In general, “efficiency” refers to the ability to accomplish a task or achieve a goal with the least wasted time, effort, energy, or resources. It measures how effectively inputs (such as time, labor, or materials) are converted into outputs (results or products).
Measuring Power Supply Efficiency and Why It’s Important
Mar 28, 2025
Power supply efficiency is critical to both performance and cost-effectiveness, impacting energy use, operational reliability, and long-term savings. Assuming a typical power supply’s operational life of five years, the wasted energy due to lower efficiencies will cost more than the original power supply!
Table of contents
The importance of power supply efficiency
There are three major reasons that power supply efficiency is important to the user.
1) The difference between output power and input power is the power being dissipated (primarily as heat).
This means that even a small efficiency difference can result in a big operational difference.
2) Temperature is the most important factor influencing device reliability.
3) High efficiency is important to reduce running costs.
The electricity consumption of a 96% efficient power supply will be 30% less than one of 94%, although there is only a 2% difference in efficiency! Further assuming a typical power supply’s operational life of five years, then the wasted energy due to lower efficiencies would easily exceed the cost of the power supply itself!
1) The difference between output power and input power is the power being dissipated (primarily as heat).
- A power supply with 90% efficiency will lose 10% as heat.
- A power supply with 95% efficiency will lose 5% as heat, which means it will get only half as hot as the first example.
- A power supply with 97.5% efficiency will lose 2.5% as heat, which means it will get only quarter as hot as the first example.
This means that even a small efficiency difference can result in a big operational difference.
2) Temperature is the most important factor influencing device reliability.
- This is because of the Arrhenius relationship, which is a formula for the temperature dependence of reaction rates in physical chemistry. The Arrhenius equation asserts that chemical reactions double for every 10°C increase in temperature.
- Because chemical reactions are involved in aging, corrosion, and electrical stress effects, higher temperatures lead to earlier failures.
- Any power supply that runs cooler will generally last longer, which means there are three ways to ensure a long operational life:
- Use higher temperature-rated components, such as capacitors rated at 105°C rather than those rated at only 85°C. This means that if the internal temperature is around 60°C, the higher-rated components have a much greater headroom and are not overly stressed.
- Use a layout that allows free convection cooling so that dissipated heat can easily escape the unit. For example, members of the RACPRO1 series are designed with two vertical air flow channels to use the chimney effect to aid passive cooling. Furthermore, heat-sensitive components (such as capacitors) are placed below hotter components (such as transformers and transistors) to reduce their thermal stress.
- Design for high efficiency to avoid generating waste heat in the first place.
3) High efficiency is important to reduce running costs.
- A 1kW power supply with 94% efficiency will waste 365 × 24 × 6% = 526kWh per year.
- A 1kW power supply with 96% efficiency will waste 365 × 24 × 4% = 350kWh per year.
The electricity consumption of a 96% efficient power supply will be 30% less than one of 94%, although there is only a 2% difference in efficiency! Further assuming a typical power supply’s operational life of five years, then the wasted energy due to lower efficiencies would easily exceed the cost of the power supply itself!
Measuring power supply efficiency
Different considerations are involved when measuring the efficiency of DC/DC and AC/DC converters. Below is a formula for calculating efficiency and a high-level summary of these differences is as follows:
For example, if a power supply draws 100 watts from an electrical source but delivers only 90 watts to its load device, the efficiency is (90W / 100W) × 100% = 90%.
Power supply efficiency formula
In the context of a power supply, efficiency refers to how effectively the supply converts electrical input power (from the source) into output power (for the load device) without losing energy as heat or other forms of waste. The formula for power supply efficiency, expressed as a percentage, is as follows:
Efficiency (%) = (Output / Input Power) × 100%
For example, if a power supply draws 100 watts from an electrical source but delivers only 90 watts to its load device, the efficiency is (90W / 100W) × 100% = 90%.
Measuring the efficiency of AC/DC converters
Measuring the efficiency of an AC/DC converter is difficult because a power analyzer is required to accurately measure the real (effective) AC power delivered to the converter, compensating for phase angle (Cos Phi) between the voltage and the current. Also, with AC/DC converters without power factor correction (PFC), any harmonics will affect the measurement. Most power analyzers will measure up to the 39th harmonic (as defined by the IEC EN 61000-3-2 standard). To obtain a truly accurate measurement, however, you must go to the 99th harmonic and average over a long time (several minutes), especially with modern controller ICs that use low-power standby techniques such as pulse skipping.
Efficiency measurements should be carried out under all the different operating conditions (low line to high line and low load to full load). Efficiency without load is always 0%. This is because when the converter runs, it will consume a small amount of "housekeeping" power to run the switching circuits while delivering no output power (zero load current). This means a typical efficiency curve for a power supply will show zero efficiency at no load.
A good power supply design will present a flat efficiency curve from around 10% load through to full load. Also, maximum efficiency is typically at higher loads, but a good design will have little reduction in efficiency at 50% load or less. Also the efficiency curve should not be very dependent on the supply voltage, although as the AC voltage drops, the input current rises, and this causes some additional I²R losses. The efficiency curves of the RACPRO1 series are exceptionally good. Not only is the efficiency independent of the supply voltage across all load conditions, but the peak efficiency of 96% is reached at 30% load and stays constant through to 100% load, thus covering all typical industrial load conditions.
Finally, standby power consumption is also important, firstly to save money in electrical costs if the unit spends a lot of time in standby mode, and secondly to meet the EU and US power consumption ecological targets and legislation. The RACPRO1-T960/24 draws only around 2W in standby, despite being able to deliver over 1150W if the load suddenly needs it.
Efficiency measurements should be carried out under all the different operating conditions (low line to high line and low load to full load). Efficiency without load is always 0%. This is because when the converter runs, it will consume a small amount of "housekeeping" power to run the switching circuits while delivering no output power (zero load current). This means a typical efficiency curve for a power supply will show zero efficiency at no load.
A good power supply design will present a flat efficiency curve from around 10% load through to full load. Also, maximum efficiency is typically at higher loads, but a good design will have little reduction in efficiency at 50% load or less. Also the efficiency curve should not be very dependent on the supply voltage, although as the AC voltage drops, the input current rises, and this causes some additional I²R losses. The efficiency curves of the RACPRO1 series are exceptionally good. Not only is the efficiency independent of the supply voltage across all load conditions, but the peak efficiency of 96% is reached at 30% load and stays constant through to 100% load, thus covering all typical industrial load conditions.
Finally, standby power consumption is also important, firstly to save money in electrical costs if the unit spends a lot of time in standby mode, and secondly to meet the EU and US power consumption ecological targets and legislation. The RACPRO1-T960/24 draws only around 2W in standby, despite being able to deliver over 1150W if the load suddenly needs it.
High-performance RACPRO1 DIN rail power supplies
To address the most demanding industrial applications, we recently introduced our RACPRO1 family of DIN rail power supplies. The RACPRO1-T240, -T480, and -T960 deliver 240, 480, and 960 watts, respectively (Figure 1).
Fig. 2: Meet the RACPRO1 family: T240 (left), T480 (middle), and T960 (right)
The members of this family provide industry-leading efficiencies for power supplies in this class, featuring power factor correction (PFC) > 0.9, these supplies offer efficiencies of 94.1% (RACPRO1-T240), 95.3% (RACPRO1-T480), and 97.1% (RACPRO1-T960).
Fig. 2: Meet the RACPRO1 family: T240 (left), T480 (middle), and T960 (right)
The members of this family provide industry-leading efficiencies for power supplies in this class, featuring power factor correction (PFC) > 0.9, these supplies offer efficiencies of 94.1% (RACPRO1-T240), 95.3% (RACPRO1-T480), and 97.1% (RACPRO1-T960).
Additional resources
The RECOM AC/DC, DC/DC, and EMC Book of Knowledge provide an extensive collection of experience and learnings necessary to implement successful AC/DC power supply designs. Chapter 12 discusses how to lower housekeeping power consumption along with some measuring techniques.
Also, RECOM’s Energy Efficiency in AC/DC Power Supplies: Quick Guide to the Essentials blog provides a helpful introduction regarding Ecodesign regulations, which are a set of standards and guidelines established by governments or regulatory bodies to ensure that products are designed to minimize their environmental impact throughout their entire life cycle. These regulations primarily focus on improving energy efficiency and promoting sustainability by reducing waste, emissions, and resource consumption.
The RACPRO1 webpage provides more information about this exciting new DIN rail mounted power supply family.
Also, RECOM’s Energy Efficiency in AC/DC Power Supplies: Quick Guide to the Essentials blog provides a helpful introduction regarding Ecodesign regulations, which are a set of standards and guidelines established by governments or regulatory bodies to ensure that products are designed to minimize their environmental impact throughout their entire life cycle. These regulations primarily focus on improving energy efficiency and promoting sustainability by reducing waste, emissions, and resource consumption.
The RACPRO1 webpage provides more information about this exciting new DIN rail mounted power supply family.
Applications
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RACPRO1-T240
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RACPRO1-T480
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RACPRO1-T960
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