I think the full-rated quantity of amps only happens when the panels are cold. Winter makes them produce more.

They may need to experience -20F to produce the full amount.

Thanks. All the testing procedures I've seen say that the multimeter should show the isc or something very close. Considering it was right around freezing when I tested, I would expect better output.

I would expect the voltage to increase with cold, the current
not to change much. That current is pretty low, but do be
sure you are really getting full sun. The slighest haze or
cloud will have considerable effect, and the slighest smoke
(from all those fires) is even more devastating.

I would want to at least have another comparable orientation
panel to see how it is doing, before writing off the used panels.
good luck, Bruce Roe

Without knowing what the irradiance is when you're measuring a panel's output you will have no way to measure/estimate/compare what a panel's performance is compared to its 1,000 W/m^2 STC irradiance measurement.

Thanks. It was a clear day, which made it more surprising. I also tested a Grape Solar 320 watt panel that's been in use for about 7 years and it was also putting out about 5 amps, instead of the rated 8+. I thought it was settled at that point, but a friend with panels on his van was getting the full rated output, so that didn't help anything... ¯\_(ツ)_/¯

Thanks but I'm not sure what to do with this advice. How can I determine the instantaneous irradiance during my test? The only info I've seen uses average irradiance over a day, and that would include the losses due to shorter days, not just the seasonal reduction in the strength of the sun.

Were your friends panels tilted up and toward the sun. I can't imagine full output from panels lying flat on a van's roof at this time of year. There must be some factor keeping your panels from reading higher current. Are they being shaded at all by a hand holding the panel while angling it toward the sun? You will only get max current when the orientation is perfect. This can be viewed on the meter while manually changing the tilt.

My NE panels peaked at 3.94A, SE at 7.55A, and SW/NW at 5.56A. Voltage was 35-36V.

What ISC are your panels rated for?

T400H, STC 11.14 and NMOT 8.97.

And for reference, back in June those values were NE 9.18A, SE 9.87A, and SW/NW 9.83A at peak.

To find the information you're looking for you will need to know the rate of energy input to a solar panel (that is, the plane of array or "P.O.A." irradiance) while simultaneously measuring the output of the panel, either the power output in watts or the voltage and current. The panel instantaneous efficiency will be the output divided by the input.

Given the season, time of day, solar position and other variables including the big and apriori unknowable atmospheric conditions which are constantly changing, you won't have much of a chance to see 1,000 W/m^2 normal (that is, perpendicular) plane of array irradiance on a PV panel.

Without ways of measuring at least the P.O.A. irradiance, the panel cell temps. and the panel output you won't be able to estimate the instantaneous panel efficiency and so whether or not there is a problem with the panels.

Sorry but I think you're making this way more complicated than it needs to be. Here's the gist of the question:

My tested ISC is about 60% of the rated ISC. Is that expected in the northern US, mid-December?

If testing by manually aiming a panel directly at the sun on a clear day, then, no. If already mounted at a less than perfect angle for today's sun elevation, then, could be.

Think or believe what you want but based on what I think I know and have learned from 50 or so years around solar energy I'm not making anything more complicated than it needs to be to give you an accurate comment to your posts.

This is not rocket science but your lack of understanding of what's actual required is making you think or wish things are simpler than they actually are.
GIGO applies here.

There is no such thing as "what is expected" with respect to short circuit current or other panel operating parameters such as voltage or circuit temp. except what you calculate from the operating conditions and other inputs that affect those outputs.

The conditions that control a panel's output parameters are hardly ever equal to those at standard test conditions. On site you measure the Isc and compare that to the calculated Isc using measured P.O.A. irradiance, cell temp. and maybe some other stuff depending on required precision while making sure the panel is clean.

Then, compare those results to the panel's STC parameters to get some estimate of the panels' current performance.

Take what you want of the above. Scrap the rest.