always puts in all that the windmills produce - and the energy companies will buy it (even though it's very expensive*) because the rules require them to. Ditto solar, hydro, etc.
So when you switch on your kettle with your greenwash tariff the result (since we do not have any excess of renewables over demand) is that the taps open ever so slightly on whatever generator is doing the dynamic load balancing at that point in time - typically it will be one of the CCGT stations. So your additional load will result in a matching additional generation from fossil fuel. The actual electrons you get will be from a mix of sources - exactly the same mix as EVERY other consumer in the country.
So seriously, there is absolutely no such thing as a true green tariff - they are all greenwash, getting you to pay ectra for nothing more than a warm fuzzy feeling.
* Wind IS very expensive. The operators will happily tell you how "cheap" wind is - but what they don't like to talk about are the direct cost (the 30-something pence/kWh subsidy**) or the indirect costs (the massive costs incurred by the rest of the supply industry in mitigating the effects of a highly variable/intermittence supply that has priority on supply).