UPS Size Calculator

Q: How do I calculate UPS VA size?

VA = Watts ÷ Power Factor × Safety Margin. For example, 800W of equipment at PF=0.8 with 20% margin: VA = 800 ÷ 0.8 × 1.2 = 1200 VA. Choose the next standard UPS size above this value.

Q: What power factor should I use for UPS sizing?

Modern UPS units often specify their output PF on the nameplate. Computer and IT equipment typically has a PF of 0.9–1.0. Older CRT monitors, printers, and some motors may be 0.7–0.8. Use 0.8 as a conservative estimate when unknown.

Q: How much runtime does a UPS give?

Standard small UPS units (600–1500VA) typically provide 5–20 minutes runtime at full load with internal batteries. Extended runtime UPS units connect to external battery packs. Use the battery runtime calculator to estimate runtime for your specific configuration.

Find the minimum UPS VA rating to protect your computers, servers, or other equipment. Enter your total load in watts and the UPS power factor to get the required VA rating.

Calculate UPS VA Rating

Total Load (Watts)Sum all equipment wattages on this UPS circuit

Load Power FactorUse 0.8–0.9 for typical IT equipment

Safety Margin (%)25% recommended — don't run a UPS at 100% capacity

UPS Sizing Result

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Minimum VA Rating

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Minimum kVA Rating

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Load at 100% (W)

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Recommended Standard Size

Formula

Required VA = (Total Watts ÷ Power Factor) × (1 + Margin ÷ 100)

Select the next standard UPS size at or above this value. Common UPS VA ratings: 600, 800, 1000, 1200, 1500, 2000, 3000, 5000, 6000, 10000 VA.

How to Read Equipment Wattage

Desktop PC: Check the PSU label — a 500W PSU doesn't draw 500W, it means max capacity. Under typical load, a desktop draws 80–200W.
Laptop: 30–90W depending on model and load. Check the adapter brick label.
Server (1U rack): 150–500W depending on CPU, drives, and RAM configuration. Use actual measured draw.
Monitor (LED): 20–50W. CRT monitors: 60–150W — much higher.
Networking (router, switch): 5–30W. Small UPS will power these for extended periods.
Network storage (NAS): 20–80W depending on drive count and activity.

UPS Types Explained

Standby (offline) UPS: Switches to battery on power failure (4–12ms switchover). Suitable for PCs and basic IT. Cheapest option.
Line-interactive UPS: Includes AVR (automatic voltage regulation) — corrects minor voltage fluctuations without switching to battery. Good for office IT and networking equipment.
Online (double-conversion) UPS: Always running from battery — zero switchover time, complete power conditioning. Required for servers, medical equipment, and sensitive industrial equipment. Most expensive.

Frequently Asked Questions

How do I calculate UPS VA size?

VA = (Total Watts ÷ PF) × margin. For 800W at PF=0.8 with 25% margin: VA = (800 ÷ 0.8) × 1.25 = 1,250 VA. The next standard size is 1,500 VA. A rule of thumb: VA ≈ Watts × 1.6 for typical IT loads at PF=0.8 with 25% margin.

What power factor should I use for UPS sizing?

Modern computers and servers: PF 0.95–1.0. Older equipment, printers, some UPS-rated appliances: PF 0.7–0.8. If you don't know, use 0.8 — it's conservative and widely recommended for mixed IT loads. Check your UPS nameplate; modern high-efficiency UPS units often state "PF = 0.9" or "0.8" as their rated load PF.

How much runtime does a UPS give?

Standard UPS units at full rated load typically give 5–10 minutes — enough to save work and shut down gracefully. At 50% load, runtime roughly doubles. Use the Battery Runtime Calculator with your UPS battery Ah rating to estimate accurately. For longer runtime, buy an external battery extension pack or a UPS with extended runtime option.

Can I connect a UPS to a generator?

Yes, but use an online (double-conversion) UPS or a generator-compatible line-interactive UPS. Standard offline UPS units may not tolerate the frequency and waveform variations of small generators — they'll continuously switch to battery. The UPS effectively cleans the generator output and protects your equipment from generator instability.