Producing electricity with on-site combined heat and power, to feed artificial lighting implies the production of large amounts of waste heat. Quite often the heat production exceeds the actual heat demand of the greenhouse. Thus combined heat and power induces heat surpluses. However, in a large span of time (yearround), reject heat from combined heat and power is less than the heat demand of an intensive utilized greenhouse. Thus, periods with surpluses must be considered temporary and storage of these surpluses for future use is advantageous. Another potential source of heat surpluses is the boiler, when its exhaust gases are used for carbondioxide enrichment of the greenhouse air. Carbondioxide addition is requested during the day, whereas the heat, associated with this way of CO2-production is mostly not required during the day. A buffer can store the heat produced during the day for use during the night. To determine the energy consumption and canopy growth of a greenhouse with artificial lighting, combined heat and power, CO2-enrichment of the canopy ambient and a heat storage facility a simulation model was built. In this paper the energy saving obtained by heat storage facilities with different dimensions is discussed. Comparison of the benefits with the costs related to the storage facility gives a possibility to determine an optimal storage capacity. It will be demonstrated that the amount of carbondioxide addition has a significant effect on the optimum storage capacity.