Confused about wattage ratings

landyacht.318landyacht.318 Solar Expert Posts: 82 ✭✭✭✭
I understand a panel's rated wattage is determined by VMPP x IMPP.

I understand MPPT is, in theory, designed to keep battery voltage from limiting this wattage.

So why is it, at noon, if I multiply my amperage x the voltage (according to my IPN pro remote ), I never get the rated wattage?

My Kyocera panel is rated at 7.39 amps, and according to my IPN pro remote monitor, I regularly see that amperage and more, before the MPPT conversion. But whenever I do the math, even with the MPPT conversion, I do not come out with 130 watts.

What am I missing?

When I see 7.4 amps in, and 8.1 amps out, I've been really happy, but when I do the math, I'm confused.


  • rplarryrplarry Solar Expert Posts: 203 ✭✭
    Re: Confused about wattage ratings

    A panels wattage is determined by the manufacturer, using a test facility that flashes a very bright light at the panel in a very cold enviornment. The output of the panel is then measured. In the real world we very seldom have the same light intensity and cold atmosphere of the test facility. Here is a good website that gives the real world wattages jof many different panels. Oops that website is gone, but as I remember when I last looked at the website, basically all the manufacturer claims of wattage for their panels were derated by about 15%. So what you see on the label is not what you can expect in the real world. I use Evergreen 190 panels and the real world rating was 165 watts.
    Hope this helps,
    I just refound the website:
    At the bottom of the page the PTC rating is explained. Your Kyocera 130 panels are rated at around 114 to 117 watts depending on which panels you have.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Confused about wattage ratings

    even if the pv is able to output its rated power, going from pv to controller to battery will never be 100% efficient as there will always be some loss of power from one end to the other. you have seen the current gains, but the end voltage still represents a loss, plus consumption by the controller itself. sounds bad doesn't it? but it is better than the loss represented by straight pwm controllers
  • landyacht.318landyacht.318 Solar Expert Posts: 82 ✭✭✭✭
    Re: Confused about wattage ratings

    Thanks for the link!

    I Usually will see at least 117 watts around noon, when I do the math.

    But I had been thinking that when my 7.39 amp panel was reading 7.5 amps in and 8.1 amps out, that my panel was exceeding it's specs, and now realize I've been wrong.

    I was happier before I did the Math.

    Now I want to shorten the circuit and fatten up the wiring, but for the most part, My New Crown Deep Cycle batteries are getting fully charged everyday. Well, they are being kept in acceptance(14.5 volts) for 2 hours daily.

    But I want 130 watts dang nab it!
  • RCinFLARCinFLA Solar Expert Posts: 1,482 ✭✭✭✭
    Re: Confused about wattage ratings

    Most panel wattage is rated at 25 deg C. This is usually much cooler then actual application useage where the sun heating can bring panel temps to 40-50 deg C, particularly in summertime.

    You can look at your panel's spec but they are usually around -0.33% to -0.34% per deg C on MPP voltage. The hotter panels are, the lower the MPP voltage. The current output for a given illumination level is pretty flat over temp so MPP voltage is the dominate effect. Going from 25 deg C to 45 deg C has a 6.7% drop in MPP output voltage. So, just stating current when talking about power output misses the main variable, that of MPP voltage versus temp.

    A PV cell is an illumination based current source clamped at a maximum voltage set by cell's inherent diode conduction voltage (about 0.6 vdc per cell). The diode has the negative temp coefficient so hotter it is, the lower the conduction point voltage (Voc). MPP point is where the panel is loaded such that only a small portion (about 3%) of the illumination generated current is shunted down the inherent diode of the cells and most of the current is yielded to the MPPT controller. This yields the highest V x I output from panel (Vmpp x Impp). A panel with no load (open circuit) will sends all the illumination generated current down the cells inherent diode. A panel is just a number of cells connected in series.

    An MPPT controller is a power converter. It has a switching power converter that converts higher voltage/ lower current to lower voltage/ higher current. The converter has some power loss which translates into a power conversion in the range of 94% to 97% of input power. The ratio of output current to input current is dependent on ratio of output voltage to input voltage on the MPPT controller. The higher the panels input voltage to MPPT controller the higher the output current will be in relation to input current to MPPT controller. An MPPT controller is continously hunting for max power point. Some contoller algorythms are better for certain situations then other controller. This hunting can cause some lose is max power output. Look for a gyrations in the panel voltage. There will be some due to clouds going by but the change should be relative smooth over time. Temp is the primary reason for a change in MPP voltage. Illumination variation has lesser degree effect on MPP voltage although there is a time lag as the panel may cool down a little as result of cloud shading. The series resistance of the panel and wiring length resistance from the panel moves the MPP knee down in voltage as illumination current increases resulting in more voltage drop at the controller.

    This is in contrast to what is called a PWM converter which is just a switch that connects and disconnects the panel from the battery to maintain maximum battery charge voltage. No power conversion is done with an PWM controller. Since the PV panel is an illumination based current source, the battery will get whatever the panel's current output is when PWM controller has it's switch 'on'. The panel voltage will drop to whatever the battery voltage is.
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