With the overall output of the system referred to as kilowatt peak or kWp (1 kilowatt is 1,000 watts) – which is the amount of electricity that the complete solar pv panel system will generate at peak performance.
One of the larger PV systems you could find on a family home would be around 4kWp (4,000 watt output) and if you use 280 watt panels you would need around 14 of them, which will peak at around 3,920 kw output per year.
You should bear in mind that the panel itself will not operate at 100% output and so a 4kWp system may actually produce 3,400 kw per year (85% efficiency). You may also have to take into account any shading that the panels suffer as this will also reduce performance.
The free electricity generated by the solar panels, when used around the home will significantly help to lower your electricity bills. Any electricity that is generated and not used in the home can be exported back to the national grid and the homeowner will receive a payment for this from the Feed in Tariff.
In terms of the Green deal & solar panels, you should bear in mind that the income from the feed in tariff may not be taken into consideration under a Green Deal Assessment when determining if the improvement meets the “Golden Rule”, just the savings that will be made on the household energy bills will be used to make that assessment.
Given a published cost of 18.9p per kwh for domestic electricity, (EDF July 2017) depending on the size, quality & output of the solar pv system you install, it could be reasonable to expect savings of a few hundred pounds per year on your domestic electricity bill.