At the heart of the Lagoon system is a 21.5-kilowatt Onan AC generator and two 72-volt propulsion-battery banks, each made up of six 8D 12-volt batteries. On the American version, the generator delivers both 220 volts AC, to power a 72-volt-DC battery charger and high-draw AC appliances such as air-conditioning compressors, and 110-volt AC for outlets. When the batteries are fully charged, the boat can run silently, without the generator running, for about two hours at 75 percent of full power. When the batteries' charge level drops below 80 percent, the generator automatically starts to prevent them from being depleted further. When the boat is sailing, the freewheeling propellers turn the electric motors, and they also become generators; at boat speeds of 4.5 knots and above, they recharge the batteries.