Which UPS topologies are suitable for extended runtimes?

On-line UPS and some line interactive UPS power supplies have inverters designed for longer duration or continuous running on mains power failure. Line interactive UPS may be limited to 1-2hours and some smaller on-online UPS to 4 hours. Most larger online UPS from 1.5kVA upwards can be installed with battery extension packs up to 7 hours or more in duration.