# What level of light intensity (lumens) do you need across a solar panel in order to obtain 15% energy efficiency?

What level of light intensity (lumens) do you need across a solar panel in order to obtain an incident-light to energy-output efficiency of 15%?

• What do you mean by "minimum optimal"? This sounds like two different operating points to me.
– LShaver
Jan 15, 2019 at 20:34
• What I'm referring to with solar is that there is an optimal energy conversion output of around 20% light to electricity created. I dont know how much lower you can go on the conversion output before its considered inefficient. So lets just assume that 15% is the minimal energy conversion output we would be willing to tolerate and go from there. Jan 15, 2019 at 21:49
• So what you're looking for is the minimum light intensity to achieve the highest efficiency from a panel? Assuming that the intensity vs efficiency curve may be parabolic? Any particular panel type/chemistry you're interested in?
– LShaver
Jan 15, 2019 at 21:52
• Please [edit[ your question to describe that you want to achieve the panel's optimal efficiency, how you're going to test the theory, and what the theory is. That way, people will be able to give you a better answer. Jan 16, 2019 at 5:45
• The question is simple, but we still need to understand it first. ;) That "minimum optimal" notion, AFAIK, is not really used with solar panels, so it can confuse people. If you mean that a) "Does efficiency depend on the amount of light?" (for one, I'm not sure, but interested!), and b) "If yes, how much light is needed to reach some arbitrary level, like 15%?", then please rephrase the (description of the) question, so that people won't need to munch through the comments to figure out what was really meant.
– Sz.
Jan 17, 2019 at 19:38

What level of light intensity (lumens) do you need across a solar panel in order to obtain an energy-output to incident-light efficiency of 15%?

This depends on the varying characteristics of different materials, so in this case I'll pick one Silicon based as they're pretty common, mass produced and cheap, and more to the point they're the only one I could find a nice graph which makes the point clear:

Accreditation A. Ibrahim, "Analysis of Electrical Characteristics of Photovoltaic Single Crystal Silicon Solar Cells at Outdoor Measurements," Smart Grid and Renewable Energy, Vol. 2 No. 2, 2011, pp. 169-175. doi: 10.4236/sgre.2011.22020.

• So, for the point of 15 % efficiency you need only provide about 900 W/m^2 - that corresponds to 90% of peak sun (with the panel flat-on to it).

A source radiating a power of one watt of light in the color for which the eye is most efficient (a wavelength of 555 nm, in the green region of the optical spectrum) has luminous flux of 683 lumens. So a lumen represents at least 1/683 watts of visible light power, depending on the spectral distribution.

Well this is specific and related to human vision rather than how solar cells are normaly measured, but we can make a conversion for a ball-park figure assuming that your light source is spectrally similar to the sun.

• 900 W/m^2 approximatley corresponds to 614700 Lumens per square metre.

This is not the whole story, temperature matters when looking at quoted power outputs in a catalogue or from a salesman:

The averaged conversion efficiency of the solar cell recorded for a period of six month is approximately 5.4%. It is 54% less than the efficiency of the same cell measured at STC.

A graph plotting temperature against efficiency is "illuminating":

Accreditation A. Ibrahim, "Analysis of Electrical Characteristics of Photovoltaic Single Crystal Silicon Solar Cells at Outdoor Measurements," Smart Grid and Renewable Energy, Vol. 2 No. 2, 2011, pp. 169-175. doi: 10.4236/sgre.2011.22020.

Recognise the trend and measurement distribution points? This corresponds very closley to the Illuminance/efficiency graph.

• Another thing you should bare in mind is that manufacturers quoted ratings are taken from flash tests provided at 25 Celsius (room temp) under what's called STC, standard test conditions, but frequent variation in lamp spectra and type used for measurement lead to variable comparison between different brands - it's just not tightly regulated.

# Conclusion

• An infra-red reflecting shield, and a heatsink to stop the thing overheating may be as important as the level of illuminance, when it comes to the panel's efficiency.
• Thanks for you answer. Your conclusion puzzled me a bit. Usually a conclusion follows from the text above, but here you introduce overheating. Could you elaborate? Jan 21, 2019 at 6:26
• @THelper Please seeSecond graph relating to temperature and efficiency and the following paragraph relating to the test method of manufacturers being at 25 celsius and not in field test conditions where the temp in full sunlight gets much higher and thus the efficiency drops (as per graph). Jan 21, 2019 at 16:13
• Perhaps I should have used a different phrase. Not overheating, but, heating beyond STC. What do you think? Jan 21, 2019 at 16:16
• Peak sun is taken as standard to be 1000W/m2, so 900 is not a third of max illumination. And the OP is asking about low levels of illumination, which your answer doesn't cover. What's the efficiency around 50 or 100W/m2? Jan 21, 2019 at 20:31
• @EnergyNumbers Oops my error, should have fact checked my memory, will edit. The OP wasn't specific (quantativley) about light levels, only about 15% efficiency, I supplied the info available about diminishing light levels, if there's other valid sources of info relating to lower levels - let me know and I'll edit my answer accordingly - but not much of quality seems to be in the public domain which is usable here. Jan 21, 2019 at 21:05

It's simple led flashlight can run a small panel because their light is high on the spectrum light scale.It produces more power than the yellowish light LEDs that mostly every one else thinks of when it comes to lighting the panel coupled with a mirrored shroud like in warehouses you can effectively use the panels all day an night.And you can stack them in a building taking less ground and controlled environment for maintenance purposes.As an added advantage just about eliminating the need for batteries

• Welcome to Sustainable Living! Thank you for your answer, but I'm not sure I understand it. I think you are not answering the question at the top of the page. Sep 2, 2019 at 8:07
• I've only done it on a small scale it's not like I have full size panels at the house, but rechargeable solar landscape lighting an solar toys I have tested have produced more when confronted with the higher spectrum scale of lights far as lumens I didn't keep up with it because I still use watts as a measurement of light. Sep 9, 2019 at 22:35