The software doesn't read the power drawn from servers directly. Instead, Viridity tested thousands of server configurations to calculate the amount of power they draw at different levels of utilization, and compiled the information in a database that gets installed on-site with the product.

EnergyCenter looks at the utilization rates, calculates the power draw using the database, and compiles the information into various graphics and reports that show energy use over time. Graphical charts make it relatively easy to identify inefficient machines that are candidates for replacement or consolidation.

Customers can also build a 3D image of their data center that includes server racks and PDUs, with their respective load capacities. They can then identify areas that have surplus power and experiment with different physical layouts to see where new servers could go.

Viridity isn't 100 percent accurate because it doesn't measure the power draw from the equipment itself, only by comparing utilization rates against its database. But it says the system is about 90 percent accurate and good enough to provide meaningful, actionable data about power usage. It will update the database with power use data for different equipment and configurations over time.

Measuring power draw precisely is something other vendors have struggled with. Rival Sentilla at one time used small devices that attached to each piece of equipment and reported power usage via a wireless mesh network. It decided that system was too cumbersome to implement at large data centers, and last year it switched to a software-only system. The reads the power draw from metered equipment and then calculates the remaining values with a triangulation system that fills in the blanks like a game of Sudoku. It claims this system is 98 percent accurate.