Nothing will ruin your day as quickly as a wide-scale power outage. While power systems in datacenters typically have redundancy in the form of battery backups and diesel generators, there are still single points of failure. Such as a generator not starting up properly, an automatic transfer switch dying, or a backhoe cutting power lines.
What we have done is utilize A+B power for the best redundancy possible. This is sometimes called 2N redundancy. With A+B power you have 2 power feeds fed by 2 separate UPSs (battery backup), 2 separate generators, and 2 separate utility feeds. We use the A+B feeds for all equipment — network switches, compute nodes, and storage nodes.
Below is a diagram that illustrates the A+B power system for our cloud hosting service.
A+B Power Diagram
As you can see, the extra redundancy provided helps ensure the highest possible uptime.