Uninterruptible Power Supply (UPS) Solutions


There are a number of design and acquisition decisions to make when considering the procurement of a UPS for a data center, computer room, server room, workstation or other mission critical facility. With many decision and strategy decision related to UPS performance such as:

  • The size of the load to be protected (usually in both kVA and kW)
  • The battery runtime required
  • The proper input and output voltages
  • Choosing the right type of system (i.e. on-line, line-interactive, etc.)
  • Varied price / performance seen within manufacturer product portfolios
  • Advances in technologies
  • The ideal configuration to achieve the desired level of redundancy (i.e., N, N+1, 2N, 2N+1, etc.)
  • The configuration needed to achieve the required output distribution
  • A thorough understanding of IEC 62040-3, the Method of Specifying the Performance and Test Requirements for a UPS

As such, purchasing a UPS is a complicated process.

UPS Types

The general categories of UPS systems are as follows:

  • Standby
  • Line Interactive
  • Ferro-Resonant
  • Double Conversion On-Line
  • Delta Conversion On-Line
  • Rotary
(Click image to view full-size)
(Click image to view full-size)

A standby UPS is among the most common type used for personal computers. It is called a standby UPS because power provided by the battery is ‘offline’ until such time that it is needed. Typically, these UPS products are small in size, low power, low cost, but highly efficient. Some versions provide additional surge suppression and noise filtering. UPSs of this type are typically effective in single-phase load ranges from 100 to 1,500 VA.

A line interactive UPS is the most common UPS sold today. It is typically used to protect individual or a limited number of servers. Like the standby UPS, a line interactive UPS is ‘offline’ until utility power is interrupted. Then, a transfer switch directs power from the battery through an inverter to create AC power. Unlike a standby UPS, however, a line interactive UPS typically provides a certain degree of voltage regulation, meaning that the output is adjusted back to a nominal operating range regardless of the input voltage. Line interactive UPSs are highly efficient, available in small rack-mountable or floor-mountable versions, and typically effective in single-phase load ranges from 0.5 to 10 kVA.

At one time, ferro-resonant UPS dominated the market in the 3 to15 kVA range. A ferro-resonant UPS relies on a unique transformer configuration that provides some voltage regulation as well as excellent noise filtering capabilities. Like the prior configurations, this too is a standby technology in that it utilizes an inverter in standby mode. They are rarely used today due to their instability when operating with the much more highly reactive electronics and power supplies of modern computer loads. Additionally, they are very inefficient and dissipate excessive heat.

(Click image to view full-size)
(Click image to view full-size)

The double conversion on-line UPS design is by far the most common type for loads above 10 kVA. Unlike the previous types, a double conversion UPS is considered ‘on-line’ because its nearly ideal output waveform is derived completely from battery power through its inverter. As such, this isolates the output from the input. The input primarily serves to continually charge the battery. Therefore, during an AC power failure, on-line operation results in no transfer time. Since 100% of the load power is converted twice, once from AC to DC (to charge the battery) and a second time from DC to AC (for the output), a double conversion UPS is inherently less efficient than offline UPS types. These UPSs are available in a broad range of sizes for three-phase loads from 10 to 1,000 kVA. Furthermore, these UPSs can be arranged in a vast array of configurations to achieve high availability for very large three-phase data center loads.

A delta conversion on-line UPS design is a more recent topology that was originally patented and utilized exclusively by APC for its Silcon-series 3-phase UPS line. This UPS family has since been discontinued.

However, its technological innovations continue on in APC’s high-power Symmetra MegaWatt UPS series. Simply stated, the advantage of this topology is its energy efficiency. It achieves high efficiency by not processing 100% of the power, 100% of the time, as is the case with a double conversion UPS. Rather, it processes only the portion of the power that is outside of the ‘window’ of an acceptable power waveform, while at the same time having an output that is on-line with load such that there is only a load step change from steady-state to full battery-load operation. As such, this technology is best suited for the highest power applications and well above 1,000 kW.

A rotary UPS is one that unlike the prior approaches does not rely on electronics to pass through or recreate an output AC waveform. Rather, a rotary UPS relies on a mechanical motor and generator to create a pure sine wave output without the need for filter capacitors. Additionally, the resulting low input impedance allows the ability to handle any type of load no matter how ‘dirty’ it may be. Also, this approach yields an ability to handle high fault currents and provide isolation from high harmonic load input currents. Finally, a rotary UPS can be configured to use direct diesel bypass. By comparison, a static (or electronic) UPS, can only source its load from a generator in a ‘break before make’ fashion. Due to the nature of its ability to handle ‘difficult’ loads, rotary UPSs are large in size and expensive. As such, they are reserved for large loads in excess of 1,000 kW.

PTS takes the mystery out of specifying, acquiring, commissioning, and maintaining complex UPS systems. As a data center facility consultancy, PTS is vendor agnostic working with clients to fully understand their Key Design Criteria within the mission critical facility and providing a range of manufacturer products.

Once the Key Design Criteria are thoroughly understood, PTS works to match the right solution to the multitudes of UPS technologies and configurations available. All these solutions are recognized for being at the forefront of power protection innovation and technology and are class-leading in terms of system reliability, efficiency, availability, scalability and flexibility – using less energy, saving valuable floor space (leaving room for revenue-earning equipment), and achieving significant total cost of ownership savings – making them ideal for IT applications.

PTS Uninterruptible Power Supply (UPS) Services

To learn more about PTS’ Uninterruptible Power Supply consulting, procurement, commissioning or maintenance services, contact us or visit:

Uninterruptible Power Supply (UPS) Products

Learn more about PTS recommended UPS solutions (in alphabetical order):


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