IT-06

UPS Sizing

IEC 62040-3 · ASHRAE thermal guidelines · EIA-310-E · ANSI/BICSI 007-2024

01 · Connected load

Load

Nothing added yet

Load power factorⓘ

Growth headroom

02 · UPS

Topology (IEC 62040-3)

Required runtimeⓘ

min

Inverter efficiency on battery

03 · Battery

Battery string voltage

Capacity per string

Number of strings

Usable depth of dischargeⓘ

End-of-life capacity derateⓘ

Awaiting input11 fields left: Load · Load power factor · Growth headroom · Topology (IEC 62040-3) · Required runtime · Inverter efficiency on battery · Battery string voltage · Capacity per string · Number of strings · Usable depth of discharge · End-of-life capacity derate

Capacity and layout

The drawing appears when the inputs are complete

01
Size a UPS from the connected load and power factor, work out battery capacity for a required runtime with depth-of-discharge and ageing derates, and classify the topology per IEC 62040-3.
02
Tip. Size the battery against usable capacity, not nameplate. Depth of discharge and end-of-life ageing together leave roughly two thirds of the rated amp-hours, so a string sized on its label will fall short of its runtime long before it is due for replacement.
03
VFI — Online double conversion
04
VI — Line interactive
05
VFD — Offline / standby
06
80 % — Typical usable depth of discharge
07
20 % — End-of-life capacity derate

UPS topology and battery sizing

Two independent decisions. Topology determines what the UPS protects against and how cleanly it transfers; battery capacity determines how long the load survives. IEC 62040-3 classifies the first with a three-letter code that says exactly what is and is not regulated.

Topology classification

The code describes the output’s dependence on the input. VFI is independent of both voltage and frequency; VFD is dependent on both and simply passes the mains through until it fails.

Use cases

VFI — online double conversion, zero transfer time
VI — line interactive, voltage regulated, 4–8 ms transfer
VFD — offline standby, 10–20 ms transfer
N, N+1, 2N and 2N+1 describe unit redundancy, not topology

Formula

VA = watts ÷ power factor

IEC 62040-3

Runtime and usable capacity

The inverter draws more from the battery than it delivers to the load, and only part of the nameplate capacity is usable before the cut-off voltage — less again as the battery ages.

Use cases

Depth of discharge typically 80 % for VRLA
End-of-life derate of about 20 %
Together these leave roughly two thirds of nameplate
Inverter losses are rejected into the room as heat

Formula

Usable Wh = V × Ah × strings × DoD × age factor

Size on usable capacity, never nameplate

Battery chemistry and topology class are independent. Lithium is a chemistry — it changes energy density, cycle life, charging behaviour and cost, but it does not make a UPS VFI or VI. Specify the topology and the chemistry separately.