For an 8-camera NVR drawing about 100-150W and a small PoE switch at 200W, plan for a 1000-1500VA UPS to support 15-30 minutes of runtime. That covers most utility outages and gives ample time for graceful shutdown if power doesn't return. For sites with frequent or extended outages, scale to 2000VA+ with an extended battery pack.

Sealed lead-acid (SLA) UPS batteries typically last 3-5 years. Lithium-ion UPS batteries (newer, more expensive) last 8-10 years. Heat is the primary aging factor — UPSs in unconditioned closets fail batteries faster. Schedule replacement at year 3 for SLA and year 7 for lithium, regardless of self-test results; runtime degrades silently and slowly before complete failure.

UPSs cover seconds-to-minutes outages and bridge to generator startup. Generators cover hours-to-days outages but take 10-30 seconds to come online. For sites that need uninterrupted operation through extended outages, both are required: UPS bridges the generator startup gap and conditions generator power, generator covers the long tail. Standalone UPS without generator works for most commercial sites where occasional brief outages are acceptable.

For small sites (8-16 cameras, single NVR, one switch) — yes, a single UPS works. For larger sites, distribute UPS protection across multiple smaller units rather than one large UPS. Reasons: failure isolation (one UPS battery failure doesn't take down everything), shorter cable runs from UPS to load (avoids voltage drop), and runtime calculation simplicity per zone.

Line-interactive UPSs run on utility power normally and switch to battery on failure. They include voltage regulation (AVR) that absorbs sags and surges without battery use. Online (double-conversion) UPSs continuously regenerate AC from battery-rectified DC — perfect output regardless of input quality. Online UPSs cost 2-3x more and are appropriate for highly sensitive loads (servers, lab equipment) or sites with very poor utility power.