The size of the photovoltaic array will first be governed by the available roof space you have. Roughly speaking the first 0.5m around the edge of a roof is not suitable. A typical household will use about 4,000 kWh per year and would need a 5.3 kWp system to provide a similar output across the year. Typically this sized array would be about 36 m2 which very few domestic properties are likely to have. Therefore it is important to maximise the generation potential of the array to be installed.
Different panel and inverter manufacturers may influence what your chosen installation company can generate through their offering. If an installation company only installs one type of panel then get another company to quote so you can compare and contrast what is being offered.
For a mains connected system all energy that cannot be used immediately in the building when it is generated will automatically find its way back into the grid. Therefore under the FiT system energy companies assume that 50% is exported and pay an additional sum, circa 3p per kWh, for this electricity on top of the generation tariff. It is therefore prudent to try and use the energy when it is generated, i.e. use washing machines and dishwashers during the peak times of the day between 12:00 and 14:00.
If your photovoltaic system is not producing enough electricity to meet your demand then the additional amount will be taken from the nation grid. For off-grid systems which are usually connected to a bank of batteries for storage of the electricity if the electricity is not used it is stored up to the capacity of the batteries.
There is not export payment but should the national grid fail then electricity could be utilised from the batteries. In both situations it is important to ensure that the photovoltaic array is sized correctly taking into consideration building electricity use and occupant behaviour patterns.