The building has an unusual, bulbous shape, purportedly intended to reduce its surface area and thus improve energy efficiency, although the excess energy consumption caused by the exclusive use of glass (in a double facade) overwhelms the benefit of shape. Despite claiming the building "demonstrates the potential for a sustainable, virtually non-polluting public building", energy use measurements have shown this building to be fairly inefficient in terms of energy use (375 kWh/m2/yr), with a 2012 Display Energy Performance Certificate rating of "E". It has been compared variously to a helmet (either Darth Vader's or simply a motorcyclist's), a misshapen egg, and a woodlouse. Former mayor Ken Livingstone referred to it as a "glass testicle", while his successor, Boris Johnson, made the same comparison using a different word, "The Glass Gonad" and more politely as "The Onion".