Firstly, to understand this article, you'll need to have a look at my post about the logging platform I'm using here.
Let me describe the rooms in which I put these platforms:

- 2 platforms in a room with artificial light only. One attached to the wall, the other on a table:

   mur_labo.JPG   table_labo.JPG

- 2 platforms attached to a wall facing a huge open space, where only reflections from outdoor light can penetrate (as well as indoor light of course) :


- 2x2 platforms in rooms having their windows facing west. The platforms are either facing north or south and are at different distances from the windows:

   mur_gauche_solaim.JPG   mur_droite_solaim.JPG
   mur_gauch_jerem.JPG   mur_droite_jerem.JPG

So between the 16th of december and 16th of january, here is the average extracted energy :


First of all, you need to keep in mind that this is more or less the worst month for energy harvesting, as it is winter and mostly cloudy.
With an average of 151uW, it is the platform facing south and placed very close to the window which wins the contest. I'll say there's no big suprise here, as I'm living in the north hemisphere.
However, the other platform also facing south placed in the other room nearly got the worst extracted energy (7.4uW). Why? I guess because it was placed a bit too high and too far from the window (nearly 2.5m away).
We also notice that the 1W solar panel platform nearly extracts twice the energy that the 0.5W solar panel platform does (50uW vs 28uW). The difference is explained by their different efficencies: 15.5% vs 17%.
And finally, don't even think about extracting solar energy when your solar panel is attached to a wall in a room where no light from the outside can penetrate. I basically extracted nothing.
The platform facing up in the "closed room" extracted an average of 54.4uW, and you'll see further down this article the frequency at which the (quite bright) lights where turned on.

So what can we get from these results?
Well, you'll still need to take care about where to place your solar panels in your home, especially if you want to extract more than 5uW.
However I think it is a fair assumption to expect more than 20uW if you place your panel close to a window with the curtains partially shut (see pictures).
But then, you can expect even more if you design your embedded system with a large battery in order to get more energy in the better months of the year (numbers coming soon ;-) ).
And finally, if you know that at some point in the day the sun will directly face your solar panel, then you can be sure you'll have more than 150uW in average.

Logged data
Here are the instantaneous extracted energies through this month period.
My logging platform actually saturated for the green data (I can't log more than 17mW). Only for a short time though.
You can also see that the time in which you can actually extract energy during the day is quite short. That's why you got to make it count....

Finally, here is the comparison between using the BQ25504 to charge your battery versus having your solar panel directly power a fixed load:

Quite impressive right? This BQ chip is really a wonder...

In case you want all the data, here is the excel file: mesures.xlsx