Power · Hardware
UPS
Also known as: Uninterruptible Power Supply
Provides battery backup that keeps equipment running through a brief power outage and allows for a controlled shutdown if power isn't restored.
A UPS sits between the utility power source and the equipment it protects, storing energy in a battery bank. When utility power fails or fluctuates outside acceptable range, the UPS switches to battery — ideally fast enough that connected equipment never even sees a power interruption.
UPS types
Standby (offline) — the battery charger is the only active component during normal operation; the inverter kicks in on power loss. Transfer time is typically 2–10ms. Least expensive. Adequate for basic equipment protection but not the right choice for servers with strict transfer time requirements.
Line-interactive — includes an automatic voltage regulator (AVR) that corrects voltage sags and surges without switching to battery. Transfer time 2–4ms. The right choice for most server room and network closet applications.
Online double-conversion — all power runs through the inverter continuously, so there is zero transfer time on power loss — the equipment is already running off the inverted battery output. Most expensive, slightly less efficient, but appropriate for the highest-availability environments and sensitive equipment.
Sizing
UPS capacity is measured in VA (volt-amperes) and watts. The difference matters: a 1500VA UPS rated at 1200W can support loads up to 1200W continuously. Don't exceed 80% of rated capacity for continuous loads.
Runtime on battery depends on load and battery capacity. A 1500VA UPS at full load might run 5 minutes. At 50% load it might run 15–20 minutes. The goal is usually to bridge brief outages and provide enough time for a clean shutdown, not to run indefinitely.
Management
Enterprise UPS units include network management cards (NMC) or USB/serial interfaces that allow monitoring battery status, load percentage, and estimated runtime, and can trigger graceful shutdowns of connected servers before the battery is exhausted. This is essential in any unattended server room.
Placement in the power chain
In a typical data room: utility power → transfer switch → UPS → PDU → equipment. The UPS handles brief outages; a generator provides extended runtime beyond battery capacity.