Panels working correctly?

[Jenx]

I have a 3.9KW system, but it only imports a maximum of 2.8KW - even on a sunny day when there are no couples in the sky!

Could there be a problem with the panels? I assumed that they would import somewhere between 3.6 and 3.9KW or is it normal for a 3.9KW system to import a number so much lower than its capacity?

Thanks in advance for any information or advice.

[SunEagle]

Well depending on what the output of each of the panel is and if there is a string inverter or micro, can determine the amount of power produced. The main reason for a lower then expected output could be the time of year and how the panels are pointed. Also a panel watt rating is determined under an ideal lab condition and not the real world. But it based on your numbers it does seem to be a problem with your output.

[Jenx]

Thanks.

They ere installed by a company (10 panels of 390w each), on a 45 degree south facing roof with no obstructions. And it’s mid summer here in UK.

I keep asking them to send someone to have a look but they keep saying they are working fine!

[J.P.M.]

Jenx:

1.) If you haven't done so already, download a free PDF of "Solar Power Your Home for Dummies". It's a bit dated but still useful. You'll find a lot of basic info you'll need to understand that will help you make sense of and perhaps troubleshoot your system or at least help you have a more informed conversation with your installer.

2.) Per a fairly reliable PV model called PVWatts (see the web for a free download), depending on your location in the UK (I used London), a 3.9 STC kW unobstructed south facing system fixed at a 45 degree angle with the horizontal might be expected to produce a maximum hourly output of something in the neighborhood of 3.25 kW. That's the max. hourly output the model produces for your system in the London environs over 8,760 (24 hrs * 365 days) hourly values. The model for the most part uses synthetically generated irradiance values but has generally showed itself to be fairly reliable in terms of long-term average output.

Picking the max. hourly system output over 8,760 hours in a "typical" year is a bastardization of the model's intent, but given the model's logic and algorithms, unless you're up in the Orkneys or some other god-awful place, and/or unless there's something very different about your system, I'd put my money on somewhere between 3.00 and 3.50 kW as a fairly reliable 1st approximation of your system's likely maximum power output.

3.) I'd suggest you read the Dummies book, then get familiar with the PVWatts model by reading all the help screens and other info and then do a few runs and see what you get.
Then understand why you got what you got.
Then, call the installer and have an informed, intelligent conversation about the state of your system's output.

Welcome to the forum of few(er) illusions.

Cheers.

[Jenx]

Will do - thanks very much!

[littleharbor2]

PV watts is only as accurate as the info input into it. Get the stats as close as possible. especially the angle and orientation of your panels and remember, if you have ANY shading it can have a substantial effect on output.

[J.P.M.]

Will do - thanks very much!

You're welcome. Just read and understand all the info before you make any PVWatts runs. GIGO rules. Ask any unresolved questions you can't answer on your own here.

[scrambler]

Thanks.

They ere installed by a company (10 panels of 390w each), on a 45 degree south facing roof with no obstructions. And it’s mid summer here in UK.

I keep asking them to send someone to have a look but they keep saying they are working fine!

You need to find out what inverter is being used.
If it is a string inverter, brand and model and number of strings
If it is micro inverters, what model.

This is needed to know if the inverter side could be causing clipping.
For example, Enphase micro inverter IQ7 are limited to 250W, so if they were used on your 10 x 390W panels, you would be limited to 2.5kW
IQ7+ are limited to 295W, that would limit the array to 2.9kW.

Another element that would allow us to see if clipping is the issue is getting the Graph output of a sunny day. If the production it a nice bell curve peaking at 2.9kW, then it probably is not clidpping.
If the curve is a Bell shape with a flat top, then it is clipping.

[Ampster]

....
Another element that would allow us to see if clipping is the issue is getting the Graph output of a sunny day. If the production it a nice bell curve peaking at 2.9kW, then it probably is not clidpping.
If the curve is a Bell shape with a flat top, then it is clipping.

Even a little clipping is not an issue in terms of affecting annual production. The important thing is to look at annual production and compare that to an estimate from PV Watts or an estimate or garanty offered by the installer. The DC to AC ratio is 1.39 to 1 which may seem agressive but it all depends on tilt, azimuth and location in terms of latitude and local weather. I had a system that was 1.5 to 1 and I saw a little clipping in Spring but none in summer because at my location the Summers are hot and panel output degrades with temperature so during the summer my panels could not even put out enough to get to 95% of my inverter's capacity. The system performed according to the garanty from my installer and some estimates I ran on PV Watts.
If the OP does inform us of his system details, hopefully he includes, tilt, orientation and latitude as well as temperature the day of the graph or data.

[scrambler]

I agree, I was not judging if clipping would be a bad thing, just trying to figure out if this is the reason his real peak is far from the nominal panel peak.