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Old 09-27-2021, 02:48 PM   #1
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New system load test

Warning long detailed post.

I have dual 360w rec panels flat on my roof and a victron 100/50 controller connected to two 200ah batteries and a 1000w inverter. I was load testing the system and at 1pm today a bright sunny day. The system under a heavy load the controller was only using or seeing 435w of solar. I had the inverter on running a 8amp load. Panels were only doing 42v 9-10amps and output was about 30amp at 14v. I could see the battery voltage dropping by the sec 12.9..12.8Ö12.7 then at 12.4 I pulled the plug. The load was only running 2-3 mins. If my math is correct in perfect situations I should see about 700w and maybe 46 amps out. The numbers I am getting at disappointing. Whatís wrong?
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Old 09-27-2021, 02:54 PM   #2
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I usually rate solar panels at around 60-65% and 12 hours of daylight is usually around 5-6 hours of productive sun. This is on a clear day
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Old 09-27-2021, 02:58 PM   #3
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If thatís the case solar numbers and ratings are a jokeÖ. But ya thatís spot on, 60% of 720.

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I usually rate solar panels at around 60-65% and 12 hours of daylight is usually around 5-6 hours of productive sun. This is on a clear day
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Old 09-27-2021, 03:20 PM   #4
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When you say you were running an 8 amp load...was that 8 amps at 120V? If so, the inverter was drawing 80+ amps from the batteries and, with a 200ah battery bank, that would be a significant draw and would show up as visible voltage drop. Might that be the issue?
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Old 09-27-2021, 03:22 PM   #5
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Yes it was 8amp load on the inverter 120v 8amps. 400ah battery bank


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Originally Posted by rossvtaylor View Post
When you say you were running an 8 amp load...was that 8 amps at 120V? If so, the inverter was drawing 80+ amps from the batteries and, with a 200ah battery bank, that would be a significant draw and would show up as visible voltage drop. Might that be the issue?
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Old 09-27-2021, 03:29 PM   #6
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Was that photo taken during the test? I can see by the shadow in the photo that the sun is at 30 degrees or so. Your panels are at 0 degrees, so you won’t see anything near the rated panel output. Add panels if you can.
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Old 09-27-2021, 03:47 PM   #7
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Nope that photo was taken install day later in the day

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Was that photo taken during the test? I can see by the shadow in the photo that the sun is at 30 degrees or so. Your panels are at 0 degrees, so you wonít see anything near the rated panel output. Add panels if you can.
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Old 09-27-2021, 03:52 PM   #8
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If thatís the case solar numbers and ratings are a jokeÖ. But ya thatís spot on, 60% of 720.

Solar Panel "STC" ratings, don't often reflect real world results, especially on vehicles, but they aren't entirely' a joke, they are just misunderstood by many, and misleading to the uninitiated.



The "STC" rating (which is the Watt rating they advertise), is a test done under standardized test conditions, to make comparing between panels somewhat of an apples to apples comparison, this rating can be thought of roughly as an ideal rating that you might see occasionally, but should not count on.


Things that affect the efficiency of your panels:
1. Heat, the hotter the panel, the lower the efficiency
2. Angle, the most efficient panels are angled directly at the sun
3. Air Gap, (this comes back to heat, without an air gap panels can get really really hot
4. Shade, (this one is obvious, but even a small bit of shade can have a devastating effect)
5. Dust/Dirt on the panels, or Dust/particulate in the atmosphere
6. Undersized wiring, an undersized controller, poor connections, an undersized lead acid battery bank, or basically anything else that adds resistance.
7. Season

8. Probably a bunch more things that I am forgetting


Long story short, STC rating is not representative of what you will see day to day, but is a useful metric, there are other (less used) metrics which try to estimate 'real world' but there are so many variables that these will be closer to realistic but still a rough estimate, since conditions vary by location/region/conditions/system. Its a complicated and involved process to estimate real world solar yield for a given system/location.


I believe STC test conditions are panel temp of 25*C (77*F), directly angled at the "sun" with an intensity of 1000W per m^2, and proper airgap (or maybe no backing).


Probably there could be more representative test conditions (for instance the 25*C panel temp is pretty unrealistic in the real world)
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Old 09-27-2021, 04:54 PM   #9
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Nope that photo was taken install day later in the day
The best fixed angle for Missoula is about 38 degrees. This time of year itís about 45. And right now you have about 4 sun hours.

The point Iím trying to make is that itís never optimal. With flat panels youíre probably getting 50%. In the summer youíll do better. In deep winter youíll do worse. Last week I spent a few days in a redwood stand. There was no way I was going to charge from my panels. Fortunately there was shore power. Still, I had to reduce the expectations I had for the system I built. And I donít have a shore power connection for my inverter-charger yet, so out came the olíextension cord for the fridge.

More panels will help. More storage will help. Juicing up from the grid or generator will help when all else fails. Rethinking how youíll use the energy you generate will help.
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Old 09-27-2021, 06:08 PM   #10
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Yes it was 8amp load on the inverter 120v 8amps. 400ah battery bank
But, to clarify, an 8 amp load on the inverter at 120V will draw over 80 amps from your battery bank on the 12V side. I just want to make sure you're accounting for this huge drain/load. I did misread the battery bank size...sorry about that...but even a 400ah bank (especially an FLA or AGM bank) will show a significant voltage drop with an 80+ amp load. With conversion loss, it's probably closer to 90 amps.
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Old 09-27-2021, 09:20 PM   #11
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Thanks guys I appreciate the info feedback very helpful. This place is always my go to for help when I am stuck
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Old 09-27-2021, 11:40 PM   #12
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8 amps at 120 volts is a load of 960 watts. Under optimal conditions your panels are incapable of supporting that and your batteries will be depleted.
Losses from the suns angle have already been discussed and there are other losses as well. I took your 720 watt rated panels and used Butte Montana (fairly central in Montana) as a location. I then plugged in the data to a PV calculator.
You can only expect an average of 2.4kWh a day from your panels in September for a full month total of only 72kWh. That's only enough to support 2.5 hours per 24 hours period of that 8amps at 120VAC load (on average).
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