# Solar Panel Ratings: 100W Pmax, 17.2V Vmp, 5.81A Imp

• John1397
In summary, solar panel ratings refer to the maximum power (Pmax) output, voltage at maximum power (Vmp), and current at maximum power (Imp). However, these ratings are based on ideal conditions and may vary in real usage due to factors such as sunlight, panel orientation, and temperature. The maximum power point (MPP) is where the panel can produce the most power, but this may not always be achievable. Overall, it is important to consider all factors and be conservative when using solar panels for optimal results.

#### John1397

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

Well, there are a few things going on here. First notice the "mp" subscripts in the panel rating.
John1397 said:
• 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, let's 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
John1397 said:
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 ?

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jim hardy said:
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 realized 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
sophiecentaur said:
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
sophiecentaur said:
@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
John1397 said:
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.

jim hardy said:
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
John1397 said:
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.
Zero or short circuit current are pretty irrelevant values for the user because they involve no Power transfer. But what can you do - apart from looking at the VI graph and relating that to your particular requirement? But then, there are so many other imponderables involved that the only safe way is to be as pessimistic as you can be - or spend a year testing a sample setup.

jim hardy