What reasonable assumptions? I'm not saying you are wrong, but the energy problem is a quantitive problem. There are many energy sources that are great in certain circumstances and certain regions, but do things actually add up when we create a plan? Without plans and numbers we are stuck with hypothetical ideas that feel kinda right, but you can't solve quantitive problems this way.
That site lets you compute the optimal energy mix under various assumptions for various locations using real historical weather data.
If that Without Hot Air site is the one I think it is, it was from ten years ago, and also assumed large biomass inputs. The latter greatly increases the land area needed, leading to the incorrect conclusion that the UK could not get by just on local renewables.
One good example worked out for Britain is: https://www.withouthotair.com/c27/page_203.shtml
This will of course differ from country to country.