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To ensure your UPS is in good working order (much like batteries and fans), your UPS AC and DC capacitors will need to be replaced during the working life of an Uninterruptible Power Supply. 

Each manufacturer varies slightly in their recommendations for replacement but typically this is between 5 & 7 years. Capacitors have a specified design working life and it is strongly recommended that they are replaced in line with the manufacturers’ guidance. 

Capacitors will degrade over time and without regular planned and preventative replacement they can fail unexpectedly, causing costly downtime and placing your critical infrastructure at risk until a repair can be carried out.


Here are some of the key features of our AC & DC Capacitor replacement services but read on for full details of how we can help.

  • Full replacement of AC & DC Capacitors
  • Replacements carried out by experienced, trained engineers
  • In hours and out of hours replacement
  • Disposal in accordance with Environment Agency Regulations
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We cover more details about our AC & DC Capacitor replacement services below. We explain what a capacitor is and how it operates in your UPS and how you can help to extend the working life of the capacitors.

But if you would rather speak to one of our experts, call us today or send us an email to discuss your capacitor replacement requirements.

How to Prolong the Working Life of UPS Capacitors

AC and DC capacitors will suffer from a loss of performance over time. There are several key things that will improve or prolong the life of the UPS capacitors:

  • Maintain the ambient room temperature as close to 21c as possible
  • Regular UPS Maintenance and testing
  • Maintain good airflow around the UPS
  • Ensuring humidity levels within the UPS room are controlled
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AC capacitor icon alongside a thermometer showing hot

The Two Main Causes For Early AC & DC Capacitor Failure

  • Working at high temperatures (whether ambient temperature or internal working temperature).

For example, oil filled capacitors exposed to excessive heat can cause the internal liquid to expand and create a dangerously high pressure increase within the component.

  • Operating under excessive load conditions.

All capacitors are designed to withstand brief periods of high load conditions but if they are continually exposed to overloading or high current situations, the sooner the capacitor will require replacement.

AC & DC Capacitor Disposal Compliancy

AC & DC Capacitors are deemed as Hazardous Waste and you must dispose of old capacitors in line with Environment Agency regulations.

As part of our ISO 14001 environmental management accreditation, Specialist Power Systems are committed to ensuring that all capacitors are disposed of and recycled accordingly.

As a certified waste carrier, we will ensure all capacitors are disposed of and recycled with a full audit trail and certificated disposal.

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What a UPS Capacitor Does

A capacitor is an electrical device that stores and releases electrical energy. These devices come in various sizes based on the amount of electrical energy they are required to absorb.

Capacitors contain two conducting surfaces, usually electrodes or metallic plates, with a third element called the dielectric medium, which acts as an insulation device between the two conductors.

The total capacitance rating (how much charge can be stored by a capacitor) is determined by the thickness of the dielectric layer and is measured in farads.

Most UPS will have three main types of capacitors and each one has their own specific job in the function of a UPS system. Read on to find out about each of these different types.

AC Input Filter Capacitor

The purpose of the AC (Alternating Current) input filter capacitors is to smooth input voltage transients and fluctuations that can be found in your incoming electrical mains supply. They clean up the input waveform and reduce the harmonic content of the supply voltage.

This is important to improve UPS efficiency and operation and helps to improve stress on the UPS input components. Input capacitors are sometimes referred to as power factor correction capacitors.

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Icon symbol for direct current

DC Capacitor

DC (Direct Current) capacitors are connected within the UPS rectifier section. Their primary functions are to smooth the DC waveform and to reduce ‘ripple current’ created by the conversion from AC to DC by the rectifier IGBT’s.

This smoothing of the DC waveform is known as supply voltage filtering. Where batteries are connected directly to the rectifier, the condition of the DC capacitors will have a direct impact on the life-span of the batteries connected to them.

AC Output Capacitor

AC output filter capacitors are connected directly to the UPS output stage and ensure that the final waveform received by the critical load is as smooth as it can be. This also contributes to the overall efficiency of the UPS.

Output capacitors also filter out the PWM (Pulse Width Modulation) frequencies generated by the control and switching of the output inverter IGBT’s.


Icon symbol for pulse width modulation
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You can download this great whitepaper from Riello UPS - one of the key manufacturers we work with.

They talk about what factors affect AC & DC capacitor lifespan and what tell-tale signs suggest a capacitor might be close to failure - Check it out !

So, if you are looking to replace your UPS AC & DC Capacitors or would like to speak to an expert about any of your back-up requirements, get in touch with us today on 01234 851155 or email [email protected] to discuss how we can help.


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For more information, please call us on +44 (0) 1234 851155 or send us a message below.

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