> You can get to approximately 50% penetration safely without storage[1], then you have to start adding factors that can deal with intermittent sources like wind and solar[2].

Keep in mind that this number will vary depending on generation mix. Specifically, it will depend on the response time of your dispatch-able generators. If your primary source of non-renewable generation is big steam turbines with a painfully slow ramp-rate (e.g. coal generation) then you're going to have a bad time.