How to choose a good UPS? UPS Buying Guide

A power failure, even for a few minutes, can bring major inconvenience to our lives.
If we work on the PC and have not saved, we may lose data or, worse, the computer or household appliances may be damaged when the electricity returns. There are areas, especially the rural ones, with strong wind/storms or old voltage lines, where voltage fluctuations are frequent and can disrupt our activity.

Power surges will in time lead to damage to home appliances, computers, central heating systems, TV, etc. Fortunately, this problem has been solved by using UPS, but how do we get the best UPS at a convenient price?

What are the common power problems that may occur?

Below are some of the most common power supply problems that can occur and affect our electronic equipment:

High-Voltage Spikes

Voltage spikes are fast, have a short duration, and may occur quite frequently.
Spikes are usually caused by lightning, but there can be other causes as well.
Frequent spikes can damage and destroy electronics.

Voltage sags / Voltage Swells

Voltage sags or dips are short duration reduction in RMS voltage which can be caused by a short circuit or overload.

Voltage sags are the most common power disturbance. Voltage sags are caused by abrupt increases in loads such as short circuits, motors starting, or electric heaters turning on.
Usually, it appears when there are big consumers on the line, especially in crowded urban areas.

In contrast, voltage swells are brief increases in voltage for a short period.
Voltage swells are most often caused by an abrupt reduction in load on a circuit with a poor or damaged voltage regulator.

Frequency Noise

Frequency noises are very common and can disrupt or degrade the performance of a circuit by injecting abnormalities into the system.
Over time, multiple frequency noises can damage home appliances.


A brownout is an intentional or unintentional drop in voltage for an extended period of time.
For example in summer, when electricity consumption increases, especially due to the air conditioners, to avoid a total blackout, power companies may lower the voltage of your electricity.

Types of ups

Ups are of three types: Line-interactive, Offline, and Online, based on what type of power protection you need.

To understand the types of ups, first, we need to see the difference between Watts vs. VA and what “Power Factor” means.

Watts vs. VA, Power Factor

A UPS has both maximum Watt and maximum VA (volt-ampere).
Neither the Watt nor the VA of a UPS may be exceeded by the attached equipment, which is named “the load”.

Watts are the real power drawn by the UPS, while volt-amps are called the “apparent power”

VA to watts calculation

The real power P in watts (W) is equal to the apparent power S in volt-amps (VA), times the power factor PF:

P(W) = S(VA) × PF.

The watt determines the actual power purchased from the utility company and the heat loading generated by the equipment.
The VA is used for sizing wiring and circuit breakers.

The ratio of the watts to VA is called the “Power Factor” and is expressed either as a number (ex 0.8) or a percentage (ex 80%).
This power factor is what really matters when choosing a UPS for your specific requirements.

Line-interactive UPS

Line-interactive UPS has a power coefficient(PF) of 0.7, providing protection regardless of the input voltage.
If there are fluctuations in voltage the UPS will switch on the battery and will operate until the voltage returns to normal limits.

Offline Sources

Offline sources have a power coefficient of 0.6, being the cheapest and most used.
An offline UPS offers the most basic features, providing surge protection and battery backup.

They are dependent on the voltage and frequency they receive from the electricity grid. This means that the device receives voltage fluctuations from the network and will transmit them to the connected devices, which does not offer any additional protection to the electronic equipment.

Online sources

Online sources have a power coefficient of 0.8 or higher, being independent of the voltage and frequency received at the input. They remain permanently connected and offer maximum protection and a very fast response time.

The online UPS is ideal for equipment that is very sensitive to power fluctuations. They are necessary when utility power sags, outages and other anomalies are frequent.

Online UPS is the most indicated, the only disadvantage being the higher price.

Factors you need to consider when you want to buy a UPS

1. The power of the UPS (Uninterruptible Power Supply)

As I specified below the watts are the real power drawn by the UPS (Uninterruptible Power Supply).
In other words, your UPS has to be large enough to support all of the consumers plugged into it.
The higher the capacity (W), the more electronic equipment can support it.

How to choose the power of the UPS?

First, you have to calculate the “load”.

Make a list of all the equipment that the ups want to support.
If you are unsure how many watts your equipment requires, consult their specifications. For example: desktop(power source 450W) + monitor(20W) + boxes(80W) Total: 550W

To determine the power requirements of your equipment, ensure that the equipment plugged into the outlet does not exceed the UPS’s rated capacity.
If the rated capacity of the unit is exceeded, the Uninterruptible Power Supply may overload and shut down or the circuit breaker may trip.

It is advisable to take a safety margin of at least 10 – 15%, some studies recommending a margin of up to 20%.

The ups are of low power (maximum 500 W), medium (with values between 500 and 1000 watts), and high (going up to 3000 Watts).

When it comes to high power, prices are generally over 800$, but in most cases, you don’t need very high power UPS for home use.

2. Runtime, how long do the ups hold the equipment when the power goes out?

Autonomy is another important factor you need to consider, which means how long the UPS can power consumers when the power goes out, which is named: runtime.

Depending on how large the UPS battery is, the runtime will vary depending on how long it will work after the power is turned off.
Usually can vary between 4 minutes and up to 10 hours.

How to calculate how long Your UPS will run for?

The runtime is inversely proportional to the power of consumers:
If the connected equipment has fewer watts, the runtime will be higher and If the connected equipment has more watts, the runtime will be shorter.

The battery is the key element in the UPS, depending on the autonomy of the product, their capacity being expressed in Ah (ampere-hours).

For example, to power, the PC is enough to choose an off-line device but to ensure continuity for a boiler, it is recommended to use on-line and line-interactive devices.

For a boiler or for a central heating system it is good to take a bigger battery!

Usually, when the power is restored, You have to charge the battery to be ready next time.

3. Autonomy of battery

Autonomy is a major factor to consider when purchasing a UPC, which means how long the UPS can power consumers when the electricity is turned off, usually autonomy is ranging from 4 minutes to 10 hours.

This feature depends on the batteries the product has and is calculated for the maximum power that the UPS can protect.
For example, to power a computer it is sufficient to choose an off-line device that will provide a power supply until the generator is switched on, but to ensure continuity for a power plant, it is recommended to use on-line and line-interactive devices.

The battery can be included with the ups or we can buy it separately, most of them being of type Valve Regulated Lead Acid (battery with lead and acid), being similar to car batteries.

The battery life is normally between 3 and 5 years, and there are more expensive variants that have batteries with a life span of 7 to 10 years.

Best UPS to buy online

Choosing a good UPS is not easy. As you can see, there are many factors to consider and the price is important. We all want to find a good UPS at a great price.

If you need a UPS, but still are not sure which UPS to choose, I recommend this article, where you have updated recommendations in 2022 with the best UPS you can find online.

If you already have a UPS I invite you to post in the comments what brand it is and what made you choose it. Help others make a wise choice.

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