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Which to Choose—Flywheel vs. Battery UPS?

As data centers, manufacturing and other facilities look to increase power quality and reliability, they are faced with a choice of UPS systems. There are a few types of UPS units that are seen in the market today including static UPS systems that can either be coupled with a flywheel or battery system.

Since only around 6% of the 3-phase UPS systems in the market are flywheel UPS systems, the technology behind the units may not be understood. However, there has been a steady growth in the flywheel energy storage market as technology has improved.

A flywheel is essentially a rotating mass that spins at incredible revolutions per minute (RPM). This spinning disc is typically housed in vacuum to reduce resistance and is used to convert kinetic energy to produce DC power.  This can provide both power conditioning and runtime in the event of a power outage.

Flywheel UPS Rotor Assembly

Conversely, a battery UPS system stores energy and converts it to electrical power through a chemical reaction.

Both systems condition power and provide reliable backup in the event of a utility failure. Here are a few considerations when comparing the two main types of units.

Footprint Requirements

In theory, a flywheel UPS system requires significantly less space than a traditional battery UPS.  Since they do not have large battery requirements, the overall weight of the UPS is substantially less than a battery UPS.  Active Power, a leading manufacturer of flywheel systems, states that the average flywheel UPS configuration should consume 75% less space compared to a conventional double conversion, battery UPS system.

However, as we discuss below, in order to achieve additional runtime, the footprint requirement of a flywheel UPS may be closer to that of a traditional battery configuration.


Here, perhaps, is the Achilles heel of a flywheel UPS.  Because it is reliant on the mechanical rotation of a cylinder to provide kinetic energy, it cannot sustain extended runtime. While a battery UPS system can provide for runtime ranging from 5 minutes to a few hours, a flywheel UPS system can typically only support critical loads from 10 – 30 seconds. Even with additional flywheel assemblies included, a flywheel cannot compete with batteries for extended runtime.

As a result, a facility may often couple a flywheel UPS with additional strings of batteries that act as standby in the event that the outage is prolonged, or the emergency generator does not start and come online within that 10 second window.

Just how much would a more efficient UPS save you? Download our energy savings calculator.

Environmental Impact

Flywheel UPS systems do not contain hazardous materials that need to be maintained or replaced. Conversely, a battery contains acid and other toxic chemicals that will need specialized disposal. If using wet cell batteries, additional requirements may include containment systems, HazMat disposal and the EPA is concerned with the environmental impact these may have.

Flywheel UPS systems are not as adversely affected by a changes in temperature and can operate in a much wider range of environmental conditions than a traditional VRLA battery. Additionally, they are not prone to the same common failures that a VRLA, Lithium Ion or Wet Cell batteries might experience such as corrosion, electrolyte breakdown, or thermal runaway.

Lifecycle Cost

The average lifespan of a static UPS system is around 15 years. During the life of the UPS, one of the main costs associated with the unit will be the maintenance and replacement of batteries. If the UPS system is using standard VRLA batteries, the end user will typically pay for at least three battery replacements during the operating life of the UPS. Often, the cost to operate and maintain the UPS system will significantly exceed the original purchase price of the UPS. For more information on the lifecycle of a static UPS, visit our blog article discussing this.

A flywheel system has a significantly higher initial capital cost than a battery UPS.  If the facility determines that batteries are also needed in addition to the flywheel in order to increase runtime, the lifecycle cost of a flywheel system will exceed the total cost of a battery system.  Even if batteries are not needed, the flywheel will need maintenance – including replacement of the internal bearing – which can also add cost. The lifespan of the flywheel itself is typically longer than that of a static UPS system, allowing for a potentially lower cost of ownership.

Flywheel UPS manufacturers have also traditionally held their software and technology close to their chest, limiting competition for the maintenance of their systems. This always tends to drive up ongoing maintenance costs since the end user is often tied to the manufacturer only for service on their critical equipment. As these manufacturers open up their software and allow the end user more freedom to choose their service team, these systems may start to see an increase in market demand.

Flywheel technology has been around for a very long time, and is an excellent, stable source of backup power. If you are considering a flywheel UPS system, contact one of our consultants to discuss if it makes sense for your environment and application.