UPS

A UPS is an electrical device that sits between the mains supply and critical equipment, supplying AC power instantly if the grid fails or voltage sags dangerously. It contains a battery (usually lead-acid, lithium, or supercapacitor based), a charging circuit, and an inverter that converts stored DC energy back to AC when needed. The entire switchover happens in milliseconds, fast enough that most equipment never notices the outage.

Three main topologies exist. Offline (or standby) UPS units monitor incoming voltage and switch to battery power when it drops below threshold; they are cheap but introduce a brief delay. Line-interactive units correct voltage sags and swells without switching to battery, extending runtime and protecting against brownouts. Online (or double-conversion) UPS continuously inverts battery power to supply the load while chargers keep the battery topped up; the incoming mains is isolated from the output, eliminating electrical noise and ensuring zero switchover time, at the cost of higher initial expense and continuous inverter losses.

Capacity and runtime ratings matter critically

Capacity is measured in volt-amperes (VA) or watts; a 1000 VA unit at 0.8 power factor delivers roughly 800 watts. Runtime depends on battery size and load; a typical office-grade UPS with a few hundred watts of load might sustain power for 5 to 15 minutes, enough to save work and shut down gracefully. Data centre units use external battery cabinets to achieve hours of runtime. Oversizing a UPS is common because loads often exceed initial estimates and because battery voltage sag under peak current draw can trigger premature switchover.

Common failure modes include battery degradation (typically after 3 to 5 years of regular use), faulty inverter circuits from power surges or thermal stress, and clogged cooling vents that allow internal temperatures to climb. Self-tests, triggered manually or automatically, discharge and recharge the battery to verify readiness; a failed self-test means the unit will not protect the load during a real outage. Replacement filters and batteries are consumables; tracking their age prevents surprises.

UPS units appear in hospitals, server rooms, industrial control systems, and increasingly in home office setups. Regulations in some jurisdictions require UPS protection for life-safety systems and emergency lighting circuits. The term itself is established across the industry, though some vendors use 'uninterruptible power source' instead of 'supply'.