I am building an energy harvesting sensor for building installation, and I have to choose between two panels:

  • Indoor, 5V, 47uA Isc @ 200 Lx

  • Outdoor, 6V, 15.9mA Isc @ 50 kLx (note the different light level rating)

Given that they have similar dimensions (negligible difference for the application), one would say that the indoor panel would be more efficient with narrow band light such as the one generated by a lamp.

The problem is that this last detail is not included in the technical data sheet, as it's not included the I-V curve, that can however be traced from measures or taken from models.

Now, the strange thing is that the outdoor panel has shown a better performance (in terms of power given at the MPP) under the same - indoor - light conditions; this test has been done in a working environment, that should be lighted at about 500 Lux; the lamps are probably fluorescent, and probably with a quite good "white" light. Also with shading (a hand above the panel) the outdoor one gives more power.

Now we decided to use the outdoor, but can you find a reason for that? And what would be (apart from price) the advantage in using the indoor panel?

  • 1
    \$\begingroup\$ I'm guessing "indoor" versus "outdoor" has to do with how well the panel is sealed, UV resistant, etc. Silicon diodes are going to have pretty much the same spectral respones. Trying to get @Russell's attention here since he probably knows for sure. \$\endgroup\$ Feb 14, 2012 at 16:40
  • \$\begingroup\$ Thank you! :) in fact, handling the panels the indoor one seems more fragile, and it's thinner...but that makes the difference maybe bigger... \$\endgroup\$
    – clabacchio
    Feb 14, 2012 at 17:46

1 Answer 1


As Olin Lathrop pointed out the durability of the panels is the major difference. If the final sensor is going to be placed indoors then considering the indoor panel is realistic.

First a comparison of efficiency based on the specs you provided: The indoor panel provides about 1.18 W/lux while the outdoor panel provides about 1.91 W/lux. If there is a concern over providing enough power for the application the outdoor panel might provide the extra power needed.

Next I took the efficiency numbers and combined them with cost to obtain a metric of (W/lux)/UnitCurrency. Since purchase volume has an effect on the price I have calculated the metric for both single unit volume and maximum discount. The indoor panel is about 46% less expensive than the outdoor panel at single unit volume. At 250+ units the cost difference reduces to about 18%.

I would seriously consider the indoor panel if it would meet the power requirements. If the final sensor product will include a durable case, the argument for the indoor panel grows stronger.


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