How Do You Find the Actual Wattage Output?

BilljustBill Solar Expert Posts: 219 ✭✭✭
I found two 120 watt panels at a flea market with a label printed with: "American Signal". One panel has some cells that appear a lighter color than the majority and the Specs sticker seems to show they were made in 2003. I Paid just $22.50 each, thinking that if they didn't work, I could salvage the thirty-six 6"x 6" moncrystalline cells. In full August sun at noon, the multimeter measured 21volts and the DC amp reading with a load was 4.90 on one and 5.25 on the other, so it turns out they do work....but at a lower rating...

If these are for a 12 volt system, would you calculate the panel wattage with the 21 volts, or the 17.5 listed on a second panel sticker, or use the 12 volts the battery is rated at?

The multimeter will only take a total of 15 seconds of loading over a 15 minute period before requiring it to cool. What do you use to measure Amperage when longer testing times are needed?



  • RandomJoe
    RandomJoe Solar Expert Posts: 472 ✭✭✭
    Re: How Do You Find the Actual Wattage Output?

    Most panels use "STC" ratings for sales, whereas most people are going to see something closer to "PTC" ratings. That is generally in the range of 70% of STC. For example, my Kyocera 135W panels have a PTC rating of 95W. They do that regularly, and on "good" days will exceed it but don't reach the full 135W. (Temperature, reflections off snow, and a variety of other things will affect their final output a bit as well.)

    So if you can get around 84W max power, that's about right.

    To try to measure "max power", you need to be running at the "Vmp" point of the panels, which is probably the 17.5V listed. There are usually several items listed: Voc = voltage open circuit, probably your 21V. Vmp = voltage at max power, Isc = current short circuit, what you would measure if you shorted the panel's wires together. And Imp = current at max power.

    If your panels' max power voltage is 17.5, then they should reach 120W / 17.5V = 6.86 amps Imp. Derate to the PTC rating, you should realistically be able to achieve 84W / 17.5V = 4.8 amps.

    However, the 17.5V is only useful if you can use that voltage. (Say, with an MPPT charge controller.) If you are going to use a more basic controller (which is a lot cheaper than MPPT!) then the panel voltage is going to drop to the battery's voltage. You should still see the same current, 4.8 to 6.86A. Your total wattage, though, is down to (at 14V) 67 to 96W. The panel is doing fine, you've just dragged the output voltage down to battery voltage.