Unless you have a stand alone system, your home solar photovoltaic system is likely to be connected to your local grid.  This means that the surplus electrical power you generate in the day time can be sold to the grid, and in the night time you can draw power from the grid.

You need to have special equipment installed for this to happen and your installer or supplier can advise you on this.  Be sure to check out any applicable regulations if you are doing this yourself.

Once set up, the outward and inward flows of electricity current happens automatically.

In some jurisdictions, when you supply power to the grid, your meter runs backwards, and when you draw power from the grid it runs forward. Your meter then records your net position. In other jurisdictions, there is a price differential in what you pay for power from the grid and what the grid pays you for your power.

You need to check the position out with your local power company or your installer.

Here is an interesting example of what problems legislators can run into.  In late 2006, California enacted a law that required home suppliers to sign up to the highest domestic daytime tariff before selling power to the grid.  The result was an overnight collapse in the domestic solar industry, as some people found they were paying more for power than previously.   Fortunately, this has now been remedied on a temporary basis with the passage of legislation on 7 June 2007. See announcement here.

In the long term, should home solar power systems become really popular, there may be other problems to consider.  At the moment, this is a hypothetical case, but it makes interesting reading.  Go to the Solar Power Article 005: Unitended Consequences (7 November 2007)