We designed it so that when the water is boiling you can still generate 10W -- in fact, when you are starting with super cold water, the device may generate MORE than 10W in the beginning, but we wanted to make sure that we communicated a claim that was achievable with hot water since that's the purpose of a kettle. So, if you bring your water to a boil, you can keep it boiling for as long as you like (just don't go below the MIN FILL line) and generate 10W of continuous power.