On the first episode of Sword Art Online we know about a microwaves device installed on the helmet. We learn that this device can damage to death the user's brain removing the limiter. Unplugging the device causes the battery to activate as fallback, using the remaining power on the battery to damage the brain of the user.
The main device of a today microwave oven is magnetron, I cannot find any example of a magnetron consuming less than 1kWh as now (I accept correction), emitting more than 600W in microwaves. As of current technology, can a portable battery provide enough power to burn a brain? Are there more details about this issue described later in the anime or in the novels?