Example financial calculations

Experience suggests that with a 10kWh battery you may be able to use about 70% of the 3,400kWh you produce even if you use only 30% directly, i.e. an extra 1,360kWh. Then, your battery will save you 1,360kWh at 34p, i.e. £460 pa. Assuming the battery costs £6,000 the payback time is about 13 years, longer than the currently expected lifetime of the battery. Taking the efficiency of the battery (about 90%) into account the saving reduces to about £420 and the payback time increases to about 14 years.

At 5p per exported unit and assuming you use 30% of what your panels produce, your excess of 2,380kWh pa will provide £118 income. Your total savings will now be about £460 pa and your payback time reduces to 13 years. If you use more than 30% in your home the time is less. 

Let’s assume that you pay 34p per kWh for grid electricity and use 30% of what your panels produce, i.e. about 1,020kWh. Then you will save about £350 pa and your £6,000 will take 17 years to pay off.  If you use 45% the payback period comes down to 12 years and for 60% it’s 9 years. Quotes from some installers will make assumptions about inflation rates and will get smaller payback periods. You have to make your own judgement on how much weight to give to this but there seems to be general agreement that electricity prices will continue to [...]