How To Estimate Energy Use

72chevy4x4

I've been asked to calculate energy use for a site which has one larger boiler and 8 split systems all using natural gas. Most of the thermostats are programmable and have a feature to tell how long the system has been on (i.e. 4hr35m for current day). The desire is to know how much to charge an organization to use parts of the building-so breaking down the rooms into hvac systems would seem logical.

any suggestions guys?