I find this bit rather baffling. I've been on "green" tariffs since I took over the electricity account here, yet I'm still subject to increases in prices in line with those of wholesale gas. The tariff I was on this time last year had me spending about £45 a month (it was a too-good-to-be-true offer, to be fair - they went bust) and I'm now paying £100 a month on a so-called 100% renewable tariff. The idea that I should be cut off because there's no current renewable supply is ridiculous, quite frankly.
It's very warm, sunny and breezy today - perhaps I should be getting my energy for free, going by the same logic?
No, I'm afraid not. To quote (the great, IMO) Dr Ben Goldacre, "I think you'll find it's more complicated than that." I will attempt to explain.
It's all about the National Grid.
Electricity cannot easily be stored in any quantity, and so in the main (99%+) it's generated as required. Overall, the National Grid constantly brings power sources on and off line as demand changes. Different sources have different flexibility- for example the Dinorwig Pumped Storage is used for "pick up" peaks, e.g. at the end of a popular TV show when everyone puts the kettle on at once, the Dinorwig station can go from zero generation to megawatts in seconds. That's unusual though, most generation sources have more lag between "required" and "ready to generate" so the Grid does a lot of prediction too- (to the point of watching for a TV show ending and powering up Dinorwig just as the music plays at the end of the show- I saw a TV documentary about it some years ago). Other small demand changes are also partly dealt with by varying the frequency up and down a bit.
OK, so once you realise that generation must = demand on a second by second basis (or the National Grid falls over i.e. mass power outage), you can see that you need to have enough capacity to maintain the demand on the grid when the wind doesn't blow.
For example, as I write this, on Gridwatch
http://www.gridwatch.templar.co.uk/ we currently have the 33GW demand on the National Grid, of which 19% being met from solar, 3% from wind, 13.5% from nuclear, 7% from biomass and 48% from gas. We have nearly 1% from coal, are importing about 1% and the frequency has dropped a bit.
So you may be on a "green" tariff, but currently about half of your electricity is coming from fossil fuels. Overnight last night with little wind and no solar, that would have been a much higher proportion (assuming you are in a grid-connected residence).
You cannot buy what is not available, so on days/nights like last night with little/no renewable energy being generated, to keep your power on you need to have power from fossil fuels. Unless you'd prefer to only have electricity when the sun was shining/wind blowing? (If so, hopefully your fridge/freezer is well enough insulated to manage the gaps in supply).
Without you being fed by fossil fuels, your electricity would have been cut off last night. You pay for renewable electricity as it costs money to install, maintain, and supply; indeed the cost of your "green" electricity is also subsidised by the green levy (tax) charged on energy unit of electricity generated by fossil fuels.
The other thing to understand is that even with gas powered generation stations (which are
relatively quick to fire up), they don't come on line instantly so some generators are held in "spinning reserve" basically powered up but not generating. Also, as there are times when neither solar nor wind happens, the fossil fuel generation system needs to have enough capacity to cope with the load of all those people that want green electricity from windmills but also want electricity when the wind isn't blowing. That's why if you want to reduce the fossil fuel generation capacity you need interruptible supplies, it's also why the remaining coal-fired generators have been given an extension.
Of course there is also importation of electricity, but at the moment a lot of French nuclear capacity is out of service for inspection, so we are exporting more electricity than importing that the moment.
The actual mechanism of selecting which additional supplies to bring on line to meet the power need is a market system, with a "strike price" i.e. minimum price per kilowatt. When there is extra demand, generators price bid to meet the demand. Where demand outstrips supply, more costly generators such as diesel-powered generators and even eyewateringly expensive supply from Belgium (as happened recently) is brought on line for a short time.
Underneath all this is the National Grid. Remember, the National Grid must constantly balance supply and demand or it falls over. When the grid falls over it's not easy to get back up, local area power supplies must all be stabilised with demand and then synchronised.
This is why the National Grid has what are effectively some big trip switches. If the supply-demand balance fails in an area it will be cut off from the rest of the grid to minimise the damage. This is what happened a year or few ago when there was a grid outage in the SE when simultaneously there was an interconnector failure and a big wind turbine station link failed.
There are also limitations on what geographically located sources can be used, as the transmission network has a limit on capacity. So in principal there might be loads of wind available in Scotland but if your transmission wires bringing power from there are already at capacity that doesn't help much. The National Grid is gradually improving the main links, but that takes time, is costly, and faces the usual planning challenges. This is quite an interesting map:
https://openinframap.org/#5.96/52.439/0.159 showing the main grid with capacities of main power transmission lines.
So to answer your question in short- you face a price increase even though on a "green" tariff because in real life only part of your electricity comes from green sources. The exact ratios of renewable:non-renewable sources will vary on a constant basis, but without the fossil fuel input you'd be cut off when it was calm and dark as there would be no green electricity being generated for you to buy.
TPO