How to Calculate Energy Savings When Upgrading to a High-Efficiency Motor

There is only one sound way to understand whether switching to an efficient motor makes sense: do the math. The sentence "a high-efficiency motor saves energy" is true but abstract; the real decision is made by seeing how many kilowatt-hours and how much money that saving amounts to. Fortunately, this calculation is far simpler than it looks, and you only need a few pieces of information: the motor's power, the annual operating hours, the efficiencies of the old and new motors, and the electricity tariff. In this article we show, step by step, how to calculate the annual energy saving when switching to an efficient motor, put it into concrete figures with a real example, and explain why the payback period depends so heavily on operating hours, from DRG's energy-focused perspective.

What Exactly Is Motor Efficiency?

A motor's efficiency is the ratio of the mechanical power it produces to the electrical power it draws. In other words, the higher the efficiency, the less electricity the motor draws to do the same work. The difference is wasted as heat, friction and electrical losses. Reducing this wasted portion is the very foundation of switching to an efficient motor. We examined where and how efficiency is lost in detail in our article on electric motor efficiency losses.

Efficiency Classes: IE1, IE2, IE3, IE4

Motors are classified according to their efficiency. IE1 represents the lowest and IE4 the highest efficiency class. When an IE1 and an IE3 motor of the same power are compared, the efficiency difference between them may be a few points. This difference looks small but turns into a large energy difference in a motor that runs all year. We addressed the importance of efficiency classes in our article on high-efficiency electric motors.

The efficiency difference also varies with the motor's power. In low-power motors, the gap between classes is more pronounced, because the ratio of losses to total power is already high in small motors. In large motors, although the percentage difference shrinks, the saving is still considerable because the absolute power is large. So there is separate logic to switching to an efficient class in both small and large motors; only the result of the calculation differs.

Why Does the Power Drawn Matter?

The key point to understand here is this: the power written on a motor's nameplate (for example 30 kW) is the mechanical power it delivers to the shaft. The electrical power it draws from the grid is more than this, because there are losses. The electrical power drawn is found by dividing the mechanical power by the efficiency. The lower the efficiency, the more electricity drawn for the same 30 kW of work.

The Logic of the Saving Calculation

When comparing two motors, we assume both do the same mechanical work. The difference appears in the electricity they draw while doing that work. The old motor draws more because it is low efficiency; the new motor draws less because it is high efficiency. The difference between these two consumptions is your annual saving.

The nice thing about this logic is that it involves no performance compromise. The new motor does the same work, at the same speed, with the same torque; it simply achieves it while spending less electricity. So the saving is achieved not by cutting production but by cutting waste. This makes the efficiency investment one of the lowest-risk kinds of investment, because your gain rests not on production conditions but on the laws of physics.

The Basic Formula

Annual energy saving is calculated with the following formula:

Annual Saving = P × t × (1/η_old − 1/η_new) × Tariff

Here P is the motor's mechanical power (kW), t is the annual operating hours, η_old and η_new are the efficiencies of the two motors (as decimals, e.g. 0.88 instead of 88%), and Tariff is the electricity price per kilowatt-hour. The heart of the formula is the expression "1/η_old − 1/η_new" in parentheses; this gives the difference in power drawn by the two motors for the same work.

A Step-by-Step Example Calculation

Let us go through a concrete example. We have an old IE1 motor with a power of 30 kW, and we are considering replacing it with a new IE3 motor.

• Mechanical power: P = 30 kW
• Old motor efficiency: η_old = 0.90 (90%)
• New motor efficiency: η_new = 0.936 (93.6%)
• Annual operating hours: t = 6000 hours
• Electricity tariff: 0.12 per kWh (example value)

Let's Find the Power They Draw

The electrical power drawn by the old motor: 30 / 0.90 = 33.33 kW. The power drawn by the new motor: 30 / 0.936 = 32.05 kW. The difference: 33.33 − 32.05 = 1.28 kW. So while doing the same work, the new motor draws 1.28 kW less electricity every hour.

Annual kWh Saving

The hourly difference of 1.28 kW accumulates over the year: 1.28 kW × 6000 hours = 7,680 kWh. This is the electricity saved from a single motor, with no loss of performance, purely thanks to the efficiency difference.

Annual Money Saving

Now let us convert this kilowatt-hour figure into money: 7,680 kWh × 0.12 = 921.60 per year. So this single motor provides an energy saving of roughly 920 per year. If you consider that your facility has dozens of motors, you can imagine how large the total picture can become.

Summary Table of the Example Calculation

Payback Period

After finding the saving, calculating the payback period is very simple: you divide the additional cost of the new motor by the annual saving. Let us say the new IE3 motor is 1,200 more expensive than its old counterpart. Payback period: 1,200 / 920 ≈ 1.3 years. So the motor pays for itself in less than a year and a half; all subsequent years are net gain. We addressed this topic in depth in our article on the high-efficiency motor payback period.

The Decisive Effect of Operating Hours

The "t" in the formula, the annual operating hours, directly enlarges or shrinks the saving. In a motor that runs only a few hours a day, the monetary value of the efficiency difference stays small and the payback can take years. By contrast, in a motor running around the clock, the same efficiency difference brings a large saving in a short time. That is why the decision to switch to efficient motors should start with the motors that run the most.

How Many Hours Count as "A Lot"?

As a general rule, switching to a high-efficiency model is almost always profitable for motors running more than 4000 hours a year. In the 2000–4000 hour range, the decision depends on power and the efficiency difference. For motors running below 2000 hours, the payback can lengthen; still, if the old motor is very low efficiency, it is worth doing the math.

Also Factor In the Power Factor

Efficiency alone is not the whole picture. A motor's power factor shows how much of the apparent power it draws actually turns into work. A low power factor leads to unnecessarily high current being drawn from the grid and, in some cases, a reactive power penalty. High-efficiency next-generation motors usually also have a better power factor; this is an extra gain reflected in the bill. When doing the saving calculation, where possible, do not forget the benefit brought by an improved power factor.

Heat and Cooling Cost

An inefficient motor releases the energy it loses into the environment as heat. In motors operating in an enclosed area, this heat warms the space and causes cooling systems to work harder. So a low-efficiency motor increases not only the direct electricity bill but also, indirectly, the cooling cost. When you switch to a high-efficiency motor, this hidden cost also decreases. Although this item is not included in the simple saving calculation, it enlarges the real gain a little more.

Don't Forget the Load Ratio

In doing the calculation, we assumed the motor runs at full load. In reality, many motors run at a load below their power. The efficiency curve changes with the load ratio; in most motors, the highest efficiency is achieved at around 75 percent of rated power. A motor running at very low load is both inefficient and mis-sized. Correct power selection is a precondition for saving.

The Cost of Mis-Sizing

A motor selected much larger than needed runs constantly at low load and low efficiency. This means both paying too much when buying and spending too much energy over its lifetime. When doing the saving calculation, you also need to take the motor's real load profile into account; sometimes the solution is to determine the correct size before replacing the motor.

The Effect of the Tariff

As the electricity price rises, the return from switching to an efficient motor also rises. The same kWh saving corresponds to more money at a higher tariff. In an environment where energy costs are continuously rising, an efficiency investment made today becomes even more valuable in the future. That is why it is more realistic to do the calculation not on today's tariff but on an average future tariff.

In many facilities, the electricity tariff changes during the day; different pricing may apply for day and night. If the motor runs mainly during expensive tariff hours, the monetary value of the efficiency difference grows even more. For this reason, taking into account not only the average tariff but also the hours during which the motor runs makes the result more realistic.

The Life Cycle Cost Window

The purchase price of a motor is a small part of its total cost. In most industrial motors, energy makes up more than 90 percent of the lifetime cost. That is why a "cheap motor" is often the most expensive option. You need to think of the saving calculation in a way that covers the motor's entire lifetime; we elaborated on this in our article on motor life cycle cost.

Multiplying the Saving with a Drive

Switching to an efficient motor is powerful on its own, but in variable-load applications the saving is multiplied when combined with a frequency inverter. Especially in applications such as pumps and fans, adjusting the speed to the load provides a large energy gain. We examined this topic in our article on frequency inverter energy saving.

Rewind the Old Motor or Replace It?

Rewinding a burnt motor keeps it in the old efficiency class; at best it stays as IE1. Yet switching to a new IE3 motor starts the saving we saw in the calculation above. Instead of expensively rewinding an old, low-efficiency motor, this simple calculation often shows that replacement is far more profitable in the long run.

The Difference in Maintenance Cost

High-efficiency motors are usually built with better materials and workmanship; this means less heating, longer insulation life and less frequent failure. This maintenance gain, often overlooked when doing the saving calculation, quietly enlarges the total saving. Less downtime, fewer spare parts and less labour; all of these are a bonus added on top of the energy saving.

Common Mistakes in the Saving Calculation

The most common mistake is to take the motor's nameplate power as the consumption directly; in fact, consumption is found by dividing the nameplate power by the efficiency. The second common mistake is to underestimate the operating hours; this makes the payback period appear longer than it is. The third is to look only at the purchase price and ignore the lifetime energy cost. Avoiding these three mistakes is enough to get the calculation right.

Do the Calculation with Your Own Data

The example values here are representative. When making the real decision, use your own motor's nameplate information, measured operating hours and current electricity tariff. The formula is the same; you just need to plug in your numbers. The result often shows that switching to an efficient motor pays for itself far faster than you think.

Effect on the Carbon Footprint

Energy saving also has an environmental dimension. Every kilowatt-hour saved is electricity that does not need to be produced; this means less carbon emission. In the example above, saving 7,680 kWh per year from a single motor corresponds to a considerable carbon reduction. Thought of at facility scale, switching to efficient motors is a strong step both economically and in terms of sustainability. For many businesses, this is not just cost but also a concrete way of reaching their environmental targets.

Scale in Industrial Applications

In continuously running heavy-industry applications such as stone crushing, pumping and ventilation, the return from the efficiency difference is highest, because these motors run for most of the year. You can examine DRG's high-efficiency motors for such applications in our article on industrial electric motors.

Put the Saving into Figures with DRG Motor

The value of switching to an efficient motor becomes visible only with a concrete calculation; and for most businesses, this calculation reveals a far shorter payback period than expected. At DRG, alongside our high-efficiency next-generation motors, we help you produce a saving calculation tailored to the real operating hours, load profile and current tariff of the motors in your facility. Get in touch with us to calculate together which motor you should start with, what your payback period will be, and how much you will save annually. DRG Motor stands by you with motor solutions that let you manage your energy costs with figures.