The problem of storing energy is true for many renewable energies but there is work being done to solve it too. An example for solar power is the use of specia

The problem of storing energy is true for many renewable energies but there is work being done to solve it too. An example for solar power is the use of special salty compounds that maintain the temperature for much longer (though I'd imagine at the cost of taking longer to reach high temperatures, i.e. they have a very high heat capacity) and hence provide a more constant output. Another one is to connect one of these to an hydroelectric plant: any spare electricity is used to pump water up the dam. Finally solar power has the advantage that it's plentiful during the day and especially in the summer which is when the electric demand is highest. In the end solar power alone won't solve anything, but bundled with other renewable and with gas/nuclear to provide a steady baseline it should be fine.