UPS Sizing Guide
Choosing The Right UPS Size
Choosing the right UPS is essential to ensure your equipment remains protected and operational during power disturbances. For mission-critical environments, online double conversion UPS systems offer the highest level of protection by continuously converting incoming AC power to DC and back to AC, isolating your equipment from power fluctuations, surges, and outages. These systems are ideal for sensitive electronics, servers, and networking gear where even brief interruptions can cause data loss or downtime.
Proper sizing ensures your UPS can handle the connected load and provide sufficient runtime. Undersizing can lead to overloads and shutdowns, while oversizing may result in unnecessary costs and inefficiencies.
Step 1: Calculate Your Equipment Load
To determine the correct UPS size, follow these steps:
1. List All Equipment
Include all devices that require backup power, such as:
- Servers
- Network switches and routers
- Workstations
- Security systems
- Storage devices
2. Find Power Ratings
Check each device’s power rating, typically listed in: Watts (W) or Volt-Amps (VA)
If only Amps (A) are listed, convert using: VA = Volts × Amps
3. Add a Safety Margin
Add 20–30% to your total VA to allow for future expansion and to prevent overloading.
Example Calculation
Let’s say you’re backing up the following equipment:
Server: 800W
Network Switch: 150W
Router: 100W
NAS Storage: 400W
Workstation: 200W
Total Load = 800 + 150 + 100 + 400 + 200 = 1650W
Convert to VA (RackPower UPS power factor ≈ 1.0): 1650W = 1650 VA. Add 20% buffer: 1650 × 1.2 = 1980 VA
Recommended UPS Size for this VA load: At least 2000 VA (or 2kVA)
Step 2: Determine Runtime Requirements
Decide how long you need the UPS to keep your equipment running during an outage:
- 5–10 minutes: Enough time for safe shutdown
- 15–30 minutes: For short outages or generator startup
- 30+ minutes: For extended operation
Runtime depends on battery capacity and load, so this will influence your final UPS selection.
Step 3: Review the UPS runtime chart
Once you have calculated your total load and defined your required runtime, the next step is to review the RackPower UPS runtime chart to confirm the correct configuration.
The runtime chart shows how long each UPS model will operate at different load levels, and how runtime increases when extendible battery modules are added.
How to Use the Runtime Chart
- Match Your Load to a UPS Model: Select a UPS with sufficient capacity based on your calculated VA or watt load.
- Identify Your Load Percentage: Compare your load against the UPS capacity to determine whether you are operating at approximately 100%, 75%, or 50% load.
- Check Standard Runtime: Review the runtime for the base UPS model without additional battery modules. For example, a 1kVA unit (USS001BR) provides approximately 5 minutes at 100% load, 9 minutes at 75% load, and 15 minutes at 50% load.
- Assess Against Your Requirement: Compare the available runtime against your required backup duration. If it does not meet your requirement, additional battery modules will be required.
- Add Extendible Battery Modules as Required: Use the runtime chart to determine how runtime increases with each additional battery module and select the appropriate configuration.
UPS sizing is not just about capacity. It is about ensuring the system delivers the required runtime under real operating conditions.
Still unsure what UPS size you require? Complete the UPS sizing form below and a UPS specialist will be in contact to assist.
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