A solar-powered AC outlet. I kid you not.

Ok, so you think you’ve seen it all, and that there’s all this massive amounts of free (well, nearly) energy just streaming down on you from that big nuclear-powered star up there. This gadget is pretty novel, and pretty lame.

It’s around a 10 cm flat cylinder with an AC outlet on one side, and a teeny-tiny solar panel on the other. The panel has enough surface area that the vendor claims it can fully charge the 1000 mA-h battery inside the gadget to a full charge in 5 to 8 hours. The vendor then claims that the battery can last 10 hours (I’m assuming in discharge mode).

Fact check – let’s try to calculate the amount of power available from the gadget’s solar panel assuming a perfectly clear day and the gadget oriented so that the solar panel remains normal to the sun all day. The panel looks to be perhaps only 0.1 m across, and using 2πr^2, with r = 5 cm, the panel surface area is around 0.015 m^2. The highest solar insolation at the earth’s surface is less than 1100 w/m^2, and the solar panel is unlikely to have more than at 15-20% efficiency – let’s give it the benefit of the doubt and say 20%. That means that the solar panel, perfectly oriented and tracking the sun can generate about 0.015 m^2 * 0.20 * 1100 W or about 3.3 w of power. If the battery is really a 1000 mA-hr battery, and the gadget takes 8 hours max to charge to full, then the voltage on it needs to be about 25 vdc to make 25 w-hr of storage. And given all the inefficiencies, the battery voltage is likely more around 3.7 vdc (single Li cell) or 7.5 vdc (2 Li cells), in order that the “charges in 10 hours” claim can hold true.

That’s a pretty generous estimate, and not even considering the deleterious effects of the 80% of the sunlight that didn’t get converted to electricity by the solar panel, and instead just added a heat load of 15 watts to the little gadget, heating up the battery and charging circuit to uncomfortably high temperatures and reducing the battery and circuit life.

Over an 8 hour day, this is only 25 or so watt-hours. And in the real world, where (as the vendor suggests) the gadget is charged by sticking it to an outside-facing window the likely energy collected is going to be way less than that. I’d say maybe 10-20% of the best-case energy. But let’s ignore that big loss in efficiency for now.

So 25 w-hr, converted to 220 vac 50 Hz using a 95% efficiency converter,  means that this gadget can supply enough power over the advertised 10-hour full charge period to power “not much”. The calculation is

0.95 * 25 w-hr / 10 hr = 2.4 w continuous at 220 vac

A PC, iPad, iPhone or other smart phone-type device can’t even be fully charged using this gadget.  Another way to look at it is that an iPhone has a 4 vdc, ~ 2000 mA-hr battery, and this gadget’s battery is about half that in capacity. So it can’t even charge an iPhone fully, and in fact throws a lot of energy out the window due to the multiple conversions.

However, I will say that it would be pretty cool to see a power outlet stuck to a glass window with no other attachments, and the ability, even for a few minutes, to be able to plug in a load (like a computer or a light bulb) and amaze your friends that it can be powered. Like all magic tricks, you can only make it work under very controlled conditions and with a minimum of scrutiny.

It’s gadgets like this that give engineering a bad name.


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