For an equivalent service, the electricity consumption of servers can be reduced by choosing low-energy equipment.
When deciding which server to buy, choose one with an 80Plus-certified power supply (Platinum or Titanium) and with an Energy Star certification.
You can also choose an appropriate hardware architecture based on the web application in question. For example, physicalization is better suited for a high-traffic website. This involves using n very low-energy CPU instead of running n virtual machines on a physical server whose processor (e.g. Xeon) is often energy-intensive.