> Averaged over a year, renewable energy sources operate at about 30% of their capacity. So even in a worst-case scenario with no storage whatsoever you would need to build 3x "overcapacity".

I don't think that calculation is valid. Even assuming perfectly sunny days solar power e.g. runs at less than 100 % of its capacity simply due to the day-night-cycle. However that doesn't mean that overbuilding solar by just "1 divided by average capacity utilisation %" will be enough to get you round-the-clock-power – in fact no amount of overbuilding will get you any solar power at all at night.

What you'd actually have to look at is define a desired reliability factor for your electric grid, calculate the minimum guaranteed power output for that reliability factor (so to remain with the solar power example – even during the daytime you need to calculate the likelihood and impact of cloudy weather, but in any case as soon you as you want more reliability than there are even theoretical sunshine hours in a day, guaranteed power output drops to zero – unless of course you finally attempt to include some storage) and then overbuild according to that calculation.