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Are 4kW solar PV installations a thing of the past as network operaters tighten the rules?

Posted by Cathy Debenham on 30 August 2011 at 5:43 pm

Q: I want to install a 4kWp solar PV system to maximise my return from the feed-in tariff, but my installer says that Western Power will only let him install a 3.68kWp system. Why is that?

A: As more and more people install solar panels, the district network operators (DNOs) are having to work harder to balance the grid. This means that they have started to strictly enforce rules, where previously there was some leeway.

Up until now most installers have just been notifying DNOs of installations of up to 4kW within 30 days of commissioning. Now they make what's called a stage 2 application to the appropriate DNO for permission to connect all systems of 3.7kW or above, before going ahead and connecting it to the grid. They should also apply if they are installing more than one system of less than 3.7kW within the same postcode. 

Scottish and Southern Energy (SSE) explain the situation as follows: "The effect which concerns us most is the rise in voltage caused by power flowing in the opposite direction when the PV output is greater than the customers own load. The electricity network in the UK has been designed for power flows from higher voltages down to lower voltages and finally to the customer. No allowance was made in the system design for power flowing in the opposite direction.

"For a single property with the maximum generation allowed under G83, of 3.7kW, there will be many times when power is being exported from a property. Most of this will be absorbed by adjacent properties and will not feed back in to higher voltage systems and will generally not cause a problem, though it might do in some circumstances. However ... where multiple installations are proposed, it is more likely that power will flow in the opposite direction to that for which the system was designed."

There is a tendency for solar PV installations to cluster in areas. That's because one person in a street gets one. Others are interested and find out more, and often recommend their installer. More people then follow suit and install a solar PV system.

Some DNOs are charging for stage 2 applications. An impact assessment for multiple systems under 3.7kW per phase from SSE costs £250 + VAT - or £100 + VAT for single installations between 3.7kW and 4kW. So far Western Power does not seem to charge, but it has only started tightening up on the rules more recently than SSE.

Both reserve the right to charge if work is needed to reinforce the network to enable a PV installation to go ahead, and these charges may be substantial.

In essence this means that you can no longer have a Grid connected renewable power system that can export more than 16Amps (3.7kW) without getting  advance permission; and that if there are lots of other solar PV installations near your property it may well be wise to check with the DNO whether you need permission before going ahead.

In the short term there may be some confusion, as installers get used to the the changes and DNOs introduce new ways of working. So it makes sense to ask your installer whether they need to apply for permission, and if in doubt, check.

In the long term it probably won't make too much difference according to one of YouGen's energy experts. "In the UK for the most part a 4kWp system hardly ever generates 4kW," says Chris Rudge. "There will be occasions on a south facing roof in midsummer, when the module temperature will be low enough to maintain efficiency, you will see the meter clock over the 4kW mark, otherwise it will lurk around 3.5kW, in bright sun, obviously less as brightness reduces. So, carry on installing those arrays of 3.98kWp, etc and connect a 3.6kW inverter. That way, you'll still be generating just about the same annually as you would by fitting a 4kW inverter."

He also warns: "If the output of the system may exceed 16A, do not install and connect the system first, or you may be issued with a disconnection notice."

More information about solar PV installations from YouGen

Solar PV information page

Don't limit your solar PV system to 4kWp

Is efficiency or cost per watt most important?

The business significance of the Government's solar step-change

Is solar PV suitable for my home?

Find a solar PV installer

Photo by Jo-h


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

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1 comments - read them below or add one

Susan Jonas

Susan JonasComment left on: 2 October 2011 at 8:59 pm

Isn't it about time some money was invested in updating the National Grid?

My 4.725 kW system was installed in May 2007 - during the summer months - even though I am at home most days - I can't use all the power generated.  Before FiTs I was encouraged to export as much as possible and was paid a decent price for it.  Now I am beginning to understand why the goal posts have moved and there is no big incentive to export - in the long run this seems crazy!

I'm being paid a rubbish price for my electricity but at least I had an export meter fitted for free!

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