"200 watts" of solar doesn't seem to generate anywhere near 200W, is this just normal

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Vannautical engineer

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I finally got around to putting solar on the roof of my van, 2 100 watt flexible panels. At 200W, after doing the math, I was expecting in nice direct sun I might see about 16 amps flowing into my battery. The thing of it is, after a little use, I have rarely seen the output from the panels reach 10A, even in direct sun. I'm using a Renogy MPPT controller with the panels in parallel. I'm pretty sure everything is functioning properly, so I'm starting to wonder, is this just how it goes?

I know most electronic components have their "max" ratings taken under optimal conditions. So are solar panels like this too? I mean, can I only realistically expect to see a full 16 amps of output if I'm parked directly on the equator on a clear day at noon on the longest day of the year?

Just curious. I'm pretty happy with the panels nonetheless, just a little disappointed to only have seen a little over half the output I expected. One caveat is that I haven't had the panels in direct sun when my battery was more than a few percent under fully charged (LiFePo4 battery.) So could it be that the charge controller is just holding back when the battery is close to fully charged? (Actually now that I think about it this is probably not the case since at times when I have been drawing a lot of load with my devices the flow from the panels still never goes over about 10 in direct sun.)
 

tx2sturgis

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You wont see full power from the panels if the battery is charged or nearly charged. Just not gonna happen. Even if you had 2000 watts of solar panels, but the battery is nearly charged, the controller may still only allow 5 or 10 amps, or whatever the battery needs. It's called a 'controller' for good reason. It 'controls' the charge rate.

If you want to experiment, deplete the charge level more than normal overnight. Not dangerously low, just use a few things like lights and fans and inverters and etc, so that in the morning when the sun comes up, the charge rate will be more aggressive.

And also, for an MPPT controller to extract the maximum amount of power, your two panels need to be wired in series, not parallel.
 

jonyjoe303

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In full sunlight you should be getting about 10 amps when the sun is overhead. According to my calculations 200 watts divided by 14.6 volts (lifepo4) equals 13.6 amps which is the max the panels would ever put out. You might only see that if you have an empty battery and the sun is overhead.

You might be able to get a little more amps if you hook up the panels in series (especially in winter) the higher the panel voltage the better for the mppt to work.

Example of the average I get with different panels ( the max is in summer with a low battery)

120 watt = 6 amps   (max about 7 amps)
240 watt = 12 amps (max about 15 amps)

365 watt = 22 amps (max about 27 amps)


Another thing I notice when charging lithium is voltage drop from controller to battery. Measure the voltage on the controller lcd screen or bluetooth, then measure the voltage at the battery terminals, they should be the same. If the controller is reading 14.6 (or whatever bulk setting you set) and the battery only reads 14 volts, then the battery is not getting fully charge but the controller thinks its fully charge and will reduce the amps.



Lifepo4 will accept max amps until it reaches its max voltage. Voltage drop will reduce the systems performance.



This is my 365 watt panel with the makeskyblue 60a mppt, putting out 26 amps. Note the makeskyblue has extracted up to 375 watts from the panel on occasion.

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This is the max amps I saw with my 240 watt panel and an ecoworthy 20a mppt

a max amps.jpg
 

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Vannautical engineer

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tx2sturgis said:
You wont see full power from the panels if the battery is charged or nearly charged. Just not gonna happen. Even if you had 2000 watts of solar panels, but the battery is nearly charged, the controller may still only allow 5 or 10 amps, or whatever the battery needs. It's called a 'controller' for good reason. It 'controls' the charge rate.

If you want to experiment, deplete the charge level more than normal overnight. Not dangerously low, just use a few things like lights and fans and inverters and etc, so that in the morning when the sun comes up, the charge rate will be more aggressive.

And also, for an MPPT controller to extract the maximum amount of power, your two panels need to be wired in series, not parallel.

Yeah, maybe I should hold off my judgment until I see how it charges from a low battery. I did not realize MPPT worked better with panels in series, but that is easy enough to change. I did parallel because of the whole shading thing. But having partially shaded panels is probably a pretty fringe case.
 

highdesertranger

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Also if you mount flexible panels flat on a surface so no air can circulate under them, that will seriously lower the output. You will never see the rated output from any panel. But like the others say deplete the battery and see what happens. Highdesertranger
 

Vannautical engineer

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They have plenty of air circulating under them. I made an aluminum frame to mount my flexible panels on, which makes me wonder why I didn't just use rigid panels. But it makes sense somehow in my own head. I'll drain the battery and maybe change them over to series if I get really ambitious.
 

tx2sturgis

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With MPPT controllers, there are pros and cons to series vs parallel wiring. 

Parallel:
Pros: resistance to partial shading, redundancy if a panel fails.
Cons: slightly less power in full sunlight. 

Series: 
Pros: usually max power can be produced from the panels
Cons: no fault tolerance in the solar array. If a panel is underproducing (due to shading, bad diodes, physical damage etc) the entire power production ability can drop to very low values.

So...pick your poison...in other words, if your system is producing enough right now to get your batteries fully recharged by around noon on a sunny day, then it's probably not gonna make much difference being wired parallel or series.

But if you need every last amp because your system is not fully charged even by mid afternoon, or you need it to perform at maximum in the low sun angle (and shorter) winter days ahead, then yeah, you might want to rearrange the wiring and end up with the panels wired in series.
 

Vannautical engineer

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Ok, if the difference in series/parallel output is not that dramatic, I will probably leave them parallel for now since they are already wired up, and changing them around would require a little shuffling around how I have the wires secured down to the roof.
 

PeterPiper

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I have two 100watt panels in parallel on my van running a 35amp Xantrex PWM controller.
When the Sun is overhead on a clear day I get 10Amps if the batteries are even a bit low.

I have a couple of MPPT controllers on hand but at this power level it wouldn't make much difference and having them in parallel is more forgiving of shade I think.
Overall all things considered, at this power level I'll prolly leave them as is since it's not a big difference. Over 200watts I'd prolly go MPPT and series.
 

Spaceman Spiff

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Vannautical engineer said:
 . . .
I know most electronic components have their "max" ratings taken under optimal conditions. So are solar panels like this too? I mean, can I only realistically expect to see a full 16 amps of output if I'm parked directly on the equator on a clear day at noon on the longest day of the year?
 . . .
Solar panels are rated under idealized laboratory conditions:
 - simulated sunlight irradiance of 1000W/m^2
 - solar panel at 25ºC (~76ºF).
You will almost never see this combination in real life.
 

XERTYX

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I have 2 100 watt rich solar panels from amazon wired in series and I get excellent charging in low light conditions. Freaking fantastic charging in full sun. I cut off the MC4 connectors and soldered them with real copper 10 gauge wire and if my battery is low I usually see around 170ish-180ish watts in full sun for the initial charge then gradually it tapers down of course as it charges.

A couple of times I've even seen higher than 200 watts from them but I suspect these are really like 125 watt panels or something and they just plonk on a 100 watt sticker when they sell a 100w and a 125w sticker when they sell a 125 or something along those lines.

As far as shading goes... that's a real problem. Just dont park it in the shade and keep an eye on it if it's near a tree when the sun moves. Just the other day it was kinda overcast and I knew I would be in the store for a while so I had a look at the app for my controller. The light pole was casting a narrow shadow across my panels so i was seeing like 27 watts. Moved out of that shadow and it jumped to like 60 something watts at about 10am on a cloudy day.

I thoroughly recommend wiring in series with an MPPT controller but I've had bad luck with flex panels. I had one melt. It was a 25 watt panel just charging direct to usb of a battery bank. I sat it on the grass so it had no airflow. Started smoking and melted the panel. So I have rigid polycrystalline panels now.
 

doublegregg

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i just started a thread on an almost identical query... mine is under dokio...not happy with 'em.

i shorted my panels, in full sun, and took readings with my MM and inductive clamp meter.... i'm a newbie at this, so whatever i do is.. worth a grain of salt???

 anyway, to my newbie way of thinking, shorting the panels is the most direct way to get a reading from them. with the controller attached --- and doing it's job, no way to no what it's doing...........  i figure you're forced to just guess.
 

izifaddag

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Spaceman Spiff said:
Solar panels are rated under idealized laboratory conditions:
 - simulated sunlight irradiance of 1000W/m^2
 - solar panel at 25ºC (~76ºF).
You will almost never see this combination in real life.

That is SO true. 
I just fooled myself and got lucky - for a while. 
I had 1000 watts (theoretically) all wired in parallel and fed into a Renogy generic MPPT controller. Dec / Jan / Feb all was well. I then had shore power for a couple of months and so they got a break. 
Back out on the road in June and the controller went berserk, resets started blowing and wires were getting hot. 
Took me a bit to understand what was going on. Firstly the controller was rated at no more than 520 watts. So I had got lucky and the controller had been generous to me. I had taxed the poor thing.
I used 1 x 100 watt and 1 x 400 watt in series which corrected everything. No more crazy stuff. 
I have ordered a 2nd charge controller and will do a repeat performance running both controllers in parallel. Unfortunately - and I have no idea why - the negative connection inside the box on the other 100 watt panel fried. 
All diodes ok and the panel is working still. The MC4 connections were cooked the cable was destroyed and the plastic box deformed.
When #2 charge controller arrives I will fix the 100 watt panel and do a series connection again and voila. 
So although its a 1000 watts in a lab, in real life not a snowballs chance in hell. I just got lucky for a couple of months.
 

izifaddag

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doublegregg said:
i just started a thread on an almost identical query... mine is under dokio...not happy with 'em.

i shorted my panels, in full sun, and took readings with my MM and inductive clamp meter.... i'm a newbie at this, so whatever i do is.. worth a grain of salt???

 anyway, to my newbie way of thinking, shorting the panels is the most direct way to get a reading from them. with the controller attached --- and doing it's job, no way to no what it's doing...........  i figure you're forced to just guess.
Firstly Dokia are expensive and no better than anybody else. I will continue with the stuff I have bought from SanTan. Trinia. Very good value for money. 
Secondly don't sell yourself short. A lot of people would have no idea how to use a MM or a clamp. 
Last thing please don't short the connections to your panels together. If you do you will, at best, destroy the diodes and at worse kill a buss in the panel itself. 
You need to know the OC (open circuit) voltage from the panel. The manufacturer will tell you this. Just measure DC voltage coming from the panel without a connection. Then connect to the controller and measure again. The voltage will drop. This is because the controller is acting as a load and current is being drawn. Ohms law. Voltage will drop and current will increase. 
You will get a feel for what the panels and controller can do. YouTube is very helpful but nothing substitutes experience.
 

doublegregg

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my understanding is the way to test the maximum amount of current a panel can produce is to basically short the positive and negative terminals.  you can do this with a MM, or a clamp meter......... with the panel in direct full sun.  

see below...

How do you test a solar panel current?



[font=Roboto, arial, sans-serif][size=medium]Testing Short Circuit Current (ISC)

Directly connect the positive lead of your multimeter to the positive wire (or terminal) of the solar panel, and the negative lead of the multimeter to the negative wire (or terminal) of the solar panel. This will show the Short Circuit Current of the solar panel.
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Testing Solar Panels and Blankets | REDARC Electronics
https://www.redarc.com.au/testing-solar-panels-and-blanketshttps://www.redarc.com.au › testing-solar-panels-and-bl...
https://www.redarc.com.au/testing-solar-panels-and-blankets
https://www.google.com/search?q=how...ome..69i57.5618j0j4&sourceid=chrome&ie=UTF-8#
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  1. https://webcache.googleusercontent....panels-and-blankets+&cd=4&hl=en&ct=clnk&gl=us
 

izifaddag

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Exactly. The way he wrote, it appeared that he was going to expose the panel to sunlight and short the 2 leads together. A big mistake. Now I see what you mean. Yes that makes sense.
 

tx2sturgis

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izifaddag said:
Exactly. The way he wrote, it appeared that he was going to expose the panel to sunlight and short the 2 leads together. A big mistake. Now I see what you mean. Yes that makes sense.

Sorry...no.

You can test the full output of any normal solar panel in full sunlight by measuring the current flow with the leads shorted and an amp meter inline with the shorted leads.
 
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