When you’re responsible for keeping mission-critical systems running, power disruptions aren’t just inconvenient, they’re costly. Whether you oversee a data centre, healthcare facility or IT network, the right Uninterruptible Power Supply (UPS) is critical. This guide covers sizing and runtime, to key features, so you can select the UPS that best fits your operation’s needs.
Why a UPS is critical
A UPS provides emergency backup power during outages and protects connected equipment from common power problems such as:
- Voltage sags and surges
- Brownouts
- Frequency variations
- Harmonic distortion
- Complete blackouts
By maintaining continuous power and voltage stability, UPS systems help prevent data loss, equipment damage and costly downtime.
Three Main UPS Types Explained
Choosing the right UPS starts with understanding the different technologies available:
1. Standby (Offline) UPS
- Ideal for basic electronics and non-critical systems
- Switches to battery when an outage occurs (few milliseconds delay)
- Budget-friendly but limited protection
2. Line-Interactive UPS
- Suitable for small servers, networking gear
- Regulates voltage fluctuations without switching to battery
- Better efficiency and moderate protection
3. Online (Double-Conversion) UPS
- Provides seamless, zero-transfer-time protection
- Constantly converts and cleans incoming power
- Essential for sensitive equipment and critical infrastructure
See our range of Centiel Online (Double-Conversion) UPS systems.
Determining Your Power Requirements
Getting the size right is one of the most important steps in UPS selection. An undersized UPS will not support your equipment when it matters most. While an oversized system adds unnecessary cost and complexity.
Start With Your Total Load
- Wattage: Total power consumption of connected equipment.
- Volt-Amps (VA): Apparent power; includes both real and reactive power.
- Power Factor (PF): Ratio of watts to volt-amps (VA). Indicates the efficiency of power usage.
How To Calculate What Size UPS You Need
- Begin by listing every device the UPS needs to support: servers, networking switches, storage systems and any peripherals.
- Add the devices power ratings in watts (W).
- Convert to Volt-Amps (VA) using the formula: VA = Watts ÷ Power Factor. Most modern equipment has a power factor of around 0.9.
For example, a load of 2,000 watts with a power factor of 0.9 requires at least 2,222 VA of UPS capacity (2,000 ÷ 0.9).
Build in a Safety Margin
Consider adding a buffer of 20–30% to your calculated VA. This will ensure flexibility and avoid running your UPS at full capacity.
Best practice: Multiply your total VA by 1.2 to account for system growth, minor surges or fluctuations.
Watch for High Inrush Devices
Some equipment, like medical imaging machines and variable-speed drives draw significantly more power at startup. If your UPS needs to support this type of load:
- Choose a model with higher surge capacity
- Consult the manufacturer specifications for startup power requirements
Planning for Future Expansion
If your infrastructure is growing, it's wise to factor in more than just the current load. Consider:
- Newer servers typically draw more power
- Virtual environments can increase rack density and load
- Battery runtime may drop as more equipment is added
For organisations with rapid expansion plans, consider sizing at 1.3–1.5x your current VA load, or explore modular systems that allow you to add capacity over time.
The STULZ UPS sizing calculator makes it easy - just enter your equipment and get instant recommendations.
Runtime Considerations
The amount of time your UPS can supply power during an outage is just as critical as capacity.
Things to Consider:
- Desired Runtime: Do you need a few minutes for safe shutdown or extended runtime until generators kick in?
- Load Impact: Heavier loads reduce runtime significantly.
- Scalable: Choose systems like the STATUS Power that offer modular battery expansion.
Key Features to Consider
When comparing UPS models, do not just focus on size. Look for features that enhance performance and longevity.
- Pure Sine Wave Output: Ensures clean power delivery for sensitive equipment.
- Battery Type: Consider Lithium-ion for longer lifespan and faster recharge, or VRLA for cost-effectiveness.
- Battery Replacement Options: Hot-swappable batteries reduce downtime during maintenance.
- ECO Mode: Improves energy efficiency during normal operation.
- Form Factor: Choose between tower or rack-mount designs depending on your available space.
STULZ’s EssentialPower X1 RT offers flexible rack/tower installation to suit various environments.
Application-Specific Recommendations
Different industries have different UPS requirements. Here’s a quick guide:
- Data Centres: Require scalable, modular UPS systems with high efficiency and zero transfer time—CumulusPower is a popular choice.
- IT and Server Rooms: Mid-range line-interactive or online UPS systems offer sufficient protection and cost-efficiency.
- Healthcare and Labs: Must comply with strict standards; the PremiumTower Medical ensures clean, un-interrupted power.
- Industrial Settings: Ruggedised units with high ingress protection and load capacity are essential.
Buyer’s Checklist
Use this checklist to evaluate your UPS options:
- Have I calculated my total power needs (Watts/VA)?
- Do I need basic, line-interactive or online UPS protection?
- What UPS runtime do I require to safeguard my equipment?
- Does the UPS support future expansion needs?
- Have I considered energy efficiency features, such as ECO mode?
- Is the UPS model compatible with the physical constraints of my space?
- Is the system compliant with industry regulations?
STULZ combines proven expertise in critical power and cooling to deliver tailored UPS solutions. We help you maximise uptime and maintain peace of mind, backed by nationwide service support.
Explore the full STULZ UPS range or contact our team for expert advice on the right UPS solution.