Web hosts are starting to claim that their web servers are “optimized for energy efficiency”. Although this may be true, this is a hard claim to dispute because standards have not yet been set for efficient operation of web servers. However, if you know something about servers, these things could help you determine if a web host has truly optimized their servers to run on the least amount of energy.
The type of server matters. Blade servers tend to be more efficient because often components that consume the most energy can be shared such as fans and motherboards/processors.
The age matters. Servers are like any other technology, they become outdated as soon as they ship from the factory. Unfortunately, web hosts make money after servers have been paid for, so there is built-in economic incentives to keep servers well past their prime.
Hard drive and storage types matter. Solid state hard drives (not often found in web hosting applications) consume much less energy because there is no spinning disk.
Power supply matters. How the host configures their power supply can increase or decrease energy consumption dramatically.
Cooling systems matter. As server farms or data centers become more advanced, more efficient methods of cooling the massive heat generated by servers advances as well. From liquid nitrogen to water cooled, the method of cooling matters. Choose hosts located in colder climates. This takes less energy from the grid to cool controlled atmospheres.
Ask your host about these things, and make them part of your web host evaluation.