The intermittent nature of renewables like wind and solar mean that they require some form of backup power to make sure the lights stay on. This is a problem that has to be solved for wind and solar to truly take off.
Promising energy storage options are in development. These include batteries, compressed air, flywheels, and hydrogen production. Without economical backup and storage options, renewable power will be unable to reach its full potential. That’s why I have referred to this as The Most Important Problem in Renewable Energy.