So much for " Cheap Green " SA Power, page-24

It's simple logic. If you have 100% renewables they have to be backed up by massive storage and that alone will probably double the cost of the energy. Then after a period of no generation(wind & sun) the backup will need to be recharged. So you need enough wind and solar to provide for current needs plus that much again to recharge the storage, so at least 3 lots of duplication.
If you have 50% renewables they will need backup by gas or diesel, it costs to have these assets doing nothing.
Wind energy is a very diluted form of energy and hundreds of generators have to be spread over large areas to collect it meaning a huge web of transmission lines to collect it and if you build them on the cheap.........well we saw what happened in SA.