# Theoretical wattage from two different panels

Help me understand this correctly, please. I have two (old) panels for when I am driving somewhere with my 4x4.

120w -

80w -

I am guessing the best way to configure these two would be in parallel due to the fact that their Imp values are so different (6.67A for the 120w, 4.65A for the 80w)?

If parallel is the best, I’d use the lowest of the Vmp values (17.2V for the 80w vs 18V for the 120w) to calculate the maximum theoretical wattage, yes? This would mean 17.2V(6.67A+4.65A)=194.7w?

Correct. As far as total Watts is concerned this is a bit theoretical since the panels never produce full power.

Close enough I would say. The lower Vmp multiplied by the combined currents. Atmospheric conditions will have a larger effect than getting the calculation slightly wrong…

I will do a few tests with a proper load, and see what we get from the panels. From memory the combined wattage was surprisingly low before - I don’t think I ever saw more than 130w.

The max power (Watt) panels are marketed at is under perfect conditions. You might see that once or twice a year. The Normal power is about 75% of that.
So with your planned setup don’t be alarmed if you get around 140W max instead of 200W.

Output will also be influenced by type of charger - MPPT vs PWM

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(I am using a Victron 100/20 MPPT).

Curious output - on the 80w panel on its own I get around 70w, which is great. And the 120w on its own is about 100w. Together, however, I only get around 130w to 140w.

Do measure the voltage the MPPT settles at, and if possible, the current in each string. Helps if you have a DC clamp meter but a good old al-cheapo multimeter with a 10A range ought to do.

I can add more to the load side, but the battery is not charged yet, so I would expect more current to flow there, and hence that should use the panels to the max.

It would be helpful if you can do this screenshot for each option. So for combined, for 80W and for 120W on their own.

I was just doing that - great idea!

The PV voltage is on the low end. I would have expected a 36-cell module to settle closer to 18V.

Ok, so they jump around a lot, and hence I made videos.

The 80w panel settles at 64w, but the solar volts jumps around a lot - 13.5, 13.8, 14.2, 14.6, 15
The 120w panel settles at 89w, and the solar volts jumps around too - 13.6, 13.8, 14, 14.2, 14.6
The combined parallel string settles at 128w, and the solar volts is more stable between 13.95, 14.05, 14.15.

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I am not sure what to make of the above. I can only assume that although the Vmp values of the panels are similar, they don’t quite react the same way for their power curves?

Seems like their Maximum Power Point is vastly different and the charger hunting for it is causing quite some loss.

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Not that I have a lot of experience with connecting different PV panels together but my experience in electronics in this low voltage DC world (PV panels and Li-Ion batteries) is that you should avoid mixing and matching these.
I accept that you might want to use this odd panel with that one but unless they are connected in parallel you’re going to have a wild goose chase…

Any new comments on this topic?
Series is a lot more complicated but what about connecting panels in parallel??
If the panels have the same Voc (no. of cells) then they should be able to deliver their combined maximum power as I understand it…