What is output, specified energy and efficiency you might ask yourself. Output is given in kW (kilowatt) and corresponds to the heat given off by the stove at a certain time. A example for Specified energy would be If the stove has an output of 4 kW for 10 hours the house has been supplied with an energy volume of 40 kWh (kilowatt hours). Efficiency is the relationship between the exploited and supplied energy in a system.