Solar panels how much output

jeepnudger · Senior Member

Solar panels how much output

Bicyclecamper · Guru

my 80 watt portable panel consistantly gives me 4.5ah on a sunny day, either your panels are no good, or your regulator is stuffed.

-- Edited by Bicyclecamper on Friday 3rd of November 2023 11:07:27 PM

Cuppa · Guru

Depends on the state of charge of the batteries. If they are close to full 3.5 amps could be quite normal as the 'charge acceptance' by the batteries drops as they get closer to full.

If however they are discharged you may have a problem. To check - disconnect the batteries (remove all connections from battery terminals) & let them stand for 30 minutes or so & then check their voltage across the terminals.

Using this chart will then give you an idea of how charged/discharged your AGM batteries are. If they are below 60% to 70% you should expect to be getting the max output (amps) from your panels. If however they are above 80% to 90% the amount of amps will have dropped down. By the time the batteries are approaching 100% the current (amps) will have dwindled to almost nothing.

To determine max output from the panels depends upon how sunny, & how they are mounted.

If mounted fairly flat on the roof - in perfect conditions - expect 70% of their rated output.

Perfect conditions = Panels at a constant 90 degrees to the position of the sun on a full sunny day.

So. in a perfect situation 3x120w 12v panels would produce max 30 amps. If flat mounted on a roof you could expect a max of around 21 amps (Between say 11am & 2pm).

Some panels are less tolerant of shade than others. Some for example if 10% shaded will put out 90% of their rated output. Some however if 10% shaded will drop their output to virtually nothing.

-- Edited by Cuppa on Saturday 4th of November 2023 08:42:45 AM

-- Edited by Cuppa on Saturday 4th of November 2023 09:54:47 AM

TimTim · Senior Member

jeepnudger wrote:
I have 3 - 120w panels giving 3.5amps (so 40amp solar çontroller shows) to two AGM batteries.
What would yyou expect to be getting when it is a sunny day.

what should o/put be

120W panels output about 6.13A under ideal conditions so 3 equals about 18.39A when perpendicular to the sun. To add to what Cuppa posted the output is also location dependent and time of year.

If your batteries are near full then a simple test is when the sun is at its highest turn on as many 12v appliances as you can to draw some current from the batteries and you should see the solar panel output increase.

Tim

Cuppa · Guru

TimTim wrote:

120W panels output about 6.13A under ideal conditions so 3 equals about 18.39A when perpendicular to the sun.

______________________________________________________________________________________________________________________________________

Hi Tim how do you arrive at those precise figures? As I have always understood approx amps are derived from the panel wattage divided by the nominal voltage. In this case 120w divided by 12v = a potential max of 10 amps per panel. With the rough rule of thumb (Collyn Rivers) reducing that figure to 70% when roof mounted on a vehicle. ie 7amps

I understand that the charging voltage into the battery is higher than the nominal 12v & wonder if this is how you get the the 6.31amps with an assumption that Jeepnudger's panels are roof mounted?

Whenarewethere · Guru

I have 120 watts (6 x 20watts), about 3.8 amps output with 2 panels in series, so basically 7.6 amps if all in parallel. Up to 11.3 amps with water cooling from controller but typically 9 plus amps going into the batteries from the controller.

My set-up:

https://thegreynomads.activeboard.com/t65231112/custom-6x20-watts-solar-setup-with-mppt/

So with 360 watts of glass (proper) panels you should be getting at least 27 amps if square to the sun in the warmer months.

TimTim · Senior Member

Cuppa wrote:
TimTim wrote:

120W panels output about 6.13A under ideal conditions so 3 equals about 18.39A when perpendicular to the sun.
______________________________________________________________________________________________________________________________________

Hi Tim how do you arrive at those precise figures? As I have always understood approx amps are derived from the panel wattage divided by the nominal voltage. In this case 120w divided by 12v = a potential max of 10 amps per panel. With the rough rule of thumb (Collyn Rivers) reducing that figure to 70% when roof mounted on a vehicle. ie 7amps
I understand that the charging voltage into the battery is higher than the nominal 12v & wonder if this is how you get the the 6.31amps with an assumption that Jeepnudger's panels are roof mounted?

Hi Cuppa,

The figures I gave you are straight off the labels of one of two 120W panels that I have ready to be mounted on my van. The way to calculate is the panel Wattage divided by the maximum panel voltage (Vmp) = Short circuit current (Imp). Or as you can see from the attached photo 200W/19.6V = 6.13A. If you take a look at any panel specifications then you will see how it is calculated.

Also I said in ideal conditions when perpendicular to the sun not flat on the roof as in Jeepnudgers case. Maybe I could have explained it little better but you had already described how the output is reduced when flat on the roof and that is why I made reference to your post.

Many panels only average about 80% in the field when perpendicular to the sun so Im unsure how anyone can can give a figure of 70% for flat on a roof when there are too many variables to consider.

At the moment we are discussing solar panel output but then solar controller output is a different matter smile .

Cheers

Tim

Whenarewethere · Guru

Working out how much solar you are getting, type panel wattage, sin & angle on sun in your smartphone calculator.

Your 120 watt panel mounted flat on the roof, in the morning at say the sun is 25° up, produces 50.7 watts.

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Assuming that the panels are clean, no stupid tiny shadows (don't forget self shadowing) that will bring it to its knees & quality system as a whole.

Cuppa · Guru

TimTim wrote:

At the moment we are discussing solar panel output but then solar controller output is a different matter .
Im unsure how anyone can can give a figure of 70% for flat on a roof

I would have thought that in the context of the OP's question the amps getting into the battery were what is relevant

The 70% rule is an 'approximate' rule of thumb based upon the nominal voltage (not max voltage) & has proven to have been pretty accurate & useful over the years when determining solar requirements mounted on vehicles. I have followed this rule of thumb with two vehicles I've built, & both have proven to have had well balanced self sufficient systems. As I said this is from Collyn Rivers, an old friend & a respected author of many books about RV power systems. https://rvbooks.com.au/

120w divided by 12v=10amps, Multiplied by 70% =7 amps.

Works for me. My current 425w roof mounted solar shows between 19 & 24 amps (depending on sun conditions) coming in to the batteries on the Victron battery monitor when the batteries are allowing full current. 425w divided 12v =35.4167A multiplied by 70% = 24.7917A ..... close enough for a rule of thumb I reckon. This system comprising of 1 125w quality panel, & 5 x60w ebay cheapie panels has been in use for over 11 years now & performance has remained consistent. Batteries (3 x 120Ah Ritar AGM's) were replaced at 9 years & 10 months when one dropped a cell. 6 years of that period had been in full time use during which time we have never *needed* to plug into mains power (except when that battery dropped a cell).

-- Edited by Cuppa on Sunday 5th of November 2023 08:37:29 AM

TimTim · Senior Member

Exactly WAWT.

Time of day, time of year and location in Australia make huge differences. Tasmania will be substantially different from Cape York and that is without even considering shadowing from AC units and antenna etc.

Tim

Scubadoo · Veteran Member

As mentioned previously it will vary enormously depending on your location and the time of day and year.

I prefer real life numbers to theories, charts, formulae, Google etc.

We are currently located here in NZ at exactly 42° 27.000' S the equivalent of somewhere in the far north of Tasmania.

We have 3x 250-270W (800W) rated solar panels fed to a Victron 50A current limited MPPT controller.

50A (700W) into our battery occurs for a couple of hours most fine or partly cloudy days from mid October to the end of March.
I have seen 62A from a larger controller.
Mid winter output is typically about half.

Today - partly cloudy.

At this latitude I would expect about 24A from 3x 120W horizontal panels and a quality solar controller into the battery on a sunny summer day for an hour or four in bulk charge mode.

-- Edited by Scubadoo on Sunday 5th of November 2023 02:17:27 PM

StormCamper · Member

It's all governed by math sadly. Some don't like to hear this and claim testing and experiment is also needed or somehow theory is just theory, but this just shows a lack of understanding in the scientific method. Indeed it's all been worked out decades ago and the equations give very accurate results. First of all you should de rate your panels to 70% of their nameplate to account for the big losses like heat (summer) and angle of incidence (much more important in winter&/or lower latitudes) That just gives a realistic expectation. The two big killers are the airmass, and angle of incidence, the airmass has a compounding effect, while the output it self is proportional to the cosine of the angle between the sun's rays and panel normal. This means in summer locations with the sun always high in sky, the angle isn't super important, but in winter low latitudes even 20degress off will be a big impact. Heat is another big killer, panels usually drop in %efficiency by 0.45% for every 1deg C above module temp of 25C, in hot summer the panels can get to 75deg C, maybe more without proper airmass. At those temps a whole slurry of other things are happening like recombination losses, high power emission. Secondly, you can't calculate the output on a calc without knowing the irradiance! Thirdly, assuming the advertised wattage is genuine, you can't be sure they even tested it properly, some may cheap out on lamps that don't match the correct spectrum. Finally, whats meant by shade tolerance is to have as much bypass diodes inside the junction box as possible. The story of thin style panels being shade tolerant is a joke, they have a higher bandgap so work better in overcast simply because they can use more of the energetic blue light.