So let's say you want to go camping and take video and pictures, but not quite leave technology behind because maybe you want a GPS or to be contactable in case of an emergency. Or maybe you love camping but also love watching movies on your HTC EVO before you go to sleep.
With traditional methods, you bring the power you use. You want enough juice to watch a couple of movies a night, so you're trekking in with a few spare fully-charged batteries and once they're gone you're not using your phone anymore.
The Voltaic Fuse 4W is a portable 1.3-pound durable solar panel with a small pouch and 3000mAh replaceable battery that you use to store generated power for later. The unit is capable of turning direct sunlight into roughly 600mAh of charging power, although the best I was able to record using Battery Monitor Widget and my EVO 3D's resting screen-on discharge rate was about 550. Your mileage may vary.
The charging rate on a cloudless day with direct sunlight means you'll have fully charged the internal 3000mAh battery in 5-7 hours, which is enough for nearly two full charges of an HTC EVO 4G with a stock battery, about one and a half for an EVO 3D, or a little over one charge for the new HTC EVO 4G LTE and its non-replaceable 2000mAh internal battery.
You lose about 20-25% in power conversion from battery to battery, so the 3000mAh battery that ships with this is not going to charge two 1500mAh batteries fully; however, that's due to electric conversion gremlins, and not any fault of the battery or manufacturer.
Note that you can also plug your phone directly into this and unplug the included charger battery. You'll bypass maybe half of the electrical conversion loss, but I'm unsure if there may be any negative effects of inconsistent voltage. Probably not, but I was fine with letting it charge the pack's battery and letting the battery later charge my phones.
The Fuse 4W comes with adapter straps if you want to slap it on the back of a backpack; however, if the solar cells are not pointed at the sun it's not running as efficiently as it's able to. In practicality, adapting the thing to a hat would be a better clothing option, but would not make you look nearly as cool as an EVO owner should at all times strive to look. If you're on the go, it's better to get a little charge than none at all.
The charger looks tough and touts that it is built to withstand abuse. I do not doubt these claims, but as I had to return it in working condition I was not really about to test the limits of the thing. It feels sturdy enough.
The first day I had this, I took it out on a fairly cloudy/overcast day (think black skies and tornado warnings). The indicator light on the front lit up, showing that it was charging the battery. What I was able to gather was that with next to no sun, it still was picking up some small charge. I could not register it above 50mAh, which would take ~60 hours to charge a battery, but I was still impressed that it was getting anything at all.
I didn't have it long enough to think about charging other batteries, but as the charger battery it comes with charges via USB, my bet is you can slap in any replacement battery that can take a 500-700mAh trickle charge, so perhaps the MyCharge Portable Power Bank 6000 or similar could be used if you needed extra power capabilities (charging for friends and family) or just wanted to take advantage of the charging opportunity.
Something I started thinking about while playing with this is how much it actually costs to charge an EVO. A 1500 mAh battery, which we can safely assume we'll use the power of every day, takes about 1875mAh to charge. That's 225 watts a day or ~7000 watts a month if I did the math and conversion right. Based on my electric bill, it takes roughly $0.70 a month to power an EVO, so if you were able to cut that out in just 15 years this bad boy would pay for itself; 7 years if you're charging a friend too.
It's probably a must-have if you're wanting to pre-cache an area on Google Maps and spend a few days exploring and photographing off-the grid and not worry about power. It's also incredibly useful if you don't have power due to, for example, a tornado or high winds taking out your power lines.
Plus, it's just neat.