Sizing a UPS (Uninterruptible Power Supply) involves determining the appropriate VA (Volt-Amps) or wattage capacity and runtime needed to protect your equipment during a power outage. Here's a step-by-step guide:
1. Identify Protected Equipment
List all the devices and equipment you want the UPS to support during a power outage. This is the foundation for accurate sizing.
2. Determine Power Requirements for Each Device
For each device identified, find its power requirements. This information is usually found on the device's label, power adapter, or in its user manual. Look for:
- Voltage (Volts - V): The operating voltage of the equipment.
- Current (Amps - A): The amount of current the equipment draws.
- Power (Watts - W): The actual power consumed by the device. This might be directly listed and is ideal.
3. Calculate Volt-Amps (VA) for Each Device (If Watts are Not Available)
If the device lists Amps (A) and Volts (V), calculate VA using the following formula:
VA = Volts (V) x Amps (A)
This gives you the apparent power the device requires.
4. Calculate Total VA
Sum the VA requirements of all the devices you want the UPS to support:
Total VA = VA (Device 1) + VA (Device 2) + ... + VA (Device N)
5. Account for Power Factor
Real-world equipment doesn't always use all of the apparent power (VA). Power Factor (PF) represents the ratio of real power (watts) to apparent power (VA). If you know the power factor of your equipment (often listed on the device or its documentation), you can refine your calculations. However, if you don't know the power factor, it's safer to assume a power factor of 0.7 or 0.8 and base your calculations on VA.
If you have Wattage data, you can convert it to VA with this formula (using an estimated power factor if the device doesn't provide one):
VA = Watts / Power Factor
A UPS should be sized to handle the total VA, even if your equipment's actual power consumption (Watts) is lower.
6. Add Safety Margin
It's crucial to add a safety margin to the total VA. This helps ensure the UPS isn't overloaded and provides headroom for future equipment additions. A common practice is to add 20-25% to the total VA:
UPS VA Rating = Total VA x 1.20 (or 1.25)
This is your minimum recommended UPS VA rating. Choosing a UPS with a higher VA rating provides even more headroom and potentially longer runtime.
7. Determine Required Runtime
Estimate how long you need the UPS to power your equipment during an outage. Factors to consider include:
- Graceful Shutdown: Enough time to safely shut down computers and servers.
- Critical Operations: Sufficient runtime to maintain essential functions until power is restored or a backup generator kicks in.
Runtime depends on the UPS battery capacity and the load it's supporting. UPS manufacturers provide runtime charts that show expected runtime at different load levels. Choose a UPS with a battery capacity that meets your desired runtime requirements at the calculated VA load.
8. Consider UPS Output Power Factor
UPS also have an output power factor that must be considered. This is the ratio of output power (watts) to the output VA. Modern UPS units often have output power factors in the range of 0.8 to 1.0. To properly size the UPS wattage (W) capacity, you should divide the calculated VA by the output power factor of the UPS:
UPS Wattage (W) Rating = UPS VA Rating x UPS Output Power Factor
Example
Let's say you want to protect:
- Computer: 300W (assume PF of 0.8)
- Monitor: 50W
- Router: 15W
- Convert Computer Watts to VA: VA = 300W / 0.8 = 375 VA
- Convert Monitor Watts to VA: VA = 50W / 0.8 = 62.5 VA
- Convert Router Watts to VA: VA = 15W / 0.8 = 18.75 VA
- Total VA: 375 VA + 62.5 VA + 18.75 VA = 456.25 VA
- Add Safety Margin (25%): 456.25 VA x 1.25 = 570.31 VA
Therefore, you'd need a UPS with a minimum VA rating of approximately 571 VA. If the UPS had a power factor of 0.9, the UPS wattage would be 571 VA * 0.9 = 514 Watts. Then choose a UPS model that meets or exceeds this VA and Wattage rating and provides the necessary runtime.
