In the United Kingdom, the optimum size of a solar pv installation is 4kw. This is typically achieved by using either sixteen 250 watt panels or alternatively, a solar pv installer may be able to achieve a 4kw solar pv system by using less panels of a stronger wattage.
The reason this size of system tends to be the most sought after size of solar pv installation in the British domestic consumer market is because it is the largest size of solar pv system that is available on a single phase supply without obtaining permission from the relevant District Network Operator. The reason for needing to apply for and then to be granted permission is to ensure that the electricity fed back into the National Grid does not overload the local substations.
Who a customer’s particular District Network Operator is depends on where they are located in the United Kingdom. The 8 District Network operators across the United Kingdom are; Scottish and Southern SSE Power Distribution, Electricity North West, ESB Networks, Northern Power Grid, SP Energy Networks, UK Power Networks, Northern Ireland Electricity and Western Power Distribution.
Should a customer want a system bigger than 4kw, they must first find out which of the above District Network Operators is relevant to their geographical area. Then they must apply for permission for the solar pv system to exceed 4kw. This process can take up to 65 days. Should permission be granted, the customer should be aware that the feed in tariff rate will be reduced from 13.88 pence for every unit of electricity produced by their solar pv system down to the rate of 12.57 pence for every unit of electricity produced from their solar pv system.
With solar pv systems becoming more and more affordable as time goes by, it does make you wonder whether the benchmark of 4kw will remain.