UPS Runtime Calculator

Uninterruptible power supplies originated in the 1940s as massive motor-generator flywheel systems for military radar and radio installations that could not tolerate a line-voltage dropout. Large 60 Hz motors spun flywheels continuously; a utility sag would let the flywheel\'s inertia carry the load through the disturbance while a backup generator spooled up. The concept was effective but physically enormous.

The transistor-based static UPS — mains-charged batteries feeding an inverter when line power failed — emerged in the late 1960s from companies like Topaz Electronics, Best Power, and Powerware (later Eaton). APC (American Power Conversion) commercialized the technology for personal computers in the mid-1980s, introducing the now-standard small SLA (sealed lead-acid) desktop UPS for home offices. The line-interactive topology patented by Trippe Lite in 1988 became dominant for small/mid-size units; larger installations still use online double-conversion for cleaner output.

Modern UPSes are transitioning from SLA to lithium iron phosphate (LiFePO4) for longer life, lighter weight, and higher DoD — extending runtime meaningfully for the same footprint. Data center-scale UPS systems are scaling into the megawatt range with modular, N+1-redundant designs that tie into building-wide battery banks delivering minutes to hours of runtime while diesel generators spool up.

Enter the UPS\'s battery voltage and amp-hours (from the model\'s spec sheet or the battery label inside), the connected load in watts, the inverter efficiency (default 0.9 is a reasonable line-interactive number), and usable depth-of-discharge. The tool returns backup runtime, usable Wh, and the effective C-rate you\'re pulling from the battery.

For conservative planning, derate the result by 20–30% to account for battery aging. Also measure your real load with a watt meter rather than trusting nameplate numbers — most equipment draws far less than rated under normal conditions. All math runs client-side.