The cost of those last few percent is absolutely eye watering. The current plan to get it to 100% net zero is by having gas for the final 5% then using carbon capture and storage to offset the emissions.
Or indeed when petrol runs out as happens occasionally (in 2022 most recently). Of course there's also the problem of gas supplies stopping
[Gas is about 11-13x cheaper than renewables with externalities like grid restructure and volatility imputed.](https://papers.ssrn.com/sol3/papers.cfm?abstract_id=4028640) That’s not to say renewables don’t have their place in modern grids. In fact an optimal allocation is around 80% renewable in the UK. However, until battery storage becomes about 95% cheaper, it would be ASTRONOMICALLY expensive to make the grid 100% renewable. Trillions of pounds in the UK. An order of magnitude more expensive than just paying higher gas prices.
People often make the mistake of thinking that because wind and sun is free, the cost of energy generation is basically free. Those people don’t understand how many fixed and marginal costs there are. If you’re interested, the paper above provides good insights into those costs.