What is kVA and How Does It Differ from kW?
kW (kilowatts) is the real power that does actual work — runs the servers, lights the monitors, spins the drives. kVA (kilovolt-amperes) is apparent power — the product of voltage and current drawn from the supply. The ratio between them is the power factor (PF).
kW = kVA × Power Factor
A traditional UPS has a power factor of 0.8, which means a 10 kVA UPS can deliver only 8 kW of real power. Modern UPS units like the Eaton 9PX have an output power factor of 1.0, so a 10 kVA unit delivers 10 kW — 25% more usable power. This is why kVA ratings alone can be misleading. Always check the kW rating alongside kVA when comparing UPS units.
The 5-Step Method to Calculate Your UPS Size
Step 1 — List All Connected Equipment and Their Wattage
Write down every device the UPS will protect, along with its power consumption in watts. Where to find this:
- Check the nameplate on the back or bottom of each device (look for "Watts" or "VA")
- Check the manufacturer's datasheet (search model number + "power consumption specifications")
- Use a plug-in power meter (Killawatt or equivalent) to measure actual draw — usually 20–40% less than nameplate on modern equipment
Step 2 — Add Up Total Watts
Sum the wattage of all connected equipment. This is your connected load in watts. If some equipment starts up with a high inrush current (motors, printers, air compressors), use the startup (inrush) wattage for those devices — it is usually 2–3× the running wattage.
Step 3 — Apply 25–30% Headroom
Never run a UPS at 100% of its rated capacity. Best practice is to keep maximum load at 70–75% of UPS capacity. This headroom accounts for:
- Future equipment additions
- Inrush current spikes on startup
- Battery efficiency losses at high load
- Manufacturer's recommended operating range (most UPS units operate most efficiently at 50–80% load)
Multiply your total watts by 1.25 to 1.30.
Step 4 — Convert Watts to kVA
Divide the headroom-adjusted wattage by your UPS's output power factor × 1000:
kVA required = Watts after headroom ÷ (Power Factor × 1000)
For 0.8 PF UPS: kVA = watts ÷ 800 | For 1.0 PF UPS: kVA = watts ÷ 1000
Step 5 — Select the Next Standard UPS Size Up
UPS units come in standard sizes: 1, 2, 3, 5, 6, 7.5, 10, 15, 20, 30, 40, 60, 80, 100 kVA. Choose the next size up from your calculated kVA. Never go down to the nearest size — always round up.
Worked Example: Office of 20 Computers
| Equipment | Qty | Watts Each | Total Watts |
|---|---|---|---|
| Desktop computer (modern) | 20 | 150W | 3,000W |
| LCD monitor (24") | 20 | 25W | 500W |
| Network switch (24 port) | 2 | 100W | 200W |
| Internet router / firewall | 1 | 60W | 60W |
| NAS / file server | 1 | 120W | 120W |
| Total connected load | 3,880W | ||
| After 25% headroom (×1.25) | 4,850W | ||
| kVA at 0.8 PF (÷800) | 6.06 kVA → Choose 7.5kVA | ||
| kVA at 1.0 PF (÷1000) | 4.85 kVA → Choose 5kVA | ||
For this office: a 7.5kVA or 10kVA UPS at 0.8 power factor, or a 5kVA Eaton 9PX (1.0 PF) would be correctly sized. The Eaton 9PX option saves money on a smaller UPS while delivering the same protection.
Quick Reference by Business Type
| Business / Application | Typical Load | Recommended UPS Size |
|---|---|---|
| Small office (5 PCs, 1 printer) | 800–1,200W | 2 kVA |
| Medium office (20 PCs, networking) | 3,500–5,000W | 7.5–10 kVA |
| Large office (50 PCs + servers) | 10,000–15,000W | 20 kVA |
| Small server room (5–8 servers) | 4,000–8,000W | 10–15 kVA |
| Medium data centre (20–40 servers) | 20,000–60,000W | 40–80 kVA |
| Clinic / diagnostic centre | 3,000–8,000W | 5–15 kVA |
| Retail store / POS system | 1,000–2,500W | 2–5 kVA |
| CCTV / security system (16–32 cameras) | 400–800W | 1–2 kVA |
| Telecom / BTS site | 2,000–6,000W | 5–10 kVA |
Not sure about your load? Our engineers can perform a free load audit — measuring actual power consumption on-site — before recommending the right UPS. This is included at no charge when you request a formal quotation.
Common UPS Sizing Mistakes to Avoid
- Using nameplate VA instead of actual watts — Nameplate VA (volt-amperes) is worst-case. Modern equipment typically draws 30–50% less than nameplate. Oversizing based on nameplate wastes money.
- Forgetting headroom — A UPS loaded to 95% has no surge capacity and runs hot, reducing battery and component life.
- Ignoring motor start loads — If the UPS protects any equipment with motors (HVAC units, lifts, compressors), the startup inrush current (2–5× running current) must be accommodated.
- Not accounting for power factor of load vs UPS — If your servers have 0.9 PF loads but your UPS has 0.8 PF output, you will get a misleading kW calculation. Match load PF to UPS output PF specification.
- Ignoring future growth — A UPS sized exactly for today's load is undersized within 12–18 months as equipment is added.