250 W/m2 is still rather optimistic -- though not absurdly so! Obviously it varies a lot depending on where you are. For example, in Germany the total sunshine

250 W/m2 is still rather optimistic -- though not absurdly so! Obviously it varies a lot depending on where you are. For example, in Germany the total sunshine you have to work with is approximately 1000 kWh/year[1], or 115W/m2 on average. I think that still assumes you find a way to perfectly align your phone to be perpendicular to the sun's rays; otherwise you get less energy still. [1] http://www.solar.lucycity.de/index.php/sonnenenergie/9-sonnenstrahlung http://www.solar.lucycity.de/index.php/sonnenenergie/9-sonne... I think the technical term may be solar irradiance, here's a global map: https://en.wikipedia.org/wiki/Solar_irradiance#/media/File:SolarGIS-Solar-map-World-map-en.png https://en.wikipedia.org/wiki/Solar_irradiance#/media/File:S...