Power Supply Peak Load – How Much for How Long?

May 9, 2025
Power control system
Choosing a power supply requires more than just ensuring the rated output is greater than the expected load and “calling it good.” The expected load is only part of the story. Most applications have a typical load, a maximum load, and brief periods with a peak load that exceeds both typical and maximum. To accommodate this, power supplies can supply peak loads above their maximum rating for a limited time. Efficiency in power supplies is crucial because it reduces energy consumption and improves overall performance by minimizing waste and heat generation.

Table of contents

Power Supply Environment

Power supplies, both AC/DC and DC/DC, cater to several conditions that can impact the power output capability, including ambient temperature, humidity, input spikes, and output spikes. Effective energy management in power supplies is crucial, emphasizing the importance of conserving energy. Under maximum rated operating conditions, a supply must immediately provide rated power and continue to do so for as long as the duty cycle rating allows.

Monitoring the operation of power supplies is essential to ensure safe temperatures and maintain performance and reliability. Most loads, however, do not operate with a steady draw. Mechanical loading of motors, back EMF, inrush current, and startup load spikes all contribute to load variation. HVAC systems significantly impact energy consumption and base load demand, underscoring the need for efficient upgrades and retrofits.

Considering Peak Load

Most power supplies can deliver regulated currents higher than the rated maximum value, especially during high-demand periods when electricity consumption peaks. However, components will begin to overheat and eventually risk thermal failure. Therefore, supplies often declare a peak load specification in addition to the rated continuous load. Managing peak loads through operational adjustments is crucial to maintaining service and controlling costs. Two other factors are critical when considering peak load: maximum duration of peak load and the cool-down period between peaks.
Peak load dwell time and cycle time graph
Fig. 1: Peak load dwell time and cycle time graph of the RECOM RACPRO1-T480 series DIN rail power supply.
When designing a supply, engineers look at the expected operating conditions and design in headroom, or excess capacity above the rated power output, to accommodate peak loads. The headroom is available for short periods to accommodate brief load spikes. Peak demand influences time-of-use pricing and load management strategies, incentivizing consumers to alter their electricity consumption habits. This can have financial implications due to peak demand charges imposed on users. The performance of power supplies in high-peak power applications is essential for reducing grid strain during these periods.

With the right peak load capability, a smaller, less expensive supply unit can serve reliably for the life of the installation. Figure 1 shows the peak load capability (also called “boost power”) for a RACPRO1-T480 series AC/DC DIN rail power supply. The graph for the product data sheet shows two different peak load capabilities.

First, the supply can provide peak power of 250% of the supply’s rated power output for as long as 20 milliseconds. The chart cautions that above 150% (coded in purple) may lead to easy fuse-tripping conditions. A peak pulse of up to 150% of the supply’s maximum rated load is reliable for as long as 7.5s.

Further examination of the chart shows that the load must drop to 100% or lower for 52.5s after a maximum duration peak load. Peak loads can occur only every 60 seconds and can last for no longer than 20ms at 250%, or 7.5s at 150%. That’s a 0.03% duty cycle at 250% and a 12.5% duty cycle at 150%.

Multiple Peak Loads

Certain applications draw multiple peak loads. For example, a conveyor motor may run at a steady load but peak when responding to an occasional heavy load. Batteries play a crucial role in stabilizing the grid during these peak demand periods by storing electricity during low-demand times and supplying it when needed.

When selecting a supply for multiple peaks, the duty cycle (time between peaks) is just as important as the peak load value. Vehicle-to-grid (V2G) technology is also significant in this context, as it allows electric vehicles to function as distributed energy storage, thus enhancing peak load management and grid stability. The concept of average power is also relevant here; the average power required in many scenarios is typically much lower than the peak demand. This allows for the specification of smaller and more cost-effective power supplies without compromising reliability.

Summary

Rated load is the maximum amount of power a supply can deliver without interruption while operating within environmental limits. Peak load is the maximum power that a supply can provide without damaging the supply or putting its lifetime expectancy at risk. It can be as much as two to three times the maximum continuous load. Peak load depends on the maximum duration and duty cycle for the peak. Peak loads come from inrush current at startup, motor start current spikes, and unconventional conditions that may occur in a power load over and above steady state draw. Power supplies are designed to handle load peaks so that a smaller supply unit can provide long-term reliable service.

Systems designers must match not only equipment continuous load to their power supply, but also peak loads, peak load duration, and time between peaks. This may take additional time with both supply and load equipment data sheets, but the information is critical for lifetime expectancy and reliability. Energy-efficient appliances play a significant role in managing energy consumption and reducing peak load demands.
Applications
  Series
1 AC/DC, 480 W, Single Output, DIN-Rail RACPRO1-T480 Series
Focus New
  • Slim Design (52mm) with 25° Push-In connectors
  • Fast tool-less mounting and demounting
  • PFC >0.9 and Active Inrush Current Limitation
  • DC-Input Range 430V to 815V/850V 10s