How can solar energy be used at night?

Well, to use the energy from the daytime sunlight at night, you have to store it.   If you do not have the means to store it, and you are connected to the electrical grid, (shown below) you do not have to be concerned about storing it because the electrical grid will supply you with all the power you need at night and during cloudy days.

The Electrical Grid (shown above, source is below) http://www.clearcreeknetworks.com/2014/05/02/electrical-grids-101-an-introduction-to-utilities/

The nice thing about being connected to the grid is that if your home produces more solar energy during the day than you actually use, the electrical grid will take that excess energy from your home, use it for other customers, and give you a credit on your electric bill to lower the cost of being connected to the grid!