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Don't limit your solar PV system to 4kWp

Posted by Gabriel Wondrausch on 27 February 2013 at 9:37 am

During a typical week at SunGift Solar I can visit a huge range of people. From young professional couples eager to gain an additional income from the feed-in-tariff, through middle aged families looking for security against increasing electricity costs, to Eco-warrior pensioners who want to save the planet and “stick it to the man” by generating their own (green) electricity. Given this diversity, it is perhaps a bit surprising that when I ask what size of system they would like, I always get the same reply: “Well ... 4kWp ... obviously”.

I’m sure that everyone who looks into PV for their home is fully aware of the feed-in-tariff (FIT) 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 best limit. This is simply not always the case.

The FIT is comprised of two parts: a generation tariff  (paid for what is generated, whether you use it or not) and an export tariff (paid for what is exported to the grid). It is the generation tariff that is particularly important in this case. Currently, the purchaser of a system sized between 0-4kWp  receives 15.44p for every kWh/unit that is produced. For systems greater than 4kWp (and up to 10kWp) this is reduced to 13.99p. I think it is this lower tariff rate that has caused people to discount systems larger than 4kWp.

Clearly in some situations (say an additional 250W module to a 4kWp array) adding more panels may not work out financially, as the benefit gained by the extra energy produced is outweighed by the reduced tariff. There is also the G83 application to consider; any system capable of putting more than 3.68kW back onto the grid needs to be approved by the electrical network operator.

However, assuming the application is approved, and upgrade costs are relatively low, it then becomes a question of economies 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 it does for one designed for 4kWp. Scaffolding costs could be the same or only slightly more, depending upon the layout, and labour cost is not directly proportional to system size.

This is in contrast to the power generated by the PV systems, 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.

The main message is: 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.

Carrying on with the example in Kent: His 4kWp system would generate £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). This is not the only financial benefit; 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 will be dictated by a number of things that I haven’t discussed here including available roof space, shading levels and budget. But If the circumstances are right, investing in a larger system may provide you with higher financial returns and more energy as well as helping the planet that little bit more.

It is my role 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 size and ensure they are getting the best possible solution.

Written by Joe Perry

YouGen has answered some common questions about feed in tariffs here, including how to calculate your return on investment, how to maximise returns, and what happens if you move house, among many others.

Related information:

Solar PV guide

Find a solar PV installer

About the author: Gabriel Wondrausch is founder and director of SunGift Solar, which installs solar thermal and other renewable energy systems in the South West of England.

If you have a question about anything in the above blog, please ask it in the comments section below.

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13 comments - read them below or add one


Darrens1973Comment left on: 8 June 2015 at 2:24 pm

I know this thread is a couple years old now but I have in the last 10 months installed 2 x 5.76kwp systems in Cornwall, WPD are pretty quick at responding too. We have fitted solar edge to them as one is East West split and the other has shading issues. On a good day we are almost hitting 40kw of generation. 

Regarding the post above from Chas - did you request all 4kwp to be put on one elevation? Splitting is always better so you get a good amount generated right through the day,  no impact on the yearly total but will make a big difference to what you use of your free self generated electricity. 

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DistantHillsBnBComment left on: 24 April 2015 at 8:31 am

I installed a typical 4kw panel system in 2012 on our South facing roof but have been hearing about improved effificences and of course lower cost of modern panels.

I have a South West facing vertical gable which seems to get afternoon sun from 3 to whenever the sun goes down (there is no shading) and I  am curious to find out if it would be worth adding further panels here. We would certainly use the electricity or dump it into a thermal store.

Questions are:

Would it be better to install an additional invertor (by late afternoon the original panels are in shade) ?

Can installers mount panels on a vertical gable with some sort of fittings to give the best angle to the sun? (is 30 degrees best?)

Would it be worth it financially?




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tony nicholson

tony nicholsonComment left on: 13 March 2015 at 8:33 pm

luckily for me my south facing roof was only big enough for a 3.3Kw system, the inverter has two imputs and tracks half the panels on each input, the system has only been operational since 3rd Feb 2015 but has produced up to 2.7 Kw and 14.4Kwh on a good day....cant wait for summer!

ps Id better buy an air conditioner put some of that 'free' power to good summer days use!

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SlotsComment left on: 14 October 2014 at 4:59 pm

I think there may be more to this than meets the eye? There are two types of inverters used with solar panels: high voltage string and individual panel microinverters.

The latter as far as I know must comply with G83/2 to be sold in the UK. This means the inverters max current output must be Watts(say 250)/230V however if the inverter is capable of handling 300W and connected to a 300W panel the inverter will clip the power to a maximum of 250W.

The limit is 230V x 16A = 3680W which means 14 panels is the limit for G83.

Not quite!

16 panels at 300W (4800W) can be connected however the maximum generated would be 3680W. At certain times of the year power clipping would take place. But consider that most of the time the power generated is below 3680W for 250W panels. With 300W panels the power would increase but still be below the 3680W limit.

Certainly G59 is an option but don't dismiss G83 as the limiting factor. This power clipping also takes place in high voltage string inverters. The MCS installer has to say whether the inverter clips or not when the installation is regist eredthere is no requirement to say which make of inverter is used.

I think I have this right but you may know better?



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ChasComment left on: 6 October 2013 at 10:39 am

I have a 4 Kw system currently installed

the Apex of my roof is almost due north -south and the system installed is on the west facing aspect of the roof

the system generates a good flow of power, but only (particularly at this time of year) only in the afternoon.

In the summer significant power is generated after about 11am

The system is also coupled to a 'Solar Boost' device which sends power to my 3kw immersion heater when the system is generating more than 3Kw spare power which on a summer afternoon is frequently the case.

The system was installed in march this year and the tarif at that time applies

I would like to know if there would be an advantage to fitting additional panels to my East facing roof which  (as I write in the morning at 10.30am) is completely bathed in sunshine but of course generating no power in the morning, which is a time at which it can most often be needed.

How would this affect my tarif and would I need a second inverter. (A neighbour even suggested putting each system on a timer, so that only one set of panels feeds to the inverter at a time ie., one in the morning and the other in the afternoon. Not sure if this is do-able or even a good or bad idea)


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SunGift Energy

SunGift EnergyComment left on: 10 June 2013 at 11:44 am

Hi Suqui, 

It would certainly be possible to install a split system feeding into two phases, but this would require two inverters and hence the associated costs.

A further limiting factor is that the feed in would then be unbalanced, something the DNO may not be happy about.

Ideally the system would be installed with a three phase inverter, splitting the load equally and therefore within G83/1. 

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SuguiComment left on: 7 June 2013 at 2:59 pm

Hi all,

I would like to know if we are installing 4.5kwp say 14 sunpower 327w, due to earning poits towards our code for sustainable homes, and we have a 3phase suply.

Could we split it into two systems maybe 2kwp and a 2.5kwp feed to two of the phases or do we need to put the15 panels and split the load evenly into the tree phases, with the intention of staying withing the standard sigle phase G83/1.

Please help !!!


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Sims Solar Ltd

Sims Solar LtdComment left on: 2 March 2013 at 6:46 am

Great article. The whole G83/G59 debate goes on. We as a company routinely install 4kW inverters on a 4kW south facing install. The additional cost if any for a 4kW inverter is marginal. My DNO (SSE) is great ant turns around applications in 5-10 working days so no material delay. 

Regarding whether to go 4+kW totally agree and we do a number of 6kW installs (which from experience SSE will allow us to install on a single phase supply without reinforcement). With the

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DimitriComment left on: 1 March 2013 at 3:45 pm

One point not often made in these discussions is that if you install your ,let's say 8kwP system as two seperate ones, you will have two seperate generation meters each of which can be at the highest tariff.

In my case I have one 6kwp system on the house roof and a 4kwP on the garage.

I'm now over two years in and generating almost £4,000.00 p.a. as well as free electricity daytimes during the summer.


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SunGift Energy

SunGift EnergyComment left on: 1 March 2013 at 2:05 pm

Thanks for your comments.

I agree that the G83 application is an important factor to consider, and as discussed in the fourth and fifth paragraphs of my blog, the viability of a project exceeding such 16A per phase limitations can often rest upon the successful DNO line diagnostics; permitting a system to be installed outside the G83 parameters.

The point of this blog however, is that for the majority of people the idea of installing a PV system larger than 4kWp isn’t even considered. Through my experience this can be attributed to a number of factors ranging from misinformation to a lack of installer knowledge or even just not taking the time to look into it at the design stage – it’s very easy for a company to just offer “the standard 4kWp” whether it’s the most appropriate system or not.

If any of you attended the Solar Power Roadshow yesterday at Exeter Racecourse, this issue was highlighted further by Cathy who discussed an instance where a “Clued-Up” customer did his research and decided that a 10kWp SunPower array was the right system for him, only to find that no companies were interested in installing it for him as he was a “Domestic” customer and as such had to stick to 4kWp. Fortunately Mr Barrowcliffe got in touch with JoJu Solar who found out that he could in fact put 10kWp down his single phase supply. His original blog and experiences can be found here.

In some circumstances, yes it’s well worth sticking to the 4kWp limit dictated by both G83 and the FiT, in other cases the variables may work in your favour and a larger system could be better suited to your requirements.

In either event the idea of a system larger than 4kWp should not automatically be ruled out without first considering whether the factors that dictate that limit are relevant to you.

Here at SunGift we design systems that best fulfil your specific requirements, whether that’s in financial terms or in energy production. All necessary applications, registrations and certificates (including the G59 application) needed for a successful installation are handled by our project management team and part of the package offered as standard.

Any reputable installer should already be considering this option for you, however more and more these days it is unfortunately being overlooked!


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Heliotricity Solar

Heliotricity SolarComment left on: 1 March 2013 at 9:33 am

Yes, we agree with Rudge Renewables. UK Power Networks (SE England) will allow 3.84kWp systems before a G59 connection agreement is required so the article, while correct in its argument for economies of scale, is misleading for domestic customers.

If you have a large garden/field, or a 3-phase supply however, do consider scaling up to a larger system. The planning applicationion and DNO agreement may well be worth the effort for the returns involved.

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NicoleYouGenComment left on: 28 February 2013 at 9:29 am

Thank you for that, Rudge Renewables. That's really helpful!

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Rudge Energy

Rudge EnergyComment left on: 27 February 2013 at 4:39 pm

If your considering this 5kWp option, check with your DNO first (Western Power in Devon And Cornwall), or best to get your installer to, as one of the main reasons 4kWp is so popular is that to install a system which can export more than 3.68kW (16Amps) is outside the single phase G83/1 standard.

We almost univerally get the response from the DNO 'no more than 3.68kW without a network upgrade'... which of course the customer has to pay for.

A few years back, this was not so much of an issue with the DNO's, but the massive uptake of PV installations over the last couple of years has hardened their attitude as PV systems have been causing quite large grid voltage fluctuations in some areas.

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