"100% renewable electricity" obviously doesn't change where the electricity you actually use comes from. We certainly don't want the lights to go out and our oven to stop working just because it's a cold still night and all the hydro was used up. You would need to check the supplier's fine print very carefully if you actually decide you care what this means. The consumer magazine "Which" have attempted that for you if you subscribe, but basically buy from "Ecotricity" or maybe "Good Energy".

The most cynical thing your supplier could do in the UK is buy REGOs (Renewable Energy Guarantee of Origin). At that point the 100% Renewable claim would be legal, without any other effort. If that's all your supplier does it isn't worth a penny, we'll get to why below, but if they charge extra for the tariff based on REGOs it's pure profit for them I assure you.

A better thing they could do is arrange to buy in bulk from a renewable generator or (more practical for larger outfits) just own the renewable generator. This means their interests would at least somewhat line up with yours - if their renewable energy generators are cheaper they get to keep more of your money while offering attractive prices.

But back to REGOs. So, when you make renewable electricity in the UK you get REGOs, and you're allowed to sell them. You could sell your electricity with the REGOs, here you go, 100% renewable electricity. Or, you could snip the REGOs off, sell those to anybody who wants them, and sell the electricity as just electricity, which people wanted anyway.

If "100% renewable" was in very high demand this wouldn't be a problem, a handful of more expensive suppliers would bid for the REGOs, and this would create pressure to deploy more renewable power.

But in the UK the vast majority of households use the incumbent supplier. What does "incumbent" mean here? Well, historically the UK had regional monopoly suppliers who both billed consumers and handled the last mile distribution infrastructure that actually means electricity works in your house. But the Conservative party believes strongly in Free Market principles, even where there's no evidence they would help. So, it privatised the industry, giving away national infrastructure for a song and creating dozens of private companies that notionally can compete to supply electricity. Except of course your actual supply is the same as ever, there is still a monopoly last mile supplier, it just doesn't deal with consumers. What had been the regional suppliers were now private companies that had "grandfathered in" millions of residential customers in their region, but were free to compete to "supply" customers anywhere.

All these companies immediately offered somewhat lower prices to anybody who'd switch, and began raising the prices for those who didn't switch. Switching is annoying (despite efforts to make it simpler it cannot be entirely painless) and so most people never switch. Sounds like you have, so immediately you're not the usual case. They also began aggressive (indeed sometimes outright illegal) campaigns to keep "their" customers and prevent defection to rivals.

So today the situation is that essentially every company advertising their prices to you offers 100% Renewable Energy, because they're buying REGOs for customers like you (and me). They can get REGOs very cheaply because they don't need very many because there are so few customers like us.

And the vast majority of households aren't on any of those supply contracts, they have a "legacy" contract that's more expensive. And so in practice when it's cold and dark the coal and gas power stations are cranked up exactly the same but on paper we can blame those millions of people not all of us with our cheaper 100% Renewable contracts...

The climate is not so easily fooled.