The racks would rarely be nearing physical capacity, but might actually be maxed out in logical capacity due to power or cooling constraints at the rack level (the mantra to avoid creating hot spots in data centers has actually made floor and rack use a lot less efficient).
They are curbing run-away energy with a combination of micro-level controls (for individual servers, power distribution units, air-flow controllers, and cooling units) as well as macro-level controls and policies (for racks of servers, rows of racks, and entire data centers).
In addition, the inherent benefits of the platform, including the helium fill, allow HGST to significantly improve datacenter TCO on virtually every level: capacity, power, cooling and storage density.