# Solar Panel Ratings

Solar Panel Ratings
• Maximum power (Pmax): 100W
• Voltage at Pmax (Vmp):17.2V
• Current at Pmax (Imp): 5.81A
When I measured volts and amps this is correct. When it is connected to 12 volt battery I get 13 volts and 3.5 Amps when it is charging under Ohm's law this would be 45.5 watt are my meters off or am I losing current some where or am I not understanding this correctly as I thought if it is rated 100 watt you could run a 100 watt X 12 volt light bulb

Tom.G
Gold Member
Well, there are a few things going on here. First notice the "mp" subscripts in the panel rating.
• Voltage at Pmax (Vmp):17.2V
• Current at Pmax (Imp): 5.81A
The "mp" refers to the Maximum Power Point of the panel, that's the voltage and current at which you can get the most power from the panel. It is marked as MPP in this graph.

(Graph from: https://www.teachengineering.org/lessons/view/cub_pveff_lesson03)

Since you didn't specify what panel you are using, lets try rescaling the above graph to the values you are seeing. If you re-label the X-axis so MPP is at your 17.2V instead of 5V, you will see that your panel is operating at the 3.74V point on this graph; not at the MPP.

What is often left unsaid is that the MPP is taken under 'standard', close to ideal conditions; there are no clouds, low humidity, the Sun is directly overhead, the panel is directly facing the Sun, and often that the panel temperature is at 25°C (77°F). None of these conditions is likely to be true in real usage.

For further information see the link for the graph, and do a Google search for 'mpp of solar panel'.

Hope this helps.

Cheers,
Tom

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anorlunda
jim hardy
Gold Member
Dearly Missed
When it is connected to 12 volt battery I get 13 volts and 3.5 Amps
As Tom said
To get rated Pmax requires full eated Rated sunshine..
What time of day was it ? Any clouds or tree limbs blocking the sun ?

Last edited:
sophiecentaur
Gold Member
As Tom said
To get rated Pmax requires full eated sunshine..
What time of day was it ? Any clouds or tree limbs blocking the sun ?
@John1397 I imagine you are planning to implement a real system somewhere. So many factors to take into account and you need to do a lot of reading round this topic first.
The ratings are only reliable as values to start off your calculations with. I suspect the quoted Current refers to current with a short circuit load. That V/I graph shows how that is not realistic. My limited experience (one '40W' panel) show a massive variation in available power from the panel under conditions that would have seemed to be Maximal. It's a really practical problem and the theory is only 'best case'. I arranged the panel to point directly at the sun at Noon in Spring and was disappointed with the performance until I realised that I had used a very pessimistic approach and it actually does the job well enough all year.
A complicated (MPPT) regulator can arrange to present the panel with a current drain which will maximise the power output under all conditions but it's often cheaper and less trouble just to put more area of cells up there.
@jim hardy - we must remember to "eat" more sunshine.

jim hardy
That V/I graph shows how that is not realistic. My limited experience (one '40W' panel) show a massive variation in available power from the panel under conditions that would have seemed to be Maximal.
I think they should divide the watt ratings on solar panels by 2 to get a more realistic rating. It looks to me if you want maximum volts you get 0 amps and if you want maximum amps you get 0 volts. If you took a transformer with those ratings it would delivery them.

tech99
jim hardy
Gold Member
Dearly Missed
@jim hardy - we must remember to "eat" more sunshine.
Thanks @AophieCentaur
- e and r are adjacent and i just missed !
happens more and more often these days....

sophiecentaur and Tom.G
I think they should divide the watt ratings on solar panels by 2 to get a more realistic rating. It looks to me if you want maximum volts you get 0 amps and if you want maximum amps you get 0 volts. If you took a transformer with those ratings it would delivery them.
No, the way they spec them is the right way to go about it. You do get that current and that voltage, but only at that point (reread post #2). A solar panel and a transformer are not the same thing.

sophiecentaur
Gold Member
Thanks @AophieCentaur
- e and r are adjacent and i just missed !
happens more and more often these days....
A good typo can brighten my day.

jim hardy
sophiecentaur