It's Practical Reality
Even the most ardent apologists for 'renewable energy' are baulking at a 20% of total energy target.
The practical reality as shown by many studies is that renewable energy (read solar and wind) are too unreliable to contribute any more than a few, at most 10, percent of total energy demand.
The ability to switch from renewable energy to other sources when the wind drops or the sun sets is the major problem.
First, the existing systems must have 100% of the capacity available so that when the wind or solar stops the other systems kick in. Most are base-load and a few are rapid kick-in systems such as hydro and gas turbine. The other sources such as Nuclear and Coal require hours to days to ramp up to meet the need.
It can quite fairly be said that to build a wind or solar power system you have to build a dam and/or a very inefficient gas turbine fossil fuel system as a backup.
Even with the overcapacity, the true capital cost of renewable energy is not quite twice the nominal price - as hydro and fossil systems are generally cheaper.
The actual benefit of renewable energy has to be measured against how much inefficient fossil fuel is burned on the off days as compared to a highly efficient base-load generators. In actual terms going renewable is not that beneficial and, by some analyses, not worth the bother.
It is no wonder that the UK Government has figured out that a 'jolly good idea to save the environment' has warts on it in all the wrong places.