Difference in output between two sets of solar panels

LittlePoached

Hello,

We have had our solar panels for about 2.5 weeks now. We have 9 panels on the front of the house (SE facing) and 6 on the side (SW facing). There is no shading on either set of panels. If I take yesterday as an example, the set of 9 on the front averaged 266W across the day. The set of 6 on the side averaged 55W across the day. Also the second set show the output going up and down all day, often dropping all the way to zero. The set on the front do fluctuate (as I would expect with clouds etc) but nowhere near as much as the ones on the side. It happens every day.

I have contacted the installers but not heard back yet. Does this sound like a fault in the system somewhere or could there be a reason for the difference in performance?

Thanks for any help

Alnat1

This is my first winter with PV, installed last March, and I have SW facing panels. I've been a little disappointed with the output Dec/Jan but realised that the sun is so low in the sky by the time it gets round to them that there's not much generation. Also seems mornings are always brighter and it clouds up more later in the day, grrrr. They did average around 200W yesterday though.

Your SW panels will make you smile in summer when they are still generating well into the evening 

Raxiel

That does seem a pretty severe difference. Having just had a check, my SW and SE arrays have less than 1% variance in their per-panel generation since installed.

Yesterday, the SW produced a bit less though, because it was sunny in the morning but got cloudy in the afternoon. Can you look at the totals to average it out and see if the pattern is still there?

You say there's no shading, perhaps not locally, but how about the horizon? If there's hills or buildings to the west, even some distance away that could result in the sun setting 'sooner' than it would if everything were perfectly flat.

Screwdriva

Do you have optimizers on either banks of panel? The difference in performance is too severe to be justified simply by orientation unless you have substantial shading. 

Do you have an image of the roof aspects taken on a sunny day around noon to share? Google Earth close up image of your roof?

LittlePoached

This is the roof before the panels. No shading as you can see. We don't have an optimizer as the installer said that they weren't necessary unless you had a lot of shading

Screwdriva

LittlePoached said:

This is the roof before the panels. No shading as you can see. We don't have an optimizer as the installer said that they weren't necessary unless you had a lot of shading

Given the distance between the neighboring property, I agree that optimizers aren't required. Something doesn't add up. I'd suggest contacting your installer by phone and forcing a revisit. Happy to help via DM on legal steps to help with this if needed.

1961Nick

I'd be inclined to wait a bit longer & gather some more data before concluding something's wrong. At this time of year, the majority of a day's output can be generated in a 2 hour sunny window with the rest of the day contributing very little. Whether that window falls in the morning or afternoon will dictate which of your arrays benefits.

Looking at my production figures for the past week, most of the production has come in the morning which would suit SE rather than SW panels.

Today: AM  83%, PM 17%

Raxiel

At this time of year, I expect the sun will dip behind the house on the bottom left of that image before it sets below the horizon, so a little bit less on that side wouldn't be unusual as the shadow of the house creeps up onto your roof.

Not enough to explain what you've described by itself though. Are you able to view the sum total generated by each panel since installation?

Edit: Just realised that without optimisers you might not have panel level output, but if they're on separate strings the sum total would still be useful for averaging out the weather.

Alnat1

Three of us have said mornings have been sunnier than afternoons recently. Maybe give it a few more weeks to see if it balances out as the days get longer.

Screwdriva

Without knowing the exact orientation and irradiance of your roof, it is impossible to be 100% certain. That said, such a stark difference in generation typically is rarely due to the reduced winter irradiance, as some have suggested, especially on a SE/SW split. You'd notice a difference of 50% in some extreme cases but in your instance, the difference is far more drastic. (See images below from two different installations, one with a daily E/S/W split w/ shading, and another with a monthly SE/SW split, like yours, as examples) 

I would definitely get a response in writing from your installer, even if it is a commitment to come and re-visit the property in a few weeks to determine if an issue does exist. This will offer some protection should you need to take them to court or take action via your credit card company.  Doing so (i.e. rejecting the goods/ installation service) within the 30 day statutory period, in writing, is extremely beneficial to you from a legal standpoint. 

ispookie666

@LittlePoached
Looks like it's just about right for the orientation. Might want to specify in degrees rather than SW for your panel orientation. I suspect it might not be true SW. 
You could use PVGIS to model what should be expected at this time of the year.