Two years ago I first had a chat with folks at Joulex on their energy efficiency solution. Their solution is an agentless approach that inventories the IP addresses of all devices on the network to identify the power uses. There are very few devices running in a data center that don't have an IP address.

And, Cisco has announced intent to acquire Joulex.

JouleX’s energy management solution, together with Cisco EnergyWise™, will provide customers with a simple way to measure, monitor and manage energy usage for network and IT systems across the enterprise, without the use of device-side agents, hardware meters or network configurations. JouleX’s software helps to reduce energy costs by monitoring, analyzing and managing energy usage of all network-connected devices and systems through a set of policies derived through analytics tailored for an enterprise’s needs.

I haven't run into many who have used Joulex in a data center, but with Cisco's acquisition there could be a new push into the data center market.