During a typical week at SunGift I can visit a huge variety of people; varying from young professional couples eager to gain an additional income from the feed-in-tariff, to middle aged families who are looking for security against the ever increasing electricity costs, and right through to “Eco-warrior” pensioners who want to save the planet and “stick it to the man” by generating their own, green electricity. Bearing this diversity in mind, it is perhaps then a little surprising that when I ask “What size of system would you like to try and achieve?”, I always get the reply of: “Well…. 4kWp…. obviously.”

I’m sure that everyone who looks into PV for their home is fully aware of the feed-in-tariff and how it is banded; in fact I’d be surprised if this wasn’t the main reason behind their interest, what does surprise me is that consumers and installers alike seem to think that for domestic projects 4kWp is the limit. This is simply not always the case.

The FIT is comprised of two parts, a generation tariff and an export tariff. It is the generation tariff that is particularly important in this case. Currently, the purchaser of a system sized between 0-4kWp would receive 15.44p for every kWh/unit that is produced. For systems above 4kWp (and up to 10kWp) this is reduced to 13.99p. It is this reduction of incentive that used to make more difference than it does now at present rates and I believe has swayed opinion to discounting systems above 4kWp.

Clearly in some situations adding, say an additional 250W module to a 4kWp array may not work out financially. As the benefit gained by the extra energy produced is outweighed by the generation tariff reducing to 13.99ppkWh for the total amount produced by the system, (not just the amount over 4kWp).

There is also the G83 application to consider; where any system capable of putting more than 3.68kW back onto the grid needs to be approved by the electrical network operator. Should the application be approved to proceed or any upgrade costs relatively low, it then becomes a question of economy of scale. A lot of the costs in photovoltaic systems are not pro-rata: it does not cost 20% more for an inverter capable of managing a 5kWp system, than one that is designed for a 4kWp input, scaffolding costs could be the same or only slightly more, depending upon the layout and again labour is not directly linked to system size.

This is in contrast to the power generated by the PV systems themselves, where broadly speaking a 5kWp array will provide you with a 20% higher yield than a 4kWp system. This is demonstrated using industry specific software where a recent installation we completed in Kent was simulated to produce 3131kWh annually if a 4kWp system was installed or 3963kWh if it was increased to 5kWp.


It then becomes a question of cost; If you can get a system that is 20% more productive for less than a 20% increase in cost it starts to become appealing. Yes you would receive a generation tariff of 10% less for every unit, but when you are producing more of those units in total, the finances start to add up.  Take our friend in Kent; his 4kWp system would provide him with £483 in the first year from the generation tariff (3131kWh x 15.44p) whereas on the 5kWp it would give him £554 (3963kWh x 13.99p). Of course this is not the only financial benefit; with a larger system your exports would increase along with your savings on your bills .Throughout the day a larger system will constantly produce more electricity than that of its smaller counterpart; when you are exporting to the grid you will have more to sell back and when you have a high electrical demand on site more of it will be fulfilled by the PV. The benefits of these larger systems are maximised if you are a particularly high user of electricity. In Kent the estimated payback period (excluding increased benefits from inflationary increases in payments and savings) was reduced by six months by spending more up front on the larger 5kWp system.

Ultimately the viability of a system is going to be dictated by a number of things that I haven’t discussed here such as; available roof space, shading levels and budget, but If the circumstances are right investing in a larger system may well provide you with higher financial returns and more energy as well as helping the planet that little bit more. It is my role here at SunGift not to pick a system off the shelf, but to ensure that our customers are given the full facts and are able to make an informed choice on what would be the most appropriate system for their requirements. By offering a cost benefit analysis customers can see the effects of reducing or increasing the size of systems and ensuring they are getting the best possible solution?