Power packed inside

IT was an invention just waiting to be patented. With electronic devices hitting the trails, someone had to put on the market a neat, portable source of electricity.

Voila! The solar backpack. It sounds ungainly, but two companies have made a go of it.

Clear Blue Hawaii offers the Blue Sun Solarpac ($260, battery not included) and Voltaic Systems offers the Voltaic Backpack ($229, battery included).

Both can charge your GPS unit and other gadgets while you hike, bike or sit using an internal battery, and both can charge direct from the solar cells in daylight. They are also the same size -- 1,850 cubic inches.

But beyond these similarities, the packs are quite different, which can be challenging for the consumer. First, Blue Sun’s solar panel, batteries and harness can be removed and used with other Clear Blue Hawaii products, saving $200 on a second bag. The same items in the Voltaic are fixed.

Second, while both packs operate with an internal lithium-ion battery, each battery option is slightly different. The Blue Sun can accept a $50 or $150 lithium-ion battery; the $150 battery stores more electricity and has a wider output (up to 12 volts) than the $50 battery (up to 7.2 volts). Voltaic’s battery, which ranges from 3.5- to 7.2-volts, can’t match Blue Sun’s $150 battery in output or output range, nor can it match Blue Sun’s $50 battery in overall output but bests it in output range.

Blue Sun’s solar panel yields twice the power of Voltaic’s three panels when tested in identical conditions. In addition, the type of panel that Blue Sun uses is more energy efficient than Voltaic’s monocrystalline panels.

All that said, given the fact that the Blue Sun has a better solar panel and batteries that yield more output, I would recommend the Blue Sun pack. With its $50 battery, you can charge most of your hand-helds, and if you purchase the $150 battery, you will be able to recharge a 12-volt device, such as a satellite phone. But this recommendation assumes you’ve got your heart set on a solar backpack.

The truth is, the limited size of these packs makes them impractical for anything beyond overnight outings, where you could easily pack extra batteries to meet your electrical needs.

But on outings lasting a week or more, these packs could save users from having to haul 10 pounds or so in batteries (even though you couldn’t use the packs because you wouldn’t have enough space for the provisions you’d need).

When Clear Blue Hawaii and Voltaic Systems produce solar packs at least three times the size of these, they will have a product with real appeal to backpackers. Until then, the Blue Sun and Voltaic backpacks are little more than novelty items with the promise of useful packs to come.