This can depend on several different circumstances including;

• The size of the system you get
• Usage rates (that change over time)
• The amount of solar vs grid power you use
• Your plan's feed-in tariff and cost of electricity

However, many clients have reported repaying there solar system within 4-8 years.

A solar panel can only produce energy when exposed to sunlight, therefore at night is not producing energy. However, with the addition of a solar battery, excess electricity is stored and available to be used at night. 

Solar power installations generally last up to 25 years. We can find solar panels delivering power which were installed more than 30 years ago. Other components such as inverters, battery, controller systems will survive a decade in most cases.