It's a subsidy when the state ends up paying the difference between the guaranteed price and the average market price.

Rappel : le président du Sénat suppléante le président de la République. Si Macron ne peut plus remplir ses fonctions, c'est Larcher qui est temporairement au pouvoir (article 7 de la Constitution de 1958).

I guess another way would be fixed prices for all nuclear and renewables since they have fixed costs anyway. The thing is, the system already works that way to an extent, as there are a lot of guaranteed prices in renewables and some in nuclear. As far as I can tell, these subsidies are why the market price sometimes goes negative when solar or wind produce a lot: the subsidized sellers don't care, they still receive their guaranteed income based on the quantity of energy they inject into the grid. It seems to me that without these subsidies the market prices would always be high, thus better reflecting the true cost of electricity.

Setting the electricity market price based on the most expensive source at any given time is pretty much universal in Europe. It's an EU thing, not a specificity of the UK.

Changing this rule would reduce the income and profits of climate friendly electricity providers, thus penalizing the investors who tried to do the right thing.

Le fait que l'Australie arrive à faire baisser les prix sur le marché de l'électricité en subventionnant l'installation de batteries est intéressant, mais attention aux comparaisons simplistes avec la France et l'Europe.

Je rappelle que l'Europe est une des pires régions au monde pour l'énergie solaire, alors que l'Australie a plutôt un bon potentiel (source : Global Solar Atlas). Malgré cet avantage naturel, l'électricité australienne est encore 10 fois plus nocive pour le climat que l'électricité française (source : Our World in Data).