100% server up-time has come to be expected by web hosts these days, and redundant internal power supplies are one of the technologies that make complete server up-time a reality.
A power supply is an internal component that converts AC utility power into DC power which computers and most other electronics require. A redundant internal power supply is a specialised version of a conventional power supply which essentials connects two compact power supplies into a single set of cables. If one of the power supply units fail the other will kick in without any loss of current being supplied to the server.
While there are other technologies that will provide continues website up-time if one server falls offline, this may not extend to all of your services. In our situations if one server shutdown there's still a backup server ready to take its place for all website content, however an unexpected shutdown can still have a big impact on some of our services. For example our servers are continuously processing images, audio, and video. Video encoding tasks run in parallel which means with shared resources across all video encoding tasks, it can take up to a week for a particular tasks to finish processing; if one of our servers unexpectedly shutdown this will cause all unfinished encoding jobs to be reprocessed that means up to a week of wasted processing. To recover from an unexpected shutdown and gain back that week of lost processing time we may have to rent additional servers to catch up.
New physical and software technologies that offer more redundancy are commonly used in addition to existing hardware. While many websites utilise caching services, or multiple servers to continuously deliver content even if one happens to unexpectedly shut down, these servers will still be equipped with redundant power supplies.
While Redundant internal power supplies are almost exclusively used in servers some can fit into the standardized ATX form factor associated with most desktop or workstations cases.