Power outages, voltage fluctuations, and sudden power losses can affect the operation of many devices—from computers and security cameras to servers and industrial automation systems. An Uninterruptible Power Supply (UPS) provides power to connected devices via its internal batteries during an outage, allowing systems to continue operating or to be shut down in a controlled manner.
So, which UPS model should you choose? What capacity (in VA) is required? How long do the batteries last? What is the difference between an Online UPS and a Line-Interactive UPS?
In this guide, we cover the technical specifications to consider when selecting a UPS in 2026, along with common use cases and how to calculate the correct capacity.
What is a UPS, and what does it do?
A UPS is a power protection system that activates when mains power fails; it converts energy stored in its internal batteries into the appropriate electrical format to power connected devices.
The primary functions of a UPS include:
Providing temporary power during electrical outages.
Regulating low and high voltage fluctuations (within its operational range).
Reducing the risk of data loss caused by sudden power interruptions.
Providing time for the controlled shutdown of computers, servers, and network systems.
Supporting the operational continuity of security systems and critical electronic equipment.
Important: A UPS does not provide an unlimited supply of energy. The runtime during an outage depends on the power consumption of the connected devices, the condition of the batteries, the efficiency of the UPS, and the system's load level.
What are the types of UPS?
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1. Standby (Offline) UPS
Typically used for applications with lower power requirements, such as home computers and basic electronic equipment. It powers devices when the mains electricity is normal; when an outage is detected, it switches to battery power.
Advantages:
Can offer economical solutions.
Provides basic protection against power outages.
Includes suitable options for home and small office use.
2. Line-Interactive UPS
These models stand out for their Automatic Voltage Regulation (AVR) feature, which compensates for voltage fluctuations. They are a popular choice for computers, network equipment, and small businesses that require an uninterrupted power supply.
Advantages:
Can regulate voltage drops and surges within specific limits.
Reduces battery usage when mains power conditions are stable.
Suitable for use in homes, offices, and security systems.
3. Online UPS (Double Conversion)
Online UPS systems convert incoming alternating current (AC) into direct current (DC) and then back into alternating current (AC), continuously supplying the load through an inverter. This architecture provides more comprehensive protection against power grid issues and ensures zero transfer time when switching from mains power to battery power.
They are particularly preferred for:
Servers and data centers
Network and communication infrastructures
Sensitive electronic devices
Industrial control systems
Businesses with low tolerance for power interruptions
Online UPS units are generally more expensive than other basic types of UPS. The right choice should be made by evaluating both the level of protection required and the total cost of ownership.
How much VA is needed when choosing a UPS?
When purchasing a UPS, looking solely at the VA (volt-ampere) rating on the device is not sufficient. VA (apparent power) and W (active power) are not the same thing.
The active power capacity of the UPS must be sufficient to cover the total power consumption of the connected devices. For example:
Device
Typical power consumption
Computer
250 W
Monitor
40 W
Modem and network devices
20 W
Total
310 W
In this example, the total load is 310 W. The selected UPS must have an appropriate capacity in terms of both watts and VA; furthermore, sufficient headroom should be allowed for inrush currents and potential future increases in load.
Actual consumption varies depending on the device models and operating conditions. Calculating based on manufacturer labels or direct power measurements will yield more accurate results.
How long will a UPS run?
The primary factors that determine a UPS's runtime are:
The capacity and age of the batteries
The total power consumption of the connected devices
The conversion efficiency of the UPS
The temperature and maintenance status of the batteries
Instantaneous load fluctuations that occur during a power outage
For instance, two 12 V 9 Ah batteries have a theoretical total energy capacity of 216 Wh. However, this does not mean that the devices can utilize the entire 216 Wh of energy. Due to conversion losses, battery discharge characteristics, and a reduction in usable capacity under high loads, the actual runtime will be shorter.
If long-duration backup is required, one should consider UPS models that support external battery connections or larger systems sized according to specific needs. External battery connections should only be used with models explicitly supported by the manufacturer and with an appropriate battery configuration.
When should a UPS battery be replaced?
The fact that a UPS is operational does not necessarily mean its batteries still possess sufficient capacity. Aging batteries may fail to provide the expected runtime during a power outage. Signs that a battery check is required include:
The UPS shutting down shortly after a power failure
Appearance of battery fault warnings
Failure of a battery test
Issues with the battery