BUSINESS CONTINUITY · SIZING GUIDE
What Size UPS Does a Small Business Need?
A small-business UPS should be sized around the electrical load of the equipment it must protect, not simply the number of computers or the size of the business. The UPS needs enough watt and VA capacity for the connected equipment while also providing the amount of battery runtime the business actually requires.
Research basis: This guide uses general UPS and electrical sizing principles. Actual UPS capacity, runtime, supported load and operating characteristics vary by manufacturer and model. Verify the specifications and runtime information for the particular UPS before purchasing or deploying it.
The short answer
Add the power requirements of the equipment that must remain protected, then choose a UPS whose watt and VA ratings comfortably exceed that load.
Do not size a UPS to its maximum published load simply because the equipment technically fits within the rating. Practical headroom can accommodate changing loads, additional equipment and temporary increases in power consumption.
After determining that the UPS can electrically support the equipment, check its expected battery runtime at your actual load. A UPS with enough output capacity can still provide less runtime than the business requires.
UPS sizing starts with the protected load
Begin by deciding which equipment actually needs uninterrupted power.
- modems and internet gateways
- routers and firewalls
- network switches
- wireless access points
- desktop computers and monitors
- point-of-sale equipment
- NAS devices
- small servers
- communications equipment
Equipment that can safely remain off during an outage does not necessarily belong on the UPS. Reducing the protected load can increase runtime and reduce the UPS capacity required.
Watts and VA are not the same thing
UPS specifications commonly include both a watt (W) rating and a volt-ampere (VA) rating.
Watts describe real power used by the connected equipment. When adding devices to a UPS, the combined wattage must remain within the UPS's supported watt rating.
VA describes apparent power. It reflects voltage and current and is another limit the UPS must be able to support.
That means a UPS labeled 1500 VA should not automatically be treated as capable of supporting a 1500-watt load. The watt rating may be substantially lower than the VA number.
Important: Check both the watt rating and the VA rating of the UPS. The connected equipment must remain within both supported limits.
Why power factor matters
The relationship between watts and VA is influenced by power factor.
Watts = VA × power factor
For UPS selection, however, a small business generally does not need to calculate power factor for every device if reliable equipment wattage information and the UPS manufacturer's published watt and VA limits are available.
The practical rule is simpler: determine the expected connected load and make sure the selected UPS supports that load under both ratings.
Step 1: List the equipment that must stay online
Do not start by choosing a UPS model. Start with the business process you are trying to protect.
For example, maintaining internet connectivity during a short outage might require only the modem, firewall, router and network switch. Protecting a workstation could add a desktop computer and monitor. A small server environment could include a server, NAS and networking equipment.
Step 2: Determine the operating wattage
Find the normal power requirement for each device. Manufacturer specifications, equipment labels, power-management tools or an appropriate power meter can provide useful information.
Avoid assuming that the power supply's maximum rating is always the device's normal operating consumption.
Once you have reasonable operating estimates, add the loads together.
Example: sizing a UPS for network equipment
| Equipment | Example planning load |
|---|---|
| Internet modem or gateway | 20 W |
| Firewall/router | 25 W |
| Network switch | 60 W |
| Wireless equipment | 25 W |
| Total example load | 130 W |
In this simplified example, the UPS must support at least the 130-watt operating load. That does not mean selecting a UPS whose maximum rating is exactly 130 watts. Additional capacity provides useful operating headroom.
Example: workstation and network equipment
| Equipment | Example planning load |
|---|---|
| Desktop computer | 180 W |
| Monitor | 40 W |
| Network equipment | 130 W |
| Total example load | 350 W |
A UPS for this example must support the 350-watt load under normal operation and remain within its VA limit as well.
These numbers are examples for illustrating the sizing process. Actual equipment consumption can differ substantially, so use measurements or specifications for your own hardware.
Step 3: Allow practical headroom
Running a UPS continuously near its maximum supported load leaves little room for changes.
A business may later add another network device, replace a workstation with more power-hungry hardware or encounter temporary load increases.
There is no universal headroom percentage that is correct for every installation. The appropriate margin depends on the equipment, expected growth, runtime requirements and the UPS manufacturer's guidance.
Step 4: Decide how much runtime you need
UPS power capacity and UPS runtime answer different questions.
The watt and VA ratings tell you whether the UPS can support the connected load. Battery runtime tells you approximately how long that equipment can continue operating after utility power fails.
Do not assume that a larger VA number automatically means a particular runtime. Battery configuration and connected load also matter.
Use the manufacturer's runtime curve
UPS manufacturers commonly provide runtime estimates or runtime charts at different load levels. These are more useful than assuming a fixed runtime from the UPS's VA rating.
Locate the expected load on the manufacturer's runtime information and determine whether the estimated operating time satisfies your requirement.
Remember that actual runtime can change with battery age, battery condition, temperature, load behavior and other operating factors.
For a deeper explanation of the relationship between stored energy, connected load and operating time, see How Long Will a Battery Backup Run a Small Business?
Example: small server and network stack
| Equipment | Example planning load |
|---|---|
| Small server | 250 W |
| NAS | 70 W |
| Firewall/router | 25 W |
| Network switch | 60 W |
| Total example load | 405 W |
A UPS protecting this example environment must support more than the 405-watt operating load while also satisfying the applicable VA requirement and desired runtime.
Do startup and changing loads matter?
Yes. Some equipment can temporarily require more power than its normal operating level, and computer or server consumption can change with workload.
Motor-driven equipment, refrigeration, pumps and other specialized loads require additional analysis and should not automatically be placed on a conventional office UPS simply because their average wattage appears to fit.
Do not forget the physical UPS configuration
Electrical capacity is only part of the selection process.
- the number of battery-backed outlets
- whether some outlets provide surge protection only
- plug and receptacle requirements
- tower versus rack-mount installation
- available rack space where applicable
- battery replacement requirements
- USB, network or management connectivity
- shutdown-software compatibility
- environmental and ventilation requirements
When a larger UPS is not the whole answer
If the primary requirement is many hours of backup power, a larger stored-energy system, generator or layered continuity strategy may be more appropriate than continually increasing the size of a conventional UPS.
For that decision, see UPS vs. Battery Power Station for Small Business: Which Is Better?
For broader backup-power planning, see Backup Power for Small Business: UPS, Battery Power Station or Generator?
UPS sizing checklist
- Identify only the equipment that requires uninterrupted power.
- Determine the expected operating wattage of each device.
- Add the protected loads together.
- Check relevant peak or changing-load requirements.
- Compare the total load with the UPS watt rating.
- Verify that the load also remains within the UPS VA rating.
- Allow practical operating and growth headroom.
- Determine the required battery runtime.
- Check the manufacturer's runtime information at the expected load.
- Verify the number and type of battery-backed outlets.
- Confirm physical, management and shutdown requirements.
Bottom line
The right UPS size is determined by the equipment being protected, its electrical load and the runtime the business requires.
Start by adding the wattage of the critical equipment. Then compare that load with both the watt and VA limits of candidate UPS systems, leaving practical headroom instead of designing around the maximum rating.
Finally, verify expected runtime at the actual load. A UPS can have enough electrical capacity to operate the equipment while still having too little battery runtime for the business requirement.
If longer-duration battery capacity is your primary concern, see How Big of a Battery Backup Does a Small Business Need?