Design principles Offices in particular have traditionally been extravagant users of energy because, in relation to all other costs, energy is a relatively minor fraction of the total annual budget. In many cases the major electricity cost is incurred by lighting. The 1980s sealed glass box may use energy at a rate of over 500 kWh/m2/year. Currently, best practice is in the region of 90 kWh/m2/year. The aim of the architect under the sustainability banner is to maximise comfort for the inhabitants whilst minimising, ultimately eliminating, reliance on fossil-based energy.