How do I size a UPS for a server room?

Add up the total kW of all IT equipment (servers, switches, storage), divide by the average power factor (typically 0.9 for modern PSUs) to get kVA, then apply a 20–25% margin. A 10kW server load at PF 0.9 = 11.1 kVA, with 25% margin = 13.9 kVA — select a 15 kVA UPS.

Why is the generator larger than the UPS?

Generators must handle motor starting inrush current, which is typically 6× the running current for direct-on-line (DOL) starting. Even a single HVAC compressor can draw 6× its rated current for several seconds. The generator must be sized for the worst-case simultaneous starting scenario.

How long should UPS battery runtime be?

For UPS systems with generator backup, 10–15 minutes is typical — enough to bridge the gap while the generator starts and stabilises (usually 10–30 seconds). Without a generator, 30–60 minutes may be needed for orderly server shutdown. Extended runtime beyond 2 hours is rarely cost-effective with battery alone.

Should I use a single large UPS or modular?

For loads above 40 kVA, modular UPS systems offer advantages: N+1 redundancy, hot-swappable modules, and scalability. A 100 kVA modular UPS might use 5×25 kVA modules, providing 100 kVA with one module as redundancy. Single UPS systems are simpler and cheaper for smaller installations.

UPS & Generator Sizing Calculator

Name: UPS & Generator Sizing Calculator
Rating: 4.8 (150 reviews)
Author: SparkyHub

Calculate UPS rating, generator size, and battery capacity for your installation

Reference Info & Formulas

Sizing Approach

UPS: Sized on continuous running load plus safety margin.

Generator: Sized on starting (inrush) load — motors draw 6× FLC on DOL start.

Battery: Sized on runtime × power at battery DC voltage.

Typical Loads

Server rack: 3–6 kW, PF 0.9

Network switch: 0.5 kW, PF 0.95

HVAC unit: 5–15 kW, PF 0.85 (motor)

Lift motor: 10–30 kW, PF 0.8 (motor)

Lighting: 1–5 kW, PF 0.95

Recommended kit

As an Amazon Associate we earn from qualifying purchases.

Neoteck Clamp Meter 1000A True RMS AC/DC, 9999 Counts, VFD, LoZ, Measures Voltage, Current, Temperature, Capacitance, Resistance, Diode, Continuity, Frequency, Duty Cycle – Multimeter with NCV, TRMS

£42.494.4

6000 Counts Digital Clamp Meter True RMS Multimeter Clamp Ammeter Voltage Meter NCV Test Universal Meter Tester

£20.995.0

UNI-T UT210E Mini Digital Clamp Meter 100A AC DC Current Voltage True RMS Pliers Ammeter Resistance, Capacitance, Diode Tester Multimeter with Alligator Clip

£59.324.6

Three Phase Efergy Elite power monitor with portable display No Wi-Fi needed.

£129.954.9

Wallbox Power Meter 3-Phase Energy Monitor - Three Phase Energy Meter 100A - Electricity Usage Monitoring LCD - DIN Rail KWH

£168.994.2

Fluke 179 True-RMS Digital Multimeter

£360.004.6

Data: IEEE 485 / IEC 62040 sizing principles

For guidance only. The responsibility for any electrical installation lies with the qualified person carrying out the work. Always verify calculations independently and apply professional judgement.

UPS vs Generator — When You Need Both

UPS provides instantaneous power during mains failure. Generators provide long-duration backup but take 10–30 seconds to start.

The typical arrangement for critical power is: Mains → UPS (battery bridge) → Generator (long-term backup). The UPS covers the gap while the generator starts and its output stabilises. For non-critical loads, a generator alone may suffice with a brief interruption.

Motor Starting Multipliers

Starting current multipliers by motor start type

BS EN 60947 / BS 7671

Start Method	Starting Current	Starting Duration	Typical Use
DOL (Direct On Line)	6–8× FLC	2–5 seconds	Motors up to 11kW
Star-Delta	2–3× FLC	5–10 seconds	Motors 7.5–75kW
Soft Starter	2–4× FLC	5–30 seconds	Pumps, fans, compressors
VFD (Variable Frequency)	1–1.5× FLC	Continuous ramp	Variable speed loads

Generator must be sized for worst-case starting scenario. Stagger motor starts where possible.

Battery Sizing Considerations

Temperature Effects

Battery capacity reduces by approximately 1% per degree below 20°C. At 10°C, a 100 Ah battery effectively delivers only 90 Ah. Install batteries in a temperature-controlled environment or add 10–20% margin for uncontrolled spaces.

Battery Life

Lead-acid UPS batteries typically last 3–5 years; lithium-ion 8–12 years. Factor in replacement costs when evaluating total cost of ownership. Replace batteries before they lose more than 20% of rated capacity.

Frequently Asked Questions

SparkyHub is a free resource built by Taro Schenker — I build websites for trade businesses too.

Related Calculators

Max Demand

Calculate total maximum demand after diversity for the EIC

Three Phase Balance

Balance loads across three phases

Transformer & Busbar

Size transformers, calculate fault levels, and select busbars

kVA/kW

Convert between kVA and kW using power factor