If you've got plenty of sunshine around and a need to charge your USB gadgets, you'll probably appreciate one of Brando's neat solar chargers. Equipped with a 3,000mAh battery that can be charged via leaf-like solar panels or AC power, the Brando Solar Tree charger lets you grow your own electricity.
It may not be the most portable or stylish of solar chargers, but it certainly makes for an interesting desktop decoration. The device is semi-modular and lets you customize the tree to your liking, but the small solar panels may not be as efficient as other charging devices. One advantage the tree does have though, is its budget-friendly price tag of $55.
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In principle any light source works fine. However, the isn't much energy in normal room lighting compared to sunlight. Sunlight has an energy density of roughly 1000W/m2 (100W/sq.ft.).
To calculate the energy density in your room lighting. Just add the wattage all the different bulbs you have in the room, and remember to divide with efficiency of each type. Incandescent lightbulbs are roughly 5% efficient (the rest is heat), fluorescent lightblub are roughly 95% efficient, and LED lightbulbs are roughly 99.8% efficient. Now you have the total energy. Then divide that by the total boundary surface, just calculate all walls, ceiling, and floor surfaces. Then you get the energy density of your room lighting. In most cases it'll be 100s if not 1000s of times less.
Now to finish off. Take the active surface area of the solar cells and multiply the energy density with that and then multiply with the solar cell's efficiency which is typically 14% for crystalline cells and 8% for thin film cells.
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