Inputs
Formula Used
- Connected load (kW): IT + mechanical + other.
- Design load (kW): Connected × (1 + margin%) × (1 + growth%).
- UPS input power (kW): Design ÷ UPS efficiency.
- Required UPS capacity (kVA): UPS input kW ÷ power factor.
- 3φ current estimate (A): (kVA × 1000) ÷ (√3 × VL-L).
- Battery energy (kWh): (Design kW × runtime hours) ÷ battery efficiency.
- Battery capacity (Ah): (kWh × 1000 ÷ DC voltage) × (aging ÷ DOD).
How to Use This Calculator
- Enter IT, mechanical, and other connected loads in kW.
- Set power factor and efficiency based on your design assumptions.
- Add design margin and growth to match your planning horizon.
- Select redundancy (N or N+1) and choose modular sizing if needed.
- Enter runtime and battery assumptions for short ride-through sizing.
- Press Calculate UPS Size and review results above.
- Export CSV/PDF for submittals, reviews, or internal documentation.
Example Data Table
| Scenario | IT (kW) | Mech (kW) | Other (kW) | PF | Eff | Margin% | Growth% | Runtime (min) | Result (kVA) |
|---|---|---|---|---|---|---|---|---|---|
| Mid-size room | 120 | 30 | 10 | 0.95 | 0.96 | 15 | 20 | 10 | ≈ 252 kVA |
| High growth pod | 200 | 40 | 20 | 0.97 | 0.95 | 20 | 35 | 8 | ≈ 405 kVA |
| Edge site | 35 | 8 | 4 | 0.92 | 0.94 | 10 | 15 | 15 | ≈ 71 kVA |
Professional Notes for UPS Sizing
1) UPS sizing goals in data centers
A UPS in a data center is designed to keep critical IT loads energized during utility disturbances and generator transfer. Good sizing balances reliability, efficiency, and maintainability. This calculator estimates required kVA, installed kVA, input current, and battery capacity for a target ride-through time.
2) Load inventory and criticality
Start with a clear load schedule. Separate IT load (racks, network, storage) from mechanical and auxiliary loads that are truly on the UPS bus (controls, pumps, security). For example, 120 kW IT + 30 kW mechanical + 10 kW other equals 160 kW connected. Keep non-critical HVAC and general lighting off the UPS when possible.
3) Margins, growth, and redundancy
Design margin covers uncertainty, diversity, and harmonics, while growth covers planned expansion. With 15% margin and 20% growth, a 160 kW connected load becomes 220.8 kW design. Redundancy (N or N+1) affects installed capacity: modular systems typically add one extra module to maintain capacity after a failure.
4) Runtime and battery capacity
Runtime depends on generator start time, transfer scheme, and operational policy. Battery energy is approximated from design kW and runtime hours, adjusted by battery efficiency. Capacity in amp-hours is then adjusted for depth of discharge and aging; for instance, 80% DOD and a 1.25 aging factor increase required Ah to protect end-of-life performance.
5) Interpreting results for design
Use the kVA result to shortlist UPS frames or module counts, and use the current estimate to size upstream feeders and protective devices. Validate final selections with manufacturer tools, including harmonic limits, overload curves, temperature derating, and battery discharge tables for your specific chemistry and string layout.
FAQs
1) Why does power factor change the UPS kVA?
UPS capacity is rated in kVA. For the same kW, a lower power factor increases kVA demand, so the UPS must be larger to supply the same real power reliably.
2) Should I use efficiency in sizing?
Yes. Efficiency affects the input power needed to deliver the design load. Lower efficiency increases upstream kW and may influence feeder, generator, and heat-rejection planning.
3) What does N+1 mean in modular systems?
N is the number of modules required to meet the load. N+1 installs one additional module so the system can lose a module and still support the design load.
4) How accurate is the battery Ah estimate?
It is a planning estimate. Final battery sizing must use the vendor discharge curves at your runtime, temperature, end-of-life capacity, and string configuration for the selected chemistry.
5) Why include depth of discharge and aging?
Limiting DOD improves battery life, while aging accounts for capacity loss over time. Together they increase required amp-hours so the UPS still meets runtime at end of life.
6) How do I choose a module rating?
Use a vendor’s available module sizes (for example 25, 50, or 100 kVA). Pick a rating that supports incremental growth while keeping typical operating load in an efficient range.
7) Is the input current value suitable for protection sizing?
Treat it as a preliminary check. Actual current depends on topology, overload, harmonics, and bypass conditions. Always coordinate with manufacturer data and local electrical code requirements.