Wednesday, May 1, 2013

Solar Wishful Thinking

You might have seen this picture, which is making the rounds on Facebook (with a link to an article at "A portable solar-powered outlet that sticks to any window", says the caption.

Cute gadget.  But let's do some estimates of how much power we might get out of it.

The intensity of bright sunlight is about 1000 watts per square meter. Assuming those two pins have a standard spacing, the diameter of this gadget is maybe 4-6 centimeters, so the area is about 12-18 square centimeters. That gives less than 2W incident radiation when oriented normal to the sunlight (the most efficient direction - less otherwise). Solar cell efficiencies are maybe 20%, so the available power is a few tenths of a watt under optimum conditions. If the gadget really contains an inverter as well (so producing AC power at the outlet) there will be further losses there. 

Let's be super optimistic and suppose that we get half a watt out of the device.  What could one do with that?  Not run your microwave or your hairdryer or your laptop, that's for sure.  According to, a single rechargeable (NiMH) AAA battery stores 3400+ joules, so it would take about two hours to recharge from this device.  The battery pack for a typical small electronic device might be four AAA cells, so think about all day to recharge it.

It's a pleasant fantasy that we can stick a simple device to the window and get power for free, but in reality, energy is not floating around our environment in so concentrated a form.  Of course that is why, when we did discover really concentrated and (temporarily) abundant forms of energy - coil, oil, uranium - we were able to make such drastic changes to the environment.  Abundant energy was "a story for which the world is not yet prepared", like hunter-gatherers suddenly introduced to Haagen-Dazs.

PS: Sherlock Holmes fans, what was the story for which the world is not yet prepared? No Googling!

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