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Is there a limit on solar PV generation? Q&A

Posted by Paul Hutchens on 11 October 2010 at 9:27 am

Q:  I am about to install a solar PV panel system on my roof and have come across a problem with my Distribution Network Operator (DNO). I want to take full advantage of the feed-in tariff and was planning a 3.995Kwh system on my roof. Unfortunately the DNO insist on increasing the cost to include a relay and a site visit. The reason I am given is because I will go over the 16 Amp limit generated (at max). 

A: The guidelines state that you can install a system of up to 16 amps per phase without asking permission of the DNO. Above that level you have to ask permission so they are within their rights to do as they suggest.

Some DNOs may take a more relaxed approach than others but we have no control over that.

If you do not want to pay for the additional site visit and relay then it may be an option to reduce the size slightly to bring it back under 16 amps.

I suggest that it is worth discussing this with your installer because you can bring down the current drawn by the configuration of the system or use of different solar PV modules.

Also bear in mind that the rule applies to each phase so you can have triple that amount - total 48 amps - if you have a 3-phase supply. However this is likely to be more appropriate for larger buildings such as schools or commercial builidings.

Photo by gustaffo89

About the author: Paul Hutchens is founder and director of Eco2Solar, which installs solar systems around the UK.

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

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8 comments - read them below or add one

Paul Hutchens

Paul Hutchens from Eco2SolarComment left on: 24 June 2013 at 8:55 am

Dear AYusif

The DNO tends to be the arbiter of these matters as they control the network. You can install up to 16 amps per phase (about 3.68kW) without askinfg their permission - beyond that and you are in their hands.

They may allow you to install more, for example they are allowing 25kW on your roof, but it is at their discretion.

If they say that that 25kW is the maximum possible then you can ask them for a quote you to upgrade from 400A breaker - but it may be expensive!

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AYusifComment left on: 23 June 2013 at 11:54 am

My dear fellow experts...

Some countries have followed the UK regulations when it comes to installation of PV systems to the LV grid. In my case. I intend to install around 60 kW on a roof top. Ideally, I would like to connect 25 kW on one roof which is connected to a long feeder line connected to other houses-loads ending with the substation. And another 35 kW on another adjacent roof which has a unique feeder line. HOWEVER; the utility refused this without clarification and asked to reduce it from 35 kW to around 25 kW or even 15 kW..this will cause problems as I dont have enough area and will have to connect with a DC cable and other there a way to convince the utility that it is ok to install the intended capacity on its designated place?


SIMPLIFIED PROBLEM: You can forget about the other roof. Can the utility tell me that they dont want me to install 35 Kw on a roof and they will allow only 25 kW , the circuit breaker at substation is 400A. How can I have a convincing answer for them?

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Steve Milesworthy

Steve MilesworthyComment left on: 5 January 2012 at 8:50 am

I queried the choice of inverter on my 3.92kW system as it limited output to 16 Amps (nominally 3.68kW). I got a standard answer from my supplier as above which I checked with Western Power (the local DNO). WP said they would have reviewed a higher than 16 amp  system for no cost (though at the time - prior to the FiT cut-off date they would not have had the time to meet the deadline).

On doing calculations of expected generation, though, I estimate I'll probably lose only about 15 pence a day (or 0.3kWatt-hour) on a good summer day if the actual summer peak would have reached 3.92kWatts, so unless I achieve more than the estimated output I'm not too concerned.

 Currently, looking at 5 minute data throughout December I've been peaking at around 2.4kW (sustained peaks). This is about 20% higher than I would have expected based on calculations of solar incidence taking into account estimates of transmission through the atmosphere. My current guess is either the panel efficiency is higher than the nominal figure (due to colder temperatures) and/or the air is clearer than average in Torquay. I'll know more by summer.

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Paul Hutchens

Paul Hutchens from Eco2SolarComment left on: 16 February 2011 at 3:02 pm

There is no switching from PV power to grid electricity. The solar energy is converted to AC electricity and fed into your home via a consumer unit; in the same way as grid electricity. As a result you will never know which you are using - it is seamless to the end user or homeowner.

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dominator99Comment left on: 1 February 2011 at 2:32 pm

I'm a 63 year old so a long pay back period for any green technology is a no-no.
I've just come across the 'rent a roof' scheme offered by Isis & Homesun & they seem an ideal solution for me.
I already run an air-source heat pump (5kW output) which draws, on average, around 1kW from the mains supply.  I use the heat pump to heat domestic hot water only (2x117 litre tanks; plus 1x117 litre preheat tank when demand is high) these tanks are normally heated in the morning.
My question is - how is the switching from solar PV power to mains power performed?  Is it an either/or solution ie drawing power from 1 source only or do both feed at the same time - ie if insufficient power is available from PV panel, is it topped up by the mains supply.
If it's either/or is the transition from PV to mains & back again rapid & smooth ie to all intents a continous supply without interuption?

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Paul Hutchens

Paul Hutchens from Eco2SolarComment left on: 25 October 2010 at 7:00 pm

Dear Ben

The figure that the District Network Operators (DNO) quote is 16amps per phase which is about 3.6kWp into to the national grid.
So that would be 10.8kWp split across three phases.

This is for connections without prior warning; you might be able to install a larger array but you would need to send a notification prior to the install so that they can give you an estimate and see the connection points and transformers in the area.



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BenComment left on: 24 October 2010 at 6:43 pm

Hi - do you know what the maximum peak generating capacity a system can have when transmitting to the grid via a 3-phase 440V connection?

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AlternativeComment left on: 11 October 2010 at 11:46 am

We had a similar size system installed a couple of years ago with no problems. We used a Sunny Boy 3800 inverter and if your voltage is 240 then the maximum current is 15.833A, but your DNO might be pedantic and insist that the voltage is 220 which is 17.27A. Worth asking your installer if the output from the inverter can be limited to 3,500 Watts which is 15.909A?

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