A UPS spec sheet headlines kVA. Reality is kW × power factor. Battery autonomy is calculated at end-of-life capacity (80% nominal), not new. And the topology you pick decides whether your data centre survives a 50 ms grid sag or only a full blackout.
Four ideas to carry into the next specification conversation you have.
UPS rated 100 kVA at 0.9 PF = 90 kW real. A 100 kVA UPS feeding a 0.8 PF load delivers only 80 kW. Catch the spec sheet trap.
Continuously regenerates output sine wave. < 4 ms transfer time effectively zero. Survives any input event. Most expensive, most protection.
Cheaper, lower efficiency, ~ 4-10 ms transfer time. Acceptable for IT in offices but not for medical or industrial.
VRLA batteries at year 5 deliver 80% of nominal Ah. A 15 min design becomes 12 min in service. Size for end-of-life or replace earlier.
Per IEC 62040-3 classification.
| Topology | Transfer time / class | Best for |
|---|---|---|
| Standby (offline) | 8-12 ms · Class 3 | Home PCs, small office |
| Line-interactive | 4-10 ms · Class 3 | Mid-office IT, retail POS |
| Online double-conversion | 0 ms (continuous) · Class 1 | Data centres, hospitals, industry |
| Delta conversion (online) | 0 ms · Class 1 | Premium high-efficiency online |
| Modular / scalable | 0 ms · Class 1 | Growing data centres, hot-swap |
| Ferroresonant | 0 ms · Class 1 | Heavy industrial, harmonic-rich |
Five steps that turn a kVA number into a working system.
Sum every device's real power consumption. Servers, routers, AC for the server room, emergency lights, security panels. Apply diversity factor (typically 0.8-0.9) for IT loads where not every device runs at peak.
Hospital ICU = online, 0 ms transfer. Office IT = line-interactive, < 10 ms acceptable. Data centre = online, often with delta conversion for 97% efficiency to cut Saudi cooling costs.
If your loads draw at PF = 0.95 (modern IT with active PFC), kVA ≈ kW / 0.95. UPS catalogue rates at output PF 0.9-1.0 depending on model, read the fine print.
Target autonomy at year 5 capacity (80% of new). 15-minute design new = 12 min EOL. Or oversize batteries 25% so 12-min EOL still meets the 15-min target.
Two 100 kVA UPS in parallel = redundancy. If one fails, the other carries the load alone. Common in data centres > 50 kVA. Adds capital cost but eliminates single-point-of-failure.
Our DC designs default to online double-conversion with parallel N+1, IEC 62040-3 Class 1 transfer, and LiFePO4 battery options for 40 °C ambient.
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