News and analysis about renewable energy in California.

Viral 'Solar Window Outlet' Cannot Possibly Work

The "solar window outlet" | Photo: Yanko Design

It's excited a whole lot of people, including some who should know better, but a recently viral "solar window outlet" making the rounds of social media would need to violate a few laws of physics to deliver as promised.

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The outlet, purported to be a photovoltaic battery charger and port that sticks to a window with a suction cup, was "announced" on the website of the design firm Yanko Design last week after being designed in 2010. The page recently started gaining traction in social media, and has since been retweeted more than 1,600 times, liked more than 18,000 times on Facebook, and lauded in usually veracious publications like Grist.

According to designers Kyuho Song & Boa Oh,

The Window Socket offers a neat way to harness solar energy and use it as a plug socket. So far we have seen solutions that act as a solar battery backup, but none as a direct plug-in. Simple in design, the plug just attaches to any window and does its job intuitively.

It's an appealing and simple design: you stick the hockey-puck-sized PV collector onto a sunny window and plug your appliance into the other side. Yanko's photos and drawings of the device show what appears to be a Europlug-style outlet on the back side of the device, and the accompanying text says:

We tried to design a portable socket, so that users can use it intuitively without training.

The most charitable interpretation of the solar outlet is that the designers "created" it as an exercise in visual design without actually consulting an electrical engineer. The thousands of people apparently anxious to shell out their cash for Yanko's solar outlets are going to be disappointed: there's no way the gadget could actually work as described. Running an AC appliance on a hockey-puck-sized photovoltaic panel just ain't gonna happen.

All solar power technology is limited by one pesky little constant: the maximum amount of solar energy striking any representative section of the planet is generally about one kilowatt per square meter. Yanko's photos seem to show a photovoltaic surface about 3 inches wide. Call it 8 centimeters in diameter, which means a radius of 4 centimeters, which according to that formula you might not have used since high school to calculate the area of a circle, πr2, means about 50 square centimeters of photovoltaic surface.

50 square centimeters is 1/200 of a square meter, which means the maximum sunlight falling on that PV surface is about 5 watts. Assume the PV cells have an efficiency of 20 percent, which is unreasonably charitable to Yanko seeing as most such run between 10 and 15 percent, and the maximum power output of the PV cells drops to 1 watt.

That's a maximum. To get that maximum, the cells would need to be aimed directly at the sun during midday, unlikely with a window suction cup mount given that most windows point at the horizon. (A design that sat facing upwards on the windowsill would likely work far better, given a sunny south-facing window.) Mounting the solar cells on the inside of a window cuts the output even more, as window glass will absorb some light.

But let's be charitable and work with that maximum output of one watt of output. A watt isn't a whole lot of power. A typical electric clock will require about 5 watts to run. An electric toothbrush runs around 10 watts. If the unit had a USB rather than an AC outlet, a watt of power output would take more than four hours of sunlight to fully charge most modern smartphones -- perhaps useful in an emergency, but there are solar chargers on the market now that do better. And forget about running your 60-watt laptop, or even your 10-watt tablet, without days-long charge times. Which would be impeded by the drain on the battery the designers mention. Yanko says the product's battery will hold a full charge only for about 10 hours, meaning that over the course of a winter's night you'd lose the power you collected the day before.

But the solar window outlet doesn't have a USB plug receptacle: it's got an AC receptacle. Equipped with photovoltaic cells and a 1,000 milliamp-hour battery, both of which provide power in direct current form, the window outlet would also need to include a power inverter to run AC appliances, if you could find an AC appliance that required less than a watt of power.

A typical "ultra-compact" power inverter can get down to about the size of a hardcover book or a large beverage cup these days. Squeezing a power inverter into a hockey-puck-sized container that already needs as much room as practical taken up by a battery would require use of technology to which we don't yet have access, perhaps along these lines:

Even if you could use Time Lord tech to put an inverter in your hockeypuck, the inverter uses energy to turn direct current into alternating current. Unless the inverter runs off a separate plug, it gets that power from the PV cells, meaning there's less left over for your hypothetical 1-watt AC appliance.

In comments on the outlet's web page, designer Kyuho Song replied to criticism of the gadget's likely disappointing power output:

Thank you.The window socket design in 2010.And has completed the registration of a patent.Technical investigation for the mass production of the prototype.The technology is possible.However, the use of electricity and charge time is not efficient.But soon, believe it will be resolved technically.Thank you once again.

Skeptics on Facebook and Twitter -- including yours truly -- have been met with replies that the product is too promising not to be supported, and that rather than pointing out the inherent problems those who doubt the product should be working on ways to make it work. And there are ways to boost the output of the product. You could assemble a bunch of mirrors and lenses to focus 50 times as much sunlight on the solar cells, though the plastic housing would need to be redesigned to be melt-proof and the suction cup would need to be a lot stronger. You could place the object in an area with much stronger sunlight that anywhere on the surface of the planet Earth: in orbit, or perhaps on Mercury. (Venus is too cloudy.) You could develop a whole new line of sub-watt-powered AC appliances, like very slow blow driers or room-temperature refrigerators or LED Easy-Bake Ovens .

Or (snark aside) you could, as some UCLA techies are actually doing, skip the suction cup part and investigate the possibility that our windows themselves could be solar panels. That way each building could have tens of square meters of PV generating power from their windows without any gadgets stuck to them.

That would be an infrastructure-based solution, and so it lacks the sex appeal of buying yet another cheap plastic gadget in the hopes that said gadget will change your life. But maybe that is, as the gadgeteers say, a feature rather than a bug.

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About the Author

Chris Clarke is a natural history writer and environmental journalist currently at work on a book about the Joshua tree. He lives in Joshua Tree.
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