One of the difficulties of working with office buildings is that there is no such thing as a "typical" level of energy use. Actual energy use varies over an enormous range with no obvious explanation. The reality is that some buildings are built and operate very efficiently, while others are built and operate inefficiently. Between these two extremes, energy use can vary across a factor of four or more.

Why is this gap so large? The reality is that much of the spread is caused by poor efficiency. This wide variation in efficiency means that, while it is possible to think of a "typical" office building, there are many apparently normal buildings that will behave differently from the average.

If we look at typical energy uses within an average building that performs to around 10,000 MJ/person per annum for tenant light and power and 500 MJ/m2 for central services. This estimation is working on the assumption that each person will be working with one computer plus a share of other equipment. All up this will probably amount to about 200 W total office equipment. In addition, each person will have around 20m2 of office, including a share of corridors and meeting rooms, lit at an average of around 18 W/m2. All up that's about 360 W of lighting. Thus if all this lighting and equipment is on, it would amount to 560 W of load. This full load would have to run for approximately 5000 hours a year to use 10,000 MJ. That's around two to three times as long as you are ever likely to be at work. However, this target is based on aggregate tenant light and power for whole departments, including computer centres and the like. Thus if you don't have any outstanding or unusual items of this nature, then you should use far less than this figure.

The pie-chart below illustrates a typical energy end-use breakdown for such a building.

Two key observations can be made from the breakdown:

• Lighting, air-conditioning and office equipment dominate, accounting for approximately 85% of total energy use
• Tenant light and power and common area services (common area lighting, air-conditioning, hot water and lifts) are each responsible for around 50% of total energy use. That is to say, your energy bill and that of your landlord, if you have one, are probably roughly equal on a per unit area basis.

It is clear from this that if major savings are to be made, these will come from the lighting, air-conditioning and office equipment areas.