When to use: Preliminary sizing for a data center, server room, or critical-load UPS system. Required UPS kVA rating is the connected kW (with an aging/derating margin) divided by the UPS output power factor. Battery runtime capacity is estimated separately using a usable depth-of-discharge and round-trip efficiency assumption for the selected battery technology — VRLA (lead-acid) vs Li-ion.
This calculator estimates the UPS kVA rating and battery energy capacity needed to protect a critical load — data center, server room, or control room — through a power interruption. It compares VRLA (lead-acid) and Li-ion battery technology at a high level, since usable depth-of-discharge and runtime characteristics differ meaningfully between the two.
Total connected critical load kW is summed across all backed-up equipment. A blended load power factor (typically 0.9 for IT and critical loads) converts this to the apparent power actually drawn. An aging/derating margin (commonly 20–25%) is applied to the connected kW to account for battery aging, future load growth, and non-simultaneous peaks. The required UPS kVA rating is then this margined kW divided by the UPS module's own rated output power factor (most modern double-conversion UPS units are rated at 0.9 output PF — e.g. a "100 kVA / 90 kW" nameplate). The calculator then rounds up to the next standard commercial UPS frame size.
VRLA (valve-regulated lead-acid) batteries have a lower upfront cost and are well understood, but typically limit usable depth-of-discharge to about 80% to preserve cycle life and have a service life of roughly 3–5 years. Li-ion batteries offer higher energy density (smaller footprint and less weight for the same runtime), a longer cycle life (often 10+ years), better high-temperature tolerance, and a higher usable depth-of-discharge (often 90–95%), at a higher upfront cost. These are commonly published comparative characteristics — exact runtime-per-Ah and discharge curves are proprietary to each battery cabinet and must come from the manufacturer's runtime tables.
List each critical load by name and connected kW. Set the blended load power factor and the UPS output power factor (both default to 0.9 for typical IT/critical-load systems). Enter the desired runtime at full load in minutes and the derating/aging margin. Select VRLA or Li-ion to see the corresponding usable depth-of-discharge assumption used in the rough battery energy estimate. The recommended UPS kVA is the next standard frame size at or above the calculated requirement — treat the battery energy figure as a starting point only, not a final battery string design.
A UPS module is nameplate-rated for both a kVA and a kW capacity at its own designed output PF (commonly 0.9 for modern double-conversion units). The kVA rating you must purchase is the required kW divided by that unit's rated output PF — if your actual load PF is lower than the UPS rated PF, the kW capacity of the UPS (not just kVA) can become the limiting factor, which is why both figures are shown.
A 20–25% margin over the calculated connected load is standard practice for UPS sizing. It accounts for battery capacity fade over its service life, load growth, and imprecise load estimates at design time — without it, a UPS sized exactly to day-one load may be undersized well before end of battery life.
Not necessarily. Li-ion offers a smaller footprint, longer service life, and higher usable DoD, but at a materially higher upfront capital cost. VRLA remains common for shorter runtimes and budget-constrained projects. The choice depends on total cost of ownership, available floor space, and required service life — not runtime performance alone.
No. This tool gives a preliminary, order-of-magnitude estimate for early design and budgeting. Final battery string configuration (cell count, strings in parallel, cabinet model) must be selected from the specific UPS manufacturer's published battery runtime tables, which account for the actual discharge curve, temperature, and cabinet losses that cannot be generalized here.
Try our Electrical Studio
More calculators, simulators, and guides for this discipline.