pyr0ball

I would guess that the wattage is being controlled by the onboard battery charge controller. The system wont pull too much wattage if the battery is full. This feature prevents over-charging and preserves the longevity of the battery. It jumps back up every few minutes when the battery has lost some of it's charge and requires a little more juice to refill. I don't know this for sure because I'm not certain what you're using to measure the wattage draw. I'm assuming you're using something like a Kill-A-Watt to measure the wattage being drawn at the wall socket.