My new 200W 18v 11a(max) solar panel reads about 24v 4a 50% efficiency at midday on my MTTP Rich solar controller. I presume this is not great.

I was wondering... I have a 48v to 12v stepdown converter, will that increase amps at 12v? https://www.amazon.com/gp/product/B00BWKXTUU/ref=ask_ql_qh_dp_hza. How can I measure available amps off that device? would I need a large load to test that?

  • \$\begingroup\$ Mid day at what latitude? Are the panels perpendicular to the sun? What are any cloud cover/haze conditions? What temperature was the panel at at time of measurement? For best results, you want to be at the equator, panels perpendicular, sun at zenith, clear sky, panel cool and clean, no additional optics, although these conditions likely represent better than the panel was actually rated at. Depending on where you live, a "200W" panel of any brand or type may almost never produce 200w regardless of model or brand, so take this into account. \$\endgroup\$ – K H Mar 3 '19 at 22:21
  • \$\begingroup\$ On the day of the test it was midday with little cloud cover and strong sun. It's February in Miami Florida. 19v@5a = 95w. \$\endgroup\$ – Hell.Bent Mar 4 '19 at 23:34

Presumably you meant "MPPT" controller? Your MPPT converter and/or your panel are suspect AND your load is probably less than fully loading the panel.

There are too many unstated factors to answer your questin well but the following should help.

What is the load?
What voltage is it rated at?
Is it capable of presenting a 200W load?

As you say the panel is 200W rated and as 18V x 11A = 198W ~= 200W it is likely that you have quoted the panel's maximum power values.
ie Vmp = 18V
Imp = 11A.

A panel with 18V Vmp and 24V (or more) Voc has a Vmp/Voc ratio of 75% which is a larger droop that you'd expect from most modern panels.

If an 18V panel is producing 24V at 4A it is far from fully loaded.
You do not say what the load is but if it is capable of loading the panel fully then the MPPT converter is not doing its job.

If you fill in some of the gaps above then a better answer can be given. Providing fuill specs and datasheet links is helpful.

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  • \$\begingroup\$ Thanks. This helps. I had about 4a 12vdc load (a 12v freezer) and the battery attached was fully charged already. I can add more load to see what the charge controller reports. \$\endgroup\$ – Hell.Bent Mar 3 '19 at 16:17
  • \$\begingroup\$ Just retested. Put about 8a load, with and without the load the MPPT controller reads around 20 volts 5.3 a in which is somewhere around 100watts. It's 42x32 monocrystalline solar panel I bought from Walmart. I have another month to return it and think I will. \$\endgroup\$ – Hell.Bent Mar 3 '19 at 17:48
  • \$\begingroup\$ @Hell.Bent From your results so far there is no reason to think that the panel is faulty. You do not have a well defined load (a freezer can draw what it draws. ). To get a proper idea you need a variable and somewhat well defied load. Using eg ~~~=multiple automotive headlamp bulbs in parallel at 12V and loading the panel until Vp = 18V in full sun should give you a better idea. eg say you can get 45W headlamp bulbs, 20W stop bulbs and 5W tail light bulbs. ... \$\endgroup\$ – Russell McMahon Mar 4 '19 at 3:48
  • \$\begingroup\$ ... Using "festoon" bulbs allow wires to be wound onto bulb without soldering or soldering without real damage. eg 45 + 45 + 45 + 20 + 20+ 5 + 5 W = 185W. ... You can trim that up and down by adding or removing bulbs. When Vp = 18V and Vout = 12V how does it work? \$\endgroup\$ – Russell McMahon Mar 4 '19 at 3:49
  • \$\begingroup\$ Well... I have the typical load I'd like to perform best for.. a 40w freezer, a 3a 12vdc fan and 1a 5vdc fan. All going hard I was drawing about 8a and my new 12v 70AH 120RC SLI battery was dropping slowly at a pace where i think in 4 hours would be under 12v which as I understanding is around 50% of the reserve which is not good it's life. My MPPT charger is suppose to be smart to charge optimal. \$\endgroup\$ – Hell.Bent Mar 4 '19 at 23:33

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