Lies and solar panels (Ohm's Law applied)

[confused2]

Picking a solar panel at random from Amazon..

https://www.amazon.co.uk/Photonic-Univer...880&sr=8-6-

Peak power: 300W
- Maximum power voltage: 32.7V
- Maximum power current: 9.17A
- Open circuit voltage: 39.8V
- Short circuit current: 9.7A
- Power tolerance: 0/+3%
- Dimensions: 164 x 99 x 3.5 cm
- Weight: 19 kg

Internal resistance = (open circuit voltage)/(open circuit current)
=39.8/9.7
=4.1 Ohms

Maximum power transfer when load resistance = source resistance
( https://www.google.com/search?q=maximum+...e&ie=UTF-8 )
so best load 4.1 ohms
current 39.8/8.2=4.85 A
Voltage across load (half is dropped across internal resistance)
= 39.8/2=19.9V
Power transferred to load (voltage*current)
=19.9*4.85
=96 Watts
Nowhere near 300 Watts and 96 Watts is the theoretical maximum you can ever possibly get.

Their maximum power voltage of 32.7 volts suggests 8 volts dropped across the internal resistance so the current (must be)
= 8/4.1
= 2 Amps
So the power delivered would be (voltage*current)
= 32.7*2
= 65 Watts .. about a fifth of the claimed 300 Watts

The 'maximum power current' is either meaningless or they have shorted the output (typo (9.17)=9.7) and no power is delivered to a load with zero volts across it.

So why do they call (at best) a 100 Watt panel a 300W panel? My guess is that they'd never sell one if they did.

In fairness they have included enough information for the buyer to work out that this solar panel is actually specced to give 65 Watts on a good day.

---

There seems to be a whole industry based on the claim that Ohm's Law is wrong.
The basis seems to be that people might buy a solar panel in the hope of getting their money back in (say) 10 years. If Ohm's Law was true it would take 50 years to never to get their money back so Ohm's Law is overridden by greed. I'm not sure physics works that way.

Looking at
"9 Best 300 Watt Solar Panel Reviews You Can Buy In 2021!"

https://solarpanelfreak.com/best-300-watt-solar-panel/

Our 'independant' reviewer chooses a similar (or identical) 65 Watt device as the top of of his list.
You can check the price on Amazon:
https://www.amazon.com/dp/B07K48KK46?tag...th=1&psc=1

Optimum Operating Voltage (Vmp): 32. 20V
Open Circuit Voltage (Voc): 38. 80V
Optimum Operating Current (Imp): 9. 32A
Short-Circuit Current (Isc): 9. 71A

[Zinjanthropos]

Wow, High efficiency and special solar cable. How do they calculate efficiency?

Under perfect conditions. Clouds and night may skew numbers. 

https://news.energysage.com/technology-u...ar-panels/

Try and get rid of it....

https://www.wired.com/story/solar-panels...xic-trash/

Having another problem for a future generation to fix seems to be the norm.

Was thinking C2... once you install monstrosity on roof and it doesn’t save you anything you still have to look at it. Does it help insulate or curb heat loss thru roof?

[Yazata]

Good post, C2 !!

So why do they call (at best) a 100 Watt panel a 300W panel? My guess is that they'd never sell one if they did...

If Ohm's Law was true it would take 50 years to never to get their money back so Ohm's Law is overridden by greed. I'm not sure physics works that way.

Don't think so.

Probably most people who buy these things don't do the numbers themselves like you did. They are just looking for a 300W panel and if the ad says "300W" that's the one that they will buy.

[C C]

[...] There seems to be a whole industry based on the claim that Ohm's Law is wrong. [...]

In addition to being another sub-culture and Earth-conscious lifestyle whose beliefs one must respect even should they collide with reality in specific areas, "Green" has arguably become a form of religiosity. And thus another retail client of faith-based marketing.

So just as companies sell various items to their theological consumers and rely on those communities to project a sacred value onto the artifacts that elevates them above their cheap gimcrack status, no doubt Green industry expects the same spiritual investment to enhance their humble products (solar panel offerings, etc).

[stryder]

Do people actually buy solar panels with the intension of "earning money back"?. I mean I know there was a thing at one point where the government had grants for people adding panels to their homes with the intension of sticking any surplus back to the grid and for the most part what bull they were fed was pretty much the same as what you've pointed out.

The only time I would be in the market for panels is if/when I'm building something to be self-sufficient or as close as possible. That means it would need to fit whatever amount of energy is required in the design. I don't consider earning money back as that's an external value outside of something self-sufficient.

In any event well spotted, its one of those things a letter to Which? is probably called for.

[confused2]

I did check that solar cells behave as a voltage source in series with a resistor
https://www.markedbyteachers.com/as-and-...D%2DrI%2BE.

There are also claims that solar cells behave as a low resistance source up to a certain point and beyond that point the voltage collapses rapidly with increasing current - in which case my analysis of internal resistance is too simplistic.

I'm not suggesting Bezos invented sponsored and fake reviews - only that he has benefited from them more than most.