The invention provides a computer simulation system for simulating a data centre. The simulation system uses a logical representation of the data centre to perform the simulation. This logical representation includes a plurality of nodes representing devices in the data centre. Each node has an input for applied load and outputs for electrical power drawn and losses in the form of heat output. Each node also has a function for calculating the outputs from the inputs. A first set of connections between the nodes represent electrical power drawn by one device in the data centre from another device in the data centre. A second set of connections between the nodes represent a thermal load applied by one device in the data centre to another device in the data centre. The simulator can be run for a series of different operating conditions to map data centre efficiency, for example, or to assess the impact of different IT devices on the data centre.
