Measuring/calculating the wattage or a solar panel

[anm]

A while back someone asked about measuring the wattage of an unknown solar panel. We can't do it in a controlled, laboratory environment but given a good sunny day you should be able to get close. You will need a DVM with an adequate amps scale (for 'adequate' it should be more than the guesstimated output of the panel else you may blow a fuse inside the DVM). I'd use the size of the Renogy 100 watt poly panel as a benchmark, since that is what I've got. The Renogy 100 watt polycrystalline panel is 40.5" X 26.7". A similar size panel should be comparable in specifications, smaller sized will probably be less watts, and larger will probably be more. A similarly sized 12 volt solar panel should have an open circuit voltage somewhere close to 20 volts and a short-circuit current of about 5 amps.

The height of summer is the best time to do this measurement since at any other time of year the sun will be lower and the sun's rays will be passing through more atmosphere, reducing their intensity. Just turning the panel to directly face the sun does not decrease the amount of atmosphere the sunlight passes through before hitting your panel! Measure the open-circuit voltage, and note the reading. Set the DVM to the 10 amp scale and briefly touch it to the leads from the solar panel. Make sure the reading was not off the scale, if so you'll need a meter with a higher amperage scale, or a 20 amp shunt and you'll have to calculate it yourself (not difficult, I did it in high school). If you have gotten valid readings for both measurements you can simply multiply them to get the wattage. Remember that this will be a real-world measurement, not a laboratory measurement, so it will be lower than the panel's actual rating. Doing it on a bright sunny summer day will produce the closest reading to the panel's specification.

If you are unsure and want to protect your meter, initially angle the panel so it faces about 60° off axis to the sun's rays, that will cut the power of the sun hitting the panel in half. Then measure the short-circuit amps and if it's below 5 amps you can then face the panel directly at the sun and measure it again. Of course if you know your panel has a higher short-circuit current than your meter can handle, you can set the panel 60° off axis and simply multiply the short-circuit current by 2. Barring optical effects that should be accurate.

 

[anm]

The optical effects mentioned above primarily consist of the following:
as the angle of incidence of the incoming sunlight to the cover glass increases, the amount of light reflected from the surface (and thus doesn't hit the solar cells) increases.
So a 60° angle of incidence (measured from the 'normal line' which is the perpendicular to the surface) would probably be too great, possibly 45°, some experimentation may be required...

I would also like to point out that this meter from Harbor Fright has a 10 amp scale so it would be capable of doing short-circuit current as well as open-circuit voltage measurements on most panels. And the best part is it costs $3.00!! Today Oct 12th, my van with two 100 watt Renogy solar panels measures 8.82 amps short-circuit current, and that's with the sunlight at about 40° angle of incidence.

 

[blars]

That meter may have a 10 amp scale, but the test leads it comes with can't handle that. It's one of the items that frequently is on their "free" coupons.

 

[anm]

That meter may have a 10 amp scale, but the test leads it comes with can't handle that. It's one of the items that frequently is on their "free" coupons.

I disagree, I have one and I have used it to measure almost 10 amps, 8.82 to be specific. You are right in that it is often free, but for someone needing an inexpensive solution it's ideal. Just make sure it works before leaving their parking lot...

 

[akrvbob]

With an MPPT conroller what the panel produces and what the controller puts into the battery are two different things. You have a high voltage panel so the controller reduces the volts and increases the amps that it puts into the battery. My controller has a switch on it that displays whats coming out of the panel and on another setting it shows what it's putting into the panel.

So if it's putting out 9 amps at 28 volts that's 252 watts which is just right. Your panel is working perfectly

You'll have to check to see what the controller is putting into the battery to be certain that the MPPT is working properly.
Bob

 

[anm]

So your panel produced about 8 amps at 20 volts? So does that mean you were producing 160 watts of energy? I have a mppt controller and it shows on mine that i am producing 8-9 amps at 28-32 volts and the most wattage i have seen so far is just 150. How can your 100 watt panel produce 160 watts and mine is supposed to produce 250+ but on the controller it only shows 150+-? I am not sure what is what.

The manufacturers ratings are not what you're going to see day in and day out. Also, I have 2 X 100 watt panels, for 200 watts total. But by my measurements I only have two 80 watt panels. If I'm getting 160 watts AND the sunlight is hitting the panels at ~40° angle of incidence I'm pleased. The sun's maximum altitude at noon today was less than 50°, and I'm in El Paso, how far north of the equator are you?

This thread was only about getting a ball-park measurement of an unknown solar panel's wattage, not how well it's going to do in any specific situation. Also there was no mention of ANY controller, PCM or MPPT since it was not relevant to the subject.

 

[anm]

Ok, i see now. I thought your measurement of 8+- amps was for a single panel. I am just trying to understand how my controllers numbers are not lining up with the volts x amps = wattage math. When i replied to this thread i was obviously misreading what you typed.

I am in so cal. Didnt try to throw your thread off topic, just had some questions involving the math of this solar stuff compared to readings displayed on my controller.

They (solar panels) can be confusing, we should probably treat the manufacturers figures as something that the panels are capable of but we're unlikely to see. When you're talking about a complete solar power system with a charge controller you also have to take the battery into consideration. Just because your panel can put out 250 watts, doesn't mean it will if the battery is already fully charged, or close to it. I think you have to go by the overall performance of the system, does it do what you want it to? If you don't have enough power to meet your needs then you have to do something, either be more conservative on your usage, or fine-tune your system, or some combination of the two. Solar power generation seems to be a very hands-on science, at least until you arrive at the point where your income is greater than your expenditures (power-wise), and then you have to spend time making sure it stays that way.