A heat pipe includes a reservoir of liquid that is connected to a horizontal portion of the heat pipe via a capillary connection. The heat pipe includes a temperature sensor in proximity to a heat interface in the horizontal portion and a controller that controls a heater for the reservoir. As power into the heat pipe increases, the controller turns on the heater, causing the temperature of the liquid in the reservoir to rise. Liquid then passes from the reservoir through the capillary connection into the horizontal portion, thereby dynamically increasing the amount of liquid in the heat pipe, which increases performance of the heat pipe at higher power levels. When the heater is off, as the heat pipe cools, the liquid condenses and flows back through the capillary connection into the reservoir. The result is a heat pipe that provides demand-based charging of the liquid based on power level.