Low Efficiency from Panels - What Numbers Should I be Seeing? 5 December 2022 at 10:13AM in Green & ethical MoneySaving 24 replies 652 views

Nsar1

I'm troubled by the efficiency readings I'm getting from my system and would appreciate some advice.

We are in the north Manchester area, the ridge line of our roof (+/- 30 degrees pitch) is exactly N/S and we have 11 panels on both sides of the roof (ie 22 panels) - JA Solar 410W Mono MBB and a 5kw Sunsynk inverter. No batteries .No shadow anywhere on the roof.

Saturday was bright sunshine all day long and I have been looking at readings I'm getting off the roof over the course of the day

09.54 - 551w = 6.1% efficiency
11.06 - 638w = 7.1%
13.31 - 698w = 7.8%
13.54 - 321 = 3.6%
14.49 - 603w = 6.7%

I appreciate that December sun is fairly low in the sky but these numbers strike me as very low.

EricMears

You appear to be expressing "efficiency" as a percentage of the total kWp of the system.  That would only work if the panels were pointing directly at the sun through a cloudless sky;  that's clearly not the case.

The only meaningful comparison you should be making is to check the pvgis webpage for an idea of the expected generation and work out your actual generation as a percentage of that.

You can find the pvgis estimation tool at https://re.jrc.ec.europa.eu/pvg_tools/en  

but that tends only to give you results for a whole month (and of course you'd need to treat your two sets separately and add the results together).  There are ways of interpolating results so that you can estimate some daily figures but it's a lot of work for little benefit.

Nsar1

EricMears said:

You appear to be expressing "efficiency" as a percentage of the total kWp of the system.  That would only work if the panels were pointing directly at the sun through a cloudless sky;  that's clearly not the case.

The only meaningful comparison you should be making is to check the pvgis webpage for an idea of the expected generation and work out your actual generation as a percentage of that.

You can find the pvgis estimation tool at https://re.jrc.ec.europa.eu/pvg_tools/en  

but that tends only to give you results for a whole month (and of course you'd need to treat your two sets separately and add the results together).  There are ways of interpolating results so that you can estimate some daily figures but it's a lot of work for little benefit.

Thanks that figure is what the app is showing

EricMears

Nsar1 said:

Thanks that figure is what the app is showing

Afraid it's not a very useful app - or at least that aspect of it isn't !

chris_n

With panels facing East - West at this time of year the sun is never anywhere near 90 degrees to them, it is also very low in the sky. If you want better generation in winter you need to fit some panels on your South facing wall. Even then light levels are much lower even on a clear day as the sun's radiation has to pass through more of the atmosphere.
I am guessing you are new to solar, this is normal for December. 

Nsar1

chris_n said:

With panels facing East - West at this time of year the sun is never anywhere near 90 degrees to them, it is also very low in the sky. If you want better generation in winter you need to fit some panels on your South facing wall. Even then light levels are much lower even on a clear day as the sun's radiation has to pass through more of the atmosphere.
I am guessing you are new to solar, this is normal for December. 

New to solar panels, not to the points of the compass or indeed that winter days are shorter. But thank you for your comment.

Waywardmike

I had to go to last year as there hasn't been a sunny day here in Bedford for well over a week. 2nd Dec 2021

My peak output was ~2.5kW at around 11.20am.  

My system is 4kW panels (3.68 Inverter) slightly south east.  I have a large building in front of my house which the sun doesn't clear until ~9am.  Looking at this, yes, I would say yours is low.

*BUT*  every system is different!  I'm significantly further south than you, and maybe my comparison day was a particularly unhazy super clear frosty day.

Nsar1

Waywardmike said:

I had to go to last year as there hasn't been a sunny day here in Bedford for well over a week. 2nd Dec 2021

My peak output was ~2.5kW at around 11.20am.  

My system is 4kW panels (3.68 Inverter) slightly south east.  I have a large building in front of my house which the sun doesn't clear until ~9am.  Looking at this, yes, I would say yours is low.

*BUT*  every system is different!  I'm significantly further south than you, and maybe my comparison day was a particularly unhazy super clear frosty day.

Thanks, it was gin clear, sunglasses weather all day Saturday and I can't imagine c. 200 miles further North is going to make a substantial difference.

JKenH

I’ve pulled up the data for my 7.8 kWp East-West system for comparison with someone in the same village (100metres away) with a 4kWp SSE facing system. I chose 25 November as it was the last day with continuous sun rather than intermittent  sun which distorts the readings. 

You will see my peak was 1.53kW compared with 2.28 KW for the SSE system. Applying the same “efficiency” yardstick you used (ignore the pedants, it was obvious what you meant), my efficiency was 19.6% compared to 57% for the SSE system. 

I should point out that my system is underperforming due to panel issues with one string on one roof down about 12% and one string on the other down 24% relative to the other. Overall about 9% down on what it should be. Without those defects I might have expected to see about 1.7kW peak.

My system 

Neighbour’s system

Edit: Having just looked back at your post and your “efficiency” I think you have a problem with your installation. It might be an idea to try and find a domestic system in your neighbourhood that posts live data online so you can compare your system on a day to day basis. I do and it’s depressing. 

ABrass

My 4kW WNW string was producing a peak of 600W today in the afternoon. Your numbers seem about right. 

JKenH

ABrass said:

My 4kW WNW string was producing a peak of 600W today in the afternoon. Your numbers seem about right. 

That’s still around twice the “efficiency” the OP was reporting around lunchtime. 

Edit: what was your SSE array producing - just curious?