Choosing the Right Efficiency Class by Running Hours

There is no single right answer to the question "Which efficiency class motor should I buy?"; the right answer depends on how much the motor runs. The most sensible efficiency class for a motor running a few hours a day is completely different from one running 24/7. In this article we explain step by step how to choose the right efficiency class based on running hours, electricity price and load profile.

To learn what efficiency classes are and the differences between them, see the electric motor efficiency classes article; here the focus is entirely on choosing the right class.

Why Are Running Hours Decisive?

A high-efficiency motor is more expensive than a low-efficiency one but consumes less energy. Paying back this extra cost depends on how much the motor runs. The more the motor runs, the more energy it consumes, and the greater the savings a high-efficiency motor provides.

This is why running hours is the most critical variable in choosing an efficiency class. While switching to IE4 or IE5 pays back very quickly in a motor running 20 hours a day, the same investment may not pay back for years in a motor running a few hours a week.

Low-Use Motors

In motors running only a few hours a day or seasonally, because energy consumption is low, the savings provided by a high efficiency class are also limited. In such applications, IE3 is usually sufficient and the most economical option. The extra cost paid for IE4 or IE5 pays back over a very long time due to low usage.

For example, a backup pump, an occasionally running crane or a seasonal irrigation motor fall into this group. In these applications, IE3 is the most sensible choice in terms of both legal compliance and cost.

Medium-Use Motors

Motors running around 8-12 hours a day are in a transition zone in terms of efficiency. While IE3 is often a balanced choice in these applications, switching to IE4 can become sensible if the electricity price is high. The decision varies depending on the motor's power and the electricity unit price.

In this zone, doing a simple payback calculation clarifies the right decision. If the extra cost of IE4 pays back within a reasonable time (for example 2-3 years), IE4 can be preferred; otherwise, IE3 is sufficient.

Continuously Running Motors

Motors running 16-24 hours a day, that is, almost without interruption, are the group that benefits most from a high efficiency class. Because energy consumption is very high in these applications, the savings provided by switching to IE4 or IE5 are very large, and the extra cost usually pays back within months.

Water and wastewater pumps, ventilation systems, compressors and continuous production lines fall into this group. In such applications, choosing the highest efficiency class is almost always the most profitable decision.

The Role of Electricity Price

Besides running hours, the electricity unit price also directly affects the decision. The more expensive electricity is, the more valuable the savings provided by an efficient motor, and the shorter the payback period. Rising energy prices make switching to a high efficiency class more attractive every day.

For this reason, when doing a payback calculation, it is wise to consider not only the current electricity price but also possible future increases. Expensive electricity makes IE4 and IE5 profitable faster.

Load Profile and Efficiency

How loaded the motor runs is also important in the choice. Motors usually deliver their highest efficiency when running close to full load; at very low load, efficiency drops. This is why, in an application constantly running at low load, a high efficiency class alone is not enough; correct sizing is also needed.

In variable-load applications, using a frequency drive significantly increases efficiency. Correctly analyzing the load profile is the key to determining both the right class and the right operating method.

A Simple Payback Calculation

The most practical way to choose the right class is a simple payback calculation. First, the extra cost between the two motors is determined. Then, the annual energy savings the efficient motor will provide is calculated: this depends on the efficiency difference, the motor's power, the annual running hours and the electricity price. Dividing the extra cost by the annual savings gives the payback period.

If the payback period is much shorter than the motor's expected life, switching to a higher class is profitable. For the details of this calculation and examples of replacing an old motor, see the replacing old motors article.

Which Class Suits Which Situation?

A general roadmap can be summarized as follows: IE3 is sufficient in low-use, low-priority applications. In medium-use applications with a reasonable electricity price, IE3 is preferred, or IE4 if the price is high. In continuously running applications with high energy cost, IE4 or IE5 is the most profitable option.

This table is a starting point; the final decision is clarified by the payback calculation.

Is IE3 Enough?

IE3 is the legal minimum standard and a balanced choice for many applications. In low and medium-use applications, IE3 is usually sufficient. For the details of IE3 and in which situations it is enough, see the IE3 premium efficiency article.

However, if the motor runs a lot, going above IE3 provides significant savings. In this case IE4 comes into play and usually pays for itself in a short time.

When to Switch to IE4?

IE4 stands out in applications with high running hours and important energy cost. In a motor running more than 12 hours a day, the extra savings IE4 provides usually cover the initial cost difference in a short time. IE4 is a very balanced higher option between efficiency and cost.

For the advantages of IE4 and the payback logic, see the IE4 electric motor article. In motors that run a lot, IE4 is almost always a sensible investment.

When Does IE5 Make Sense?

IE5 is the most profitable option in applications where energy cost is very high and motors run without interruption. Especially in pump, fan and compressor systems where a frequency drive is already needed, IE5 provides the lowest energy consumption with ultra premium efficiency.

For the details of IE5 and synchronous reluctance technology, see the IE5 ultra premium motors article. In very heavily running applications, the extra cost of IE5 pays back quickly through the savings it provides.

Don't Forget Correct Sizing

A topic as important as the efficiency class is choosing the motor at the right power. Even the highest efficiency class does not provide the expected savings if it is oversized, because the motor constantly runs at low load, in its least efficient region. This is why correct sizing should be done together with the class choice.

The ideal is to choose the motor so that it runs in the region where its typical load is most efficient. When the right class and the right power come together, the highest savings are achieved.

Factor in the Frequency Drive

In variable-load applications, the operating method is as important as the efficiency class. A frequency drive provides large savings by adjusting the motor's speed to demand; especially in pump and fan loads, because power is proportional to the cube of speed, this saving is very large. The combination of the right class and a drive offers the lowest energy consumption.

For this reason, when making a choice, you must evaluate not only the motor's class but also how it will be operated. An efficiency-class decision considered together with a drive provides the highest return.

Deciding by Total Cost

The right decision is made by looking at the total cost of ownership, not the purchase price. In a continuously running motor, the vast majority of the lifetime cost comes from energy consumption. For this reason, a difference of a few points in efficiency can turn into savings many times greater than the purchase price over the motor's lifetime.

Choosing the right class by running hours means exactly minimizing this total cost. You can evaluate all options in the high efficiency motors section and get support from the DRG Motor team to make the most suitable decision for your application.

Sectoral Examples

A concrete example clarifies the decision. Consider a large water pump running 22 hours a day; here, switching to IE5 pays back the extra cost very quickly because of the high running hours. By contrast, for a workshop crane running 2 hours a day, IE3 is more than enough; the difference paid for IE4 would not pay back for years.

The continuously running fans in a textile factory or the conveyors in a mining plant are the applications that benefit most from a high efficiency class. These examples show that the decision must always be made according to the application's operating profile.

Evaluating the Existing Motor

When buying a new motor, it is important not only to choose a class but also to evaluate existing motors. Knowing which motors in a facility run how much and consume how much energy determines which ones should be renewed first. The most heavily running and oldest motors are the candidates that pay back fastest from renewal.

This evaluation can be done with a simple energy audit. Focusing first on the most heavily running motors is the smartest way to achieve the highest savings with a limited budget.

Budget and Prioritization

Renewing all motors at once may not always be possible. In this case, prioritization comes into play: the most heavily running, highest-power and lowest-efficiency motors are placed at the top of the list. When these motors are renewed, the savings provided can finance the next investment.

In some countries, there are also incentives supporting the switch to an efficient motor; these supports shorten the payback period even further. Managing the budget correctly makes the efficiency investment sustainable.

Efficiency Class and Maintenance Plan

As important as choosing the right efficiency class is preserving that efficiency. Even a high-efficiency motor loses its performance with poor maintenance. This is why a maintenance plan should be considered together with the class choice: regular bearing checks, cleaning and correct alignment allow the motor to maintain its rated efficiency for years.

In continuously running critical motors, periodic maintenance should not be neglected. Good maintenance guarantees that the savings provided by the right class choice are permanent.

Common Mistakes When Deciding

There are a few common mistakes in choosing an efficiency class. The first is looking only at the purchase price and ignoring the energy cost; this leads to large losses in motors that run a lot. The second is buying an oversized motor "just in case"; this destroys the efficiency advantage. The third is deciding without taking running hours into account.

To avoid these mistakes, a simple payback calculation and correct sizing are enough. A conscious decision prevents both overpaying and missing the savings opportunity.

Expert Support and Energy Audit

In complex facilities, an energy audit is very useful for determining which motor should be in which class. This audit reveals the most correct decision by measuring the motors' real running hours, loads and consumption. A choice made with correct data is far more profitable than decisions based on guesswork.

To determine the most suitable efficiency class for your application together, you can get support from the DRG Motor team and review all high-efficiency options in the high efficiency motors section.

Frequently Asked Questions

What is the most important factor in choosing an efficiency class? The motor's annual running hours; the more the motor runs, the faster a high efficiency class pays back.

Which class is enough for a low-use motor? Usually IE3 is sufficient and the most economical option; the extra cost of IE4/IE5 pays back over a long time at low usage.

Which class for a continuously running motor? IE4 or IE5 is the most profitable option in applications running 24/7; the extra cost usually pays back in a short time.

How is the payback period calculated? The extra cost between two motors is divided by the annual energy savings of the efficient motor.

How does electricity price affect the decision? The more expensive electricity is, the more valuable the savings and the shorter the payback period.

Is the class alone enough? No; correct sizing and using a frequency drive when needed are as important as the efficiency class.

Finding the Right Class

The secret to choosing the right efficiency class lies in a single question: how much does the motor run? While IE3 is enough in low-use applications, IE4 or IE5 pays for itself very quickly in continuously running applications with high energy cost. Electricity price, load profile and correct sizing also shape the decision. A simple payback calculation clearly reveals which class is most profitable for you, so that you neither overpay nor miss the savings opportunity.