[Aussie Mike]

If there is a power outage the system is designed to shut down. It's a safety thing I believe. If a linesman is working on the lines he won't get hit by any power coming back from the houses.

As for the usage I think the best analogy is you have a "tank" of electricity capacity available at your home. It holds as much capacity as your system produces at that instant. For example All the appliances and stuff in your home fridge, TV washing machine is drawing 2.5kW and your PV system is delivering 3KW. At that moment you are using 2.5kW of that capacity and the rest is fed into the grid. Your smart meter will record that you are producing more than you are using and you will be credited 8c per kW/h as it goes back into the grid (500W feed back in for 2 hours is 1kW/h). You are effectively selling that unused electricity back to the company wholesale.

After the sun goes down the solar system will automatically shut down. Now you are pulling direct from the grid. Your lighting, TV, the oven cocking dinner are all drawing on the grid. You are then pulling electricity from the grid and paying 24c kW/h retail for it.

This is why it can be beneficial to do most of your electric use during daylight hours. Run the washing machine, pool pump etc. Your own capacity to produce electricity is used first and the rest goes to the grid. If you are using more than the capacity of your PV system then the extra is pulled from the grid at retail price.