UPS & Generator Sizing Calculator
Calculate UPS rating, generator size, and battery capacity for your installation
Reference Info & Formulas
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.
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
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 Life
Frequently Asked Questions
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