What Size UPS Do I Need for a Small Data Center?
Even a 30-second power dip can crash servers, corrupt databases, and send customers running. Uptime Institute reports that 39% of outages now cost more than $100,000. For small data centers and edge rooms, the uninterruptible power supply (UPS) is your first and last line of defense, but sizing it correctly can feel like guesswork.
Buy too little capacity and you’ll face an expensive midnight call-out. Oversize the system and you’ll waste the budget on batteries you never use. After designing, installing, and maintaining critical power systems for clients like Disney, Nike, and City of Hope Hospital, Camali Corp has refined a simple, repeatable sizing process that hits the Goldilocks zone every time.
Use this step-by-step guide to calculate the perfect UPS size for your small data center — including runtime, redundancy, and growth headroom — and learn how to avoid the hidden traps that sink IT budgets.
Quick Glossary (skip if you’re a power pro)
Watts (W) – Real power your equipment actually uses
Volt-Amperes (VA) – Apparent power; UPS units are rated in VA or kVA
Power Factor (PF) – Ratio of watts to VA (e.g., 0.9)
Runtime – How long batteries must support the load (5, 15, 30 minutes)
Redundancy (N, N+1, 2N) – Extra capacity to survive failure or maintenance
Headroom – Extra capacity for growth (+25%) and battery aging (+10%)
Why UPS Size Matters More Than You Think
The hidden cost of undersizing
A single minute of downtime now averages $9,000 in lost revenue across industries. Undersized UPS systems crash first because batteries discharge too quickly. Restarting servers can take longer than the outage itself, multiplying losses.
The waste of oversizing
Conversely, buying a UPS that’s way larger than your load means paying for unnecessary battery strings, floor space, and cooling. Over five years those idle watts turn into thousands on your utility bill, money that could fund an N+1 redundant module instead.
“Under-sizing is the #1 reason we’re called at 2 a.m. A 20% headroom buffer costs little up front and saves six figures in lost sales.” — Hadi Fakouri, CEO, Camali Corp
How to Calculate UPS Size for a Small Data Center
Getting the right UPS size means fewer outages, longer equipment life, and better ROI. Here’s the fastest way to do it:
- Add Your Total Watts
- List every device the UPS will protect and total their watt ratings.
- Convert to VA
- Divide watts by 0.74 (typical power factor). Example: 1,350 W ÷ 0.74 ≈ 1,816 VA.
- Add a Safety Margin
- Multiply VA by 1.35 for future growth. Example: 1,816 VA × 1.35 ≈ 2,450 VA → choose a 3 kVA UPS.
- Choose Runtime
- 5–10 min: Safe shutdown only
- 20–30 min: Ride out short outages
- 60+ min: Wait for generator
- Check Form & Features
- Select rack or tower style, confirm voltage, outlet type, and consider extras like hot-swappable batteries or N+1 redundancy.
Step 1 – List and Total Your Critical Load
Pull the nameplate data (or use a power meter) to capture the maximum watts for every device that must stay online long enough to shut down gracefully or until the generator starts.
| Rack | Device | Amps | Volts | Watts (A × V) | PF | VA (W ÷ PF) |
| 1 | 2× 1U servers | 3 A | 208 V | 624 W | 0.9 | 693 VA |
| 2 | Core switch | 1 A | 208 V | 208 W | 0.95 | 219 VA |
| 3 | Storage array | 4 A | 208 V | 832 W | 0.92 | 904 VA |
| Totals | 8 A | — | 1,664 W | — | 1,816 VA |
Quick formula: Watts ÷ PF = VA
Step 2 – Decide on Runtime and Redundancy
Runtime cheat sheet
- < 5 minutes – IT shuts down immediately (rare for production)
- 15 minutes – Ride through brief utility blips or transfer to generator
- 30 minutes – Gives facilities time to start diesel gensets and stabilize load
Growth buffer: the +35% rule
Small data centers rarely stay small. We recommend +25% for projected load growth and +10% for battery aging = 35% total headroom.
Pick a redundancy tier
- N – Just enough capacity (risky)
- N+1 – One extra module or unit online for fail-over (best practice ≤ 50 kW)
- 2N – Dual independent UPS paths (reserved for Tier III/IV facilities)
For our sample load (1,816 VA), choose:
- Runtime: 15 minutes
- Redundancy: N+1
- Headroom: 35%
Adjusted VA = 1,816 VA × 1.35 ≈ 2,450 VA.
With N+1, plan two modules ⇒ 2 × 2.5 kVA.
Step 3 – Run the Numbers (real-world example)
Disney edge analytics lab
When Camali upgraded Disney’s 12-rack edge data center, the measured IT load was 11 kW (13.5 kVA). Disney required a 15-minute ride-through and N+1 redundancy.
Sizing math
13.5 kVA × 1.3 (headroom) = 17.6 kVA ⇒ Select a 20 kVA modular frame with three 10 kVA hot-swappable modules (two active, one spare). Battery cabinet rated for 15 minutes at 20 kW. Six months later Disney added a fourth module without downtime — proof that proper headroom pays off.
Selecting the Right UPS Topology and Form Factor
Line-interactive vs online double-conversion
Line-interactive UPSs are budget-friendly but only correct minor voltage sags. Once you’re protecting virtualization hosts or storage arrays, step up to online double-conversion for isolated, clean power.
Modular vs monolithic
| Modular (5–50 kVA) | Monolithic (> 60 kVA) | |
| Scalability | Add 5 kVA blocks as you grow | Fixed frame size |
| Maintenance | Swap a failed module in minutes | Entire UPS offline for repair |
| Efficiency | Up to 96% | 92–94% |
Battery chemistry: VRLA or Lithium-ion?
| Chemistry | CapEx | Life | Operating Temp | Recharge | Footprint |
| VRLA | $$ | 3-5 yrs | 20-25 °C | 8 hrs | Large |
| Lithium-ion | $$$ | 8-10 yrs | 0-40 °C | < 2 hrs | 30% smaller |
If space is tight or total cost of ownership is king, lithium-ion’s 10-year life often beats two VRLA refresh cycles.
UPS Sizing Checklist
✔️ List every must-run device, record watts & PF
✔️ Convert to VA and sum
✔️ Add 25% growth + 10% aging
✔️ Pick runtime (5, 15, 30 minutes)
✔️ Choose redundancy (N, N+1)
✔️ Select topology (online double-conversion for IT)
✔️ Decide battery chemistry (VRLA vs Li-ion)
✔️ Plan installation & preventive maintenance schedule
✔️ Document everything for audits and insurance
Need help? Our electrical and IT teams work as one to design, install, and maintain your critical power.
Frequently Asked Questions
What is a UPS?
A UPS (Uninterruptible Power Supply) is a battery-powered bridge that keeps servers alive when utility power dips or fails, buying you time for generators to start or for a graceful shutdown.
How do I calculate UPS size in VA?
Multiply each device’s watts by 1 ÷ power factor, add them together, then add 30% headroom. The final number in VA (or divide by 1,000 for kVA) is the minimum capacity your UPS must deliver.
Do I need N+1 redundancy in a small data center?
Yes. An N+1 UPS means one extra power module is always on standby, allowing maintenance or failure without taking your data center offline.
Next Steps: Protect Your Business Before the Next Outage Hits
Choosing the right UPS is only the first move. Correct installation, acceptance testing, and quarterly preventive maintenance keep that investment performing for a decade or more. Camali Corp handles the entire lifecycle:
- Design & Engineering – Load studies, breaker coordination, single-line diagrams
- Installation & Commissioning – Turnkey electrical, IT cabling, and battery build-outs
- 24/7 Maintenance – IR scans, load bank testing, and replacement scheduling
Ready to safeguard uptime and sleep easier? Talk to our power specialists today.
