MSE guide: plug-in solar panels

QrizB

The article compares (unfairly) the claimed unrealistic savings figures for plug in solar with real world savings for rooftop PV. To get anywhere near the claimed savings you'd have to use every kWh generated at the time it was generated.

I don't think you're correct on that point .

The article says:

The Department for Energy Security & Net Zero (DESNZ) says plug-in solar could save households about £70–£110 a year on energy bills.

Based on these costs and savings, the average break-even point is about four to six years.

An 800 watt plug-in solar PV system installed at an appropriate angle, away from shade and with a south-facing aspect will generate between 600 and 800 kWh per year, on average (800 in the south of England, 600 in the north of Scotland).

You might reasonably expect to use half of that, so 300-400 kWh a year.

With single-rate electricity currently priced around 25p/kWh, that's a saving of £75-£100 a year. And with systems expected to cost £4-500 you'll be looking at a payback time of 4-7 years. And that's a pretty close match to what the article states.

Why do you think the article is wrong?

NedS

https://forums.moneysavingexpert.com/discussion/comment/82107274#Comment_82107274

Personally I think the savings of £70-£110 per year, quoted from DESNZ, are quite conservative.

At my location (southern half of UK), the predicted PVGIS generation is 836kWh per year, which at my SVR equates to £220 per year, assuming I can (a) use everything generated, and (b) mount the panels such that they get a decent amount of unshaded irradiation. Obviously those with higher baseload usage will benefit more, but I think the assumption of a third to a half (£70-£110 per year) usage is not unreasonable.

For reference, when we had our original 3.6kWp roof top array installed, we managed to consume a third of our generation in summer rising to 75-80% in winter, so for a small 800W array an assumption of 50% annual usage and a saving of £110 are reasonable. If you know that your use case will be significantly different, then you can factor that into your buying decision.

FFHillbilly

https://forums.moneysavingexpert.com/discussion/comment/82105881#Comment_82105881

seems bizzarre that the DNO's are asking for G98 paperwork for these things that come on a plug and re designed to be portable. do they really think thats going to be followed stricltly, because I don't.

So Paul991 asked about installing an additional 800w of panels for example facing due east to get early morning sun probably before any sun is even on his rooftop array, and once the sun is up and the rooftop array is genereating near it's peak the new plug in solar is likely not generating much so it seems unlikely to ever exceed the 3.68kw.
and his inverter is likely already set with an export limit of 3.68kw anyway

Eightseconds

I expect Amazon and other marketplaces will be full of addon kit if all possible qualities to wire these into your flat. Can you comment that we expect some very unsuitable and even dangerous products among these?

NedS

https://forums.moneysavingexpert.com/discussion/comment/82107505#Comment_82107505

You are probably right that many people may not complete the G98 and notify the DNO, but this has the potential to create issues for the DNO if they are unable to plan local grid upgrades effectively because they do not have accurate records of what is installed on the grid locally. The net result is that everyone suffers potentially higher voltages that may then exceed the statutory range (253v max).

The issue with adding an 800W plug in solar system to an existing 3.68kW system is that the majority of 3.68kW systems have no control or monitoring of export - they are limited by virtue of their max output rating, nothing more. If you add an additional 800W system then they have the potential to generate 4.48kW, and that requires G99 approval regardless of what you may or may not export to the grid.

Eightseconds

You have made the common mistake of dividing annual output by 365 to get daily output. This is not how solar works. Summer and winter will differ. This means:

If your panels are accessible on a balcony you can improve performance if you have two different angles and change the tilt at the spring and autumn equinox. ( you could be fancier too).
In summer you will struggle to use all the power, though aircon in the day could soak some up.
Small home batteries (same size as a dehumidifier) are about to be launched and these will allow you to use power 24 hours until you run out, improving payback for some. These will also allow access to cheap overnight power in winter. ( Also need a warning that DIY battery kit is dangerous so you must always get something designed for the job. )
Many modern flats have electric heating and this has completely different annual usage patterns skewed to the winter months. These people will be very interested in balcony solar.

Predicting annual performance and payback is hard for solar. But that’s even more reason to take time to explain the seasonal variations and the ways to make it work well for the widest range of use patterns. Simple monthly performance models help, Averages are extremely misleading.

prowla

I have a solar system installed and, because of the angle of my house its yield drops off towards the end of the day.

I intend to acquire a plugin system with its panels located on a fence to augment the main system by picking up the sun for that last 1-2 hours.

My total output won't exceed my G99, but it will help extend my systems effective working day.

gjd0496

I was aware these were coming so have done some research of my own.

One thing you don't mention is a safety issue that, I believe, held up the approval for these panels in the UK.

This is the fact that the original panels generated power even if your feed from the national grid was dead, for instance during a power cut. Although this could be seen as a benefit, it is also a danger as your mains wiring would have been live even though the mains power was off. Anyone working on the electrical installation either at your house or even up a pole trying to fix the fault could have received a shock with dire consequences.

I understand that this issue was addressed and that the panels about to be released are safe but it wold be good to know how this works and in any case it deserves a mention.

Fredwriter

I have a roof system that is now about 14 years old. Because of its position it is virtually impossible to clean and also has lichen growing on it. Would I be allowed to add portable solar panels simply to restore the system to its legal limit of generation and feed in potential? Questions concerning permanent rooftop solar panels are really important considering how many homes already have this form of generation. Hopefully MSE will come up with answers in this area.

QrizB

@Eightseconds wrote:

Small home batteries (same size as a dehumidifier) are about to be launched

"About" is doing some heavy lifting in that sentence. They're not included in the current IPS and all government has said is that they'll continue to investigate them.

@gjd0496 wrote:

This is the fact that the original panels generated power even if your feed from the national grid was dead, for instance during a power cut.

You must be going back a long way with your recollections here. While the panels will always try to generate electricity whenever sunlight falls on them, UK approved grid-tied inverters (the thing that turns that DC from the panel into usable AC) haven't done this for at least 25 years. The first issue of engineering recommendation G59 was in 1991.