: Practical steps you could take (to make a tiny difference): Use a web hosting company who offsets the carbon emissions caused by the energy it uses. (Search for 'green web hosting'.) Reduce

Practical steps you could take (to make a tiny difference):

Use a web hosting company who offsets the carbon emissions caused by the energy it uses. (Search for 'green web hosting'.)
Reduce your site's loading time (because there will be a small correlation between speed, server load, and energy used.)
Use shared or VPS servers wherever possible because they offer more efficient use of energy and rack space than dedicated servers.
Reduce dependence on video and other 'heavy' media that requires encoding, storing, streaming etc. and focus on lightweight text-led design.

For novelty value alone you could...

Create a website with opening hours. The server could power down during the closing hours. Set the opening hours as slim as you dare: consider a site that's only available one hour a day, or one that only opens on Mondays.
Create a website as an installation/art piece that's only available if the visitor is pedalling on an exercise bike. When they stop pedalling, the touchscreen/keyboard ceases to function.

Thinking bigger

The best way to save energy with web design is probably not to fiddle with server architecture or website structure (both of which will likely have small effects, even for moderately busy sites), but instead build something to:

Make heaps of money so that you can spend a considerable amount on energy-related research, development, and investment that has far-reaching potential. (e.g. Google's 0m green fund.)
Design an energy-related campaign to promote sociopolitical change (e.g. Lighter Later).