Choose a double-conversion online UPS with 1500VA to 5000VA capacity for small to medium servers. Prioritize models with 10-30 minutes runtime at full load, hot-swappable batteries, and scalable runtime via external battery packs. Brands like APC Smart-UPD, Eaton 9PX, or CyberPower OL series offer network-grade surge protection, automatic voltage regulation, and remote management via SNMP or network cards.