As technology has developed, server hardware has naturally become more efficient at using power, and tasks that once took entire server rooms may now be accomplished by one box. Nevertheless, the increasing demand for scarce resources should make saving energy a priority for anyone with one or more server.
One way to reduce energy usage and cost is to not use more than you actually need. Scalability is something that you should definitely plan for, but you should also be aware of current limitations and short term expense. A server that is exclusively used for websites may not have the same power demands as an application server in a large business. Therefore, you may be able to use a smaller machine with a lower-powered processor.
Similarly, a web server does not need a high-powered graphics accelerator and other superfluous peripherals. Removing any hardware or even software you do not use will help to reduce energy costs.
Another possible way to reduce energy costs is to use virtualization. If you need to run multiple operating systems, purchasing multiple servers is not your only option. By running more than one OS in virtual machines on a single server, you can greatly reduce hardware and energy costs.
Completely shutting down your server when not in use is obviously not an option, but you can configure it to reduce power usage during low-traffic periods. Some new processors and other hardware may even have these features by default.
Energy conservation is very important for both environmental and economical reasons. With a few modifications and careful considerations, your server can be energy efficient and more affordable.