I live in the U.S. and selling power back to the grid depends on the company in the area but as far as the solar panels they all work the same. I will explain what I know. The panels generate electricity as long as there is light. If there is a little bit of light then they generate a little bit of electricity. Therefore, always be careful not to mess with the connections on the panels because they could be generating electricity and you can get electrocuted. Most installations aim at meeting only around 70% of your electric needs and if that is the case then you will still need to get electricity from the grid. Regardless if you are making enough electricity to cover your needs you have to pay fees each month for being connected to the grid. Some solar installations have batteries, which is expensive, to store electricity which you can use at night or when it is cloudy. If you do not have batteries then you need to be hooked up to the grid to get electricity when it is night or cloudy. If you are going to sell power back to the company then you need to be connected to the grid. Some power companies buy back electric and some only credit you which you can use for electric you use off the grid.
So to answer your first question you have to be connected to the grid unless you know you have batteries, can generate enough electricity to be self sustained, and are not selling electricity back to the power company.
For your second question. Water can be heated with heat from the sun, electricity, or gas. So you will need to figure out which source heats your water. If it is gas you will see gas lines going to the water heater. If you have a water heater and electricity is running to it then it is electric. I am not sure if a water heater setup that gets heated by the sun has a water tank in the house.