Rough guide to Solar Panel Output

There is currently a lot of solar panel’s available on fleabay and from other sources. Some are good and some aren’t so good. One aspect that I’ve come across is some poetic license on panel output power.

A lot of the panels offered are a bit light on with specifications so it can be hard to see what it is you are getting for your money. Provided you can obtain the physical size of the panel, the Voc, the Isc and the panel power rating you can get a rough comparison for the panel output.

With some assumptions made, 1kW/m2 radiation and 17% conversion efficiency you can get a fair idea of how the panel compares to other similar products.

Utopia would be having the actual VmaxP and ImaxP but they aren’t allways provided.

What we calculate is the theoretical maximum power that can be extracted from a physical panel size. So if the claimed rating is greater than or close to this then we’ve got reason to be doubtful of the claims.

 Here is some calculations I’ve done for a few panels.

  efficiency w/sqm max w/m2            
  17% 1000 170              
                     
Claimed Voc(V) Isc(A) P(W) Vmp(V) Imp(A) Pmax(W) H(mm) W(mm) theory W  
80 22.1 4.8 106.08 17.6 4.55 80.08 534 1209 110   actual 80W
80 21.6 5.17 111.672 17.2 4.65 79.98 534 1220 111   actual 80W
80 21.6 4.8 103.68 18 4.44 79.92 534 1220 111   actual 80w
80 20.5 2.9 59.45       534 840 76   claimed 80W
80 19.5 5 97.5       534 1220 111   Replacement for above
60 21.6 3.6 77.76 18 3.33 59.94 534 840 76   for comp with claimed 80W

The first three panels are 80W, similar size and similar Voc, Isc and powers. Note the relationship with the power between multiplying Voc x Isc and the theoretical power

 ( theoretical power = efficiency * W/m2 * m2 of panel ), they are roughly the same.

The actual output power is ~70% of the theoretical.

Panel 4 was what i was sent when I ordered and paid for an 80W panel. It arrived with a specification sticker claiming to be an 80W panel. Obviously it wasn’t 🙁 After discussions with the seller and the proof above a replacement panel was sent ( #5 above ).

And for comparison panel 6 is a 60W panel.

My original panel didn’t really pass the laughter test 😉

mounting a solar panel regulator

Some thoughts and concepts to consider when deciding upon where to mount a solar regulator.

There are a lot of portable solar panel kits being sold at the moment and for varying prices.

One aspect that has been questioned, is where should you mount the solar regulator? On the back of the solar panels or close to the batteries?

Here is a simplistic description as to what is going on to help you see what the pro’s and con’s are.
The regulator adjusts the input volts to suitable levels to charge ( and prevent overcharge ) of the battery. ie solar panel output volts is typically more than you want for the battery.
The regulator can only see it’s output voltage at the regulator terminal ( for most simple regulators, there are remote voltage sensing regulators but i’ll ignore them for the moment ).
Now if you have any current flowing through a piece of wire there will be a voltage drop ( all practical pieces of wire have resistance ).

so:
solar panel ( Vin ) —– wire ( Vloss-in ) —– regulator ( Vcharge ) —– wire ( Vloss-out ) — battery ( Vbattery ).

Depending upon where you put the long piece of wire will determine where we lose some voltage. As we want the regulator to look after the battery it’d be better if we kept Vloss-out to a minimum.

Also electronics don’t like lots of heat. the back of a solar panel sitting in the sun can be a tad warm. 🙂
Another guesstimate but, I would expect that temperatures of 50 degC  plus to be fairly easily achieved in full sunshine. Perhaps more isn’t unreasonable.
Most of the regulators will use commercial grade components that are only rated to ~ 60 degC. And most regulators will generate some heat in normal operation so you could be operating outside the devices temperature limits.
The other issue with temperature is that ( once again ) a lot of regulators are specified at 25 deg. They have built into them temperature compensation ( Batteries should be charged at differing voltages depending upon the temperature. Higher temperatures require lower voltage ). I hope you can see where this is leading 🙂
Now, if the regulator is parked in the direct sun and the battery attached to it is in the shade in a battery box under your camper, it’s a pretty fair bet to say that they will be at differing temperatures.
So with the regulator mounted on the back of the solar panel you now have a voltage drop between the regulator output and the battery due to cable losses and a regulator reducing it’s output voltage due to an increase of temperature. A double whammy for lower volts at the battery.

This are all factors that should be taken into consideration when you build a good solar system.
The impact they may or may not have depend on various factors such as current, wire resistance etc. which comes back to my original answer of yes and no.
More information is required to calculate if these factors are significant or not in your situation. I couldn’t say that ‘yes’ is an ok answer to your question with the information given.
I would suggest that if you have the option with an unmounted regulator that you do not mount it on the back of the solar panels, mount it near your batteries if practical.