There are times where you absolutely need to charge up your iPhone 4 or 4S, but you're nowhere near a power outlet. In those situations, you might fall back upon an iPhone battery case like the trusty Mophie Juice Pack Air I carry with me, but what happens when that battery runs out? No problem, as long as you're carrying the Etón Mobius (US$80) rechargeable battery case with a solar panel and it's a sunny day.
The Mobius has a hefty capacity -- 1800 mAh -- and can be charged the time-honored way by plugging it into a USB port. But for those times where you're hiking, biking, camping or just not near a power outlet, you can place the case into the sunlight to charge the battery pack.
Since your iPhone (and any other mobile phone) can react badly to exposure to laying in the sun, Etón recommends taking the Mobius off of the iPhone before setting it in a sunny place for charging. Especially with the black Mobius case, you'll want to take care picking it up after it's been sitting in the sun for a while as it can get quite warm.
One hour of sunlight is enough to help you out of a jam, providing 25 minutes of talk time, 20 minutes of 3G internet use, 35 minutes of video playback, or 145 minutes of audio playback. If you leave the Mobius in the sun long enough to get a full charge (Etón does not give an estimate of full charge time as it is dependent on a number of factors), your iPhone gets another 5 hours of talk time, 8 hours of 3G internet, 8 hours of video playback, or 32 hours of audio playback.
As you'd expect with any device that has an internal battery, the Mobius has some bulk to it. The case is 2.44" x 5.1" x 0.875" (61.9 x 129.5 x 22.2 mm) in size and adds 3.4 ounces (96.4 gm) to the weight of your iPhone. By comparison, that Mophie Juice Pack Air I was referring to earlier has the same price tag, only a 1500 mAh battery, doesn't offer solar charging, adds 2.5 ounces (71 gm) of weight, and is only about .2" (5 mm) thinner.
The case comes in black and white; as you can see from the small gallery above it's a pretty standard-looking battery case. That blue high efficiency monocrystal solar panel is the one outside feature that really makes this case stand out.
I did run into some issues trying to charge the Mobius by placing it in a window. Although the window was in full direct sunlight, the device failed to charge. I'm assuming that it was because of the anti-UV coatings on my windows, since it did charge nicely when I put it outside in the bright Colorado sunshine.
Even if you don't use the Mobius as your regular day-to-day iPhone case and battery pack, it can be a useful addition to an emergency preparedness kit. Having the ability to charge your iPhone when the power's out or you're stuck in the car somewhere could be a lifesaver.