Wood pellets can be used to fuel stoves (with or without back boilers) and pellet burning boilers especially designed for the purpose. Pellet stoves require less attention compared to log appliances and are the most convenient wood fuel to use in a domestic setting. Integral fuel hoppers store enough pellets for 1 to 3 days operation and the ash pan only needs emptying between once a month and once a year. Wood pellet boilers are fully automatic and almost as convenient as using gas or oil. They are well suited to meet variable load demands and can be operated on a timer. Pellet stoves and boilers operate at high efficiencies of around 90%. Being a very dense fuel pellets require less storage space than logs or chips.