Solar Panel Placement Testing

Hi Everyone! Question for ya. I have a couple spots I could mount my 6 190w solar panels. One is on the roof of a shed and the other would be on a ground mount rail system. What I would like to do is place a single panel in each area temporarily and log the voltage throughout the day when I am away. Is there something I could connect to that would record the energy it sees throughout the day along with the time of the day? I hope this makes sense.

example log entry possibly:

13:00 - 21.4 volts
14:00 - 22.2 volts
15:00 - 20.9 volts
-- --
21:00 - .04 volts

This would really help me determine how and where to mount my strings.

Thank you!
Greg

Voltage alone will tell you nothing helpful. You want a controller that can measure accumulated energy harvested for a day. I suppose you could also just short the panel outputs and log current instead of voltage. That might result in numbers that more closely follow power output. Not sure how practical that might be... anyone try it before?

I see what you’re saying about total power. I get it, but my thought is if a panel is rated 24open volts and it’s hitting that mark then the spot I have the panel set up is a prime spot. I just would like to monitor things and review the logs after a day or so for each area. Maybe I could rig up a Pi or something idk. Thank you for your response!!

Agreed with @bolhuijo that voltage alone is worthless.
See my charts from today below.
I'd recommended running your location at suncalc.org, don't forget to check the visual for December 21st...

I'm not aware of any type of logger, you could pick up a small Victron MPPT (with Bluetooth) that will work with your planned system voltage, connect a panel, charge a battery, and run a small DC load to see how your locations perform.

TeeShizzle said:

I see what you’re saying about total power. I get it, but my thought is if a panel is rated 24open volts and it’s hitting that mark then the spot I have the panel set up is a prime spot. I just would like to monitor things and review the logs after a day or so for each area. Maybe I could rig up a Pi or something idk. Thank you for your response!!

Click to expand...

You should hook a meter up to a panel and observe the voltage while you move it around. You will find that open circuit voltage rapidly rises with daylight. I'm looking at one of my panels right now that is putting out 27 Volts (37 Voc rated) in the evening but is only producing 3 Watts (405 Watt rated)
You will likely find that as you move the panel toward direct sunlight, the open circuit voltage will rapidly approach the Voc rating long before you have achieve the optimum angle for power.

The small Victron MPPT with a small battery is the way to go. Or just pick a spot facing south and use the correct angle. No need to conduct original research. Lots of calculators based on 40 years of data. Nrel pvwatts

Thank you everyone! Great info!

You need to measure current. Panel should be loaded with a resistor like a water heater element with a resistance value considerably lower than that of ideal. Then measure the voltage across the load. That will give a relative indication of power to compare one location vs another.

And all this testing isnt much good if you dont do it for a whole year. The sun moves in the sky different every day.

For me in heavy woods I build my cabin roof facing south, cleared a bit more trees that way and put the pannels on.

For some people aiming pannels may just mean aiming them at the hole in the tree canopy. If this is your situation then you may actualy get more sun spring and fall than summer.

I agree that measuring as framed in OP is not that useful. That’s what I originally thought I would do.

What I ended up using was the output from a shading-based simulator (Aurora) and a basic simulator (PVwatts). Then I went on the roof as well as used my drone (and its orientation/azimuth angle measurements) to confirm the direction to obstructions in the sky.

If you have no obstructions, you should just put 90% of the time you would have spent measuring the real world into making sure you use PVwatts correctly. The remaining 10% you should apply towards learning how solar panels behave electrically , since it’s not super obvious how bad shade is / how different shade mitigation techniques work.

That’s probably hose you more than 20% deviation of reality from simulation (and, let’s be honest, you’re not going to take as many real datapoints as the simulation was going to generate, so this is a false dichotomy)

You can use a commercial data logger like Hobo data logger.