The AGA is known for its longevity, with many cookers still operating after more than 50 years of service. They operate on the principle that a heavy frame made from cast iron components can absorb heat from a relatively low-intensity but continuously-burning source, and the accumulated heat can then be used when needed for cooking. Customers often talk about how the AGA actually makes their homes more energy efficient, as the AGA does a number of jobs, such as replacing several radiators, and is not simply a cooker. Today, there is even a smaller AGA (the 60cm wide City) intended for compact spaces. It has two ovens and a hotplate, with the main oven being fully programmable. Like previous models, in the winter it can be left on all the time as a heat source.