Summary of benefits and payback for all systems

An approximate indication of system returns that can be expected from the Feed-in Tariff and savings in electricity bills are summarised below. These estimates are provided by Wivey Solar Club and not by Solar Systems, who will provide individual estimates after visits to survey properties and to discuss homeowner requirements.

System Cost inc. VAT Annual return Payback period 25-year return
2 kW £3,679.09 £391 – 456 8.3 – 9.7 years £7,532 – 9,025
2.5 kW £4,111.97 £488 – 570 7.4 – 8.7 years £9,415 – 11,281
3 kW £5,296.22 £586 – 684 7.9 – 9.3 years £11,298 – 13,538
4 kW £6,137.75 £781 – 912 6.9 – 8.1 years £15,065 – 18,050

These estimates apply to south-facing roofs with a slope of about 40 degrees, which give the best solar output and returns. It is important to be aware that few roofs match these optimal conditions, so your returns may be less than shown above, as discussed below.

The table below shows how both generation and returns will vary with different roof directions and slopes (click on image to enlarge). If the value shown for your roof direction and slope is 95%, this is an indication of how much of the return shown above you can expect for an installation on your roof (so for a 3 kW with a roof with a direction and slope value of 95%, it would be expected that the annual return would be £557 – 650 (£586 – 684 x 95%)). Shading of panels is another factor that can significantly affect solar output and, of course, there is also some variation in the amount of sunshine we enjoy each year. Shading and low levels of sunlight would further reduce returns.

Wivey solar roof table.xls

Further details for the estimates given are shown by clicking on the system sizes in the summary table of returns above or by following the links shown on the top right of Wivey Solar Club pages. As described on these system pages, the ranges relate to different levels of energy use while the solar panels are generating, as electricity savings will be higher where people are at home more during the day.

Readings from already installed systems in Wiveliscombe show that the solar outputs being achieved are close to those indicated by these estimates.

Assumptions and values for variables used in calculating the estimated returns should also be noted, which are:

  • Feed-in tariff paid at 14.4p per unit for generation and 4.8p per unit for export, which are the rates that apply from 1st April to 30th June 2014.
  • No adjustment is made for inflation so future returns are shown at current values. The Feed-in Tariff increases by inflation, using the Retail Price Index (RPI) each April, and grid electricity costs are expected to increase by more than inflation, so future returns could be higher than current values. If inflation is 2% per annum, this could reduce payback periods by about 6 months and add to 25-year returns.
  • Panel output performance reduces by 0.8% per year.
  • Grid electricity is assumed to cost 13.52p per unit.
  • Annual solar generation for a south-facing 40 degree roof is assumed to be 971 kWh (units) per kWp of installed capacity (so 2,913 units for a 3 kW system), which is the output shown for a south-facing roof with 40 degree slope in our zone from Irradiance Datasets published by the Microgeneration Certification Scheme (MCS).
  • No additional costs arise for maintenance, insurance or anything else, apart from the cost of replacing the inverter once after 10-15 years at a cost of £1,000.


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