Voltaic solar panels and extension wires are completely waterproof and can be left outside in any weather.
Voltaic solar charger backpacks and Fuse systems use waterproof-backed fabric and waterproof solar panels, but the batteries are not completely waterproof. In modest rain, simply adjusting the zips so that they are not open at the top of the bag should do. If submerged or in a heavy, extended rainstorm, the fabric will absorb water and water will eventually leak in through the zippers.
Mainly, you'll want to keep the Voltaic battery dry just as you would your phone or camera. The panels can be submerged and still function, they've even floated out in the Indian ocean for a month and came out in great condition.