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BFL13's avatar
BFL13
Explorer II
May 20, 2014

Yet Another PWM vs MPPT Comparison Test

I was able to run a couple tests with blue sky at same time of year and location as my test from two years ago (flat, tilted, tracking results one) that :
A. compares PWM with MPPT, and
B. compares MPPT at 14.0v with MPPT at 12.2v (battery)

First test:
0900 battery 13.3v-- 130w panel and PWM at 7.3a (same as two years ago at 0900) Scaling this to 230, should be 7.3 x 230/130 = 12.9a

The actual 230w MPPT was 13.1a and 176w

Second set with sun higher at 1000.

Test one-- battery 14.0v

130w panel rated Isc is 8.2a but the IV curve drops this at mid-13s on up, Got actual Isc as 8.21 but with the 14.0v battery got 7.8a, scaled up to 230 should be 13.8a

MPPT was 13.59. 230w Panel rated Isc is 8.3a, 37Voc. Got 8.26/34.7

Test two-- a few minutes later with loads running and battery voltage now down to 12.2 holding there.

PWM 8.2a-- would be 14.5a at 230. 230 panel now at Isc 8.42 and 34.7Voc.

Still at 189w with 12.2v and 15.48a.

So as shown in the previous set of comparisons last month, the MPPT "performance" is all about battery voltage as to how many amps you get.

Summarizing the above on the same 189w (of 230w) you can have
13.59a with 14.0v, or
15.48a with 12.2v.

At high noon MPPT should do better than here at 1000 but some clouds coming in, not sure will get a noon reading. Will edit that in if available.

20 Replies

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  • Salvo's avatar
    Salvo
    Explorer
    May 20, 2014
    What you say is correct. But it doesn't matter. 12V panels are specifically designed for pwm use. The panel spec is optimized to provide maximum efficiency when operating in pwm mode. The performance difference is less than 10%. On hot days the difference is negligible.

    Sal

    Wayne Dohnal wrote:
    The PWM controller doesn't have the capability to force the panels to operate at the optimum voltage-current combination.
  • BFL13's avatar
    BFL13
    Explorer II
    May 20, 2014
    PT I don't get any special MPPT advantage that I have noticed during the shoulder hours or in low light compared with my PWM results. Both are poly panels if that matters.

    Of course it is easy to confuse what is making the MPPT advantage you do get in the morning between the low battery voltage (as above shows) then and maybe thinking that is from the supposed low light advantage you hear about?

    Anyway I have not seen anything like that, but I have not been everywhere and done everything either :)

    BTW once again I have too much solar. I guess it is my hobby now, buying and selling the things. I might end up being just an MPPT guy! Horrors.
  • BFL13's avatar
    BFL13
    Explorer II
    May 20, 2014
    Just an update--it was clear at high noon here at 1315ish, so I took some readings but they don't change the OP results.

    Ambient temp 21C

    130w panel and 230w panel both pointed South tilted up 30 degrees (at 49.3N lat)

    Panel temps : 130 was 52C under and 49C on top, 230 was 45C under and 42C on top using IR therm.

    130w panel rating 8.2/21.9 was 8.51/20.4
    230w panel rating 8.3/37.0 was 8.59/34.4

    Indicating above ratings' STC today, so with low battery voltage you would expect best performance from panels.

    130w got the usual Isc or a bit more, 8.3a

    230w (had to run loads to drop batt voltage) 12.6v/15.33a/196w on controller readout. Was 12.1v/ 15.72a/190w

    Seems my 230w Hanwha panel and Eco-Worthy 20a MPPT controller can do about 15 1/2 amps normally expected. ( I have seen 18.x amps but only for a moment in edge of cloud effect where it sat in the 17s for several minutes. Also showed near 230w---Drat! I was hoping for 17a every day. Oh well, I suppose MPPT 15.5a is better than PWM and 14.5a Whoopee-do! :(
  • Almot's avatar
    Almot
    Explorer III
    May 20, 2014
    BFL13 wrote:
    Other considerations may still prevail, where 24v panels cost less than 12s, or you want to use thinner wire, or you want more watts per square foot of panel with your limited RV roof space.

    That's it. More wattage, more total AH, thinner wire, easier install. Get more panels :)
  • BFL13's avatar
    BFL13
    Explorer II
    May 20, 2014
    The above comments are mis-reading the results shown.

    The MPPT "advantage" is all at the lower battery voltages.

    This means the RVer who is just using his solar to recharge his battery bank gets essentially no advantage. His bank will be in the 13s right away in the morning and go up from there.

    The MPPT advantage would go to those who run loads with their solar, where battery voltage remains in the 12s all day. This way, you get the advantage in higher amps from MPPT to run the loads.

    You can run loads after your batteries are full with either PWM or MPPT too, using "spare solar" but now the batts are full and floating.
    You could lower your solar float to 13.2v and get some (but only a little) MPPT advantage at that voltage instead of at 13.8 or whatever.
    "Depends" whether that would be worth the bother most days.

    Other considerations may still prevail, where 24v panels cost less than 12s, or you want to use thinner wire, or you want more watts per square foot of panel with your limited RV roof space.

    But just going with 12s PWM vs 12s or 24s MPPT and what happens with the amps in each case for the same wattage, what you see in the OP is how it is. You do not "lose" anything with PWM when both get you the same amps!!!! Forget the watts.
  • Wayne_Dohnal's avatar
    Wayne_Dohnal
    Explorer
    May 20, 2014
    The heart of a MPPT controller is a dc-dc converter with a variable voltage input that's controlled by a microprocessor of some sort. The controlled dc-dc converter forces the solar panels to operate at the optimum voltage-current combination for the current light conditions. It uses some sort of algorithm to find the optimum operating voltage for the panels, which changes as the light conditions change. The PWM controller doesn't have the capability to force the panels to operate at the optimum voltage-current combination.
  • rk911's avatar
    rk911
    Explorer
    May 20, 2014
    mlts22 wrote:
    Nice comparison. PWM is pulse width modulation. It "lops" the incoming voltage coming in to whatever it thinks the batteries need, then eventually just pulses a charge. MPPT (maximum point power tracking) will fiddle with the amps/volts to get the right voltage coming in. PWM is cheaper, but you lose a good chunk of energy due to the fact that what it can't use, it discards. For example, a 24 volt, 300 watt panel will essentially be a 12 volt, 150 watt panel, while with a MPPT controller, it can use the higher voltage and downconvert it.

    For a large area of solar panels, a PWM controller might be useful. For the limited real estate on most RVs, you want every single watt you can get, thus a good MPPT controller.


    thanks for the clarification.
  • mlts22's avatar
    mlts22
    Explorer II
    May 20, 2014
    Nice comparison. PWM is pulse width modulation. It "lops" the incoming voltage coming in to whatever it thinks the batteries need, then eventually just pulses a charge. MPPT (maximum point power tracking) will fiddle with the amps/volts to get the right voltage coming in. PWM is cheaper, but you lose a good chunk of energy due to the fact that what it can't use, it discards. For example, a 24 volt, 300 watt panel will essentially be a 12 volt, 150 watt panel, while with a MPPT controller, it can use the higher voltage and downconvert it.

    For a large area of solar panels, a PWM controller might be useful. For the limited real estate on most RVs, you want every single watt you can get, thus a good MPPT controller.

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