Will AC Efficiency Skyrocket with Inverter/Variable-Capacity Technology?

Do inverter-driven compressors use 50% less energy?

Various HVAC manufacturers claim inverter technology cuts energy use anywhere from 50% to 64% compared to single-stage compressors. Are these just wild marketing claims or are they research-proven?

The latter.

Air conditioners and heat pumps with an inverter-driven compressor dramatically reduce energy use. The tradeoff is a higher price tag for the unit.

Is an inverter-driven AC or heat pump worth the cost?

How soon will lower energy bills pay for the high-priced equipment? The payback period varies from a few years to “never” based on how heavily the AC or heat pump is used.

Indoor comfort is another factor to consider along with cost. Many homeowners want the precise temperatures and better humidity control inverter systems offer regardless of price.

What ACs and Heat Pumps Have Inverter Technology?

Inverter technology was developed for mini split air conditioners and heat pumps in the 1970s and 1980s. Almost all mini split systems from all leading brands have inverter-driven compressors. It is the industry norm.

In about 2013, a few brands began introducing standard AC and heat pump models that included inverter technology. Carrier Greenspeed was among the first, followed by units from Lennox, Trane/American Standard and Bryant (Carrier’s sister brand). The technology is also called variable-capacity, variable-speed and modulating when applied to standard split systems. The industry has also begun to use the same terminology – inverter-driven – as in the mini split market.

Inverter technology is not yet the norm for standard split systems. Most brands still produce single-stage and two-stage models too.

Note for clarification: You’ve likely noticed that we are using the term standard split system to refer to full-size central ACs and heat pumps. We use mini split to refer to smaller units that don’t require ductwork but do require one or more indoor air handlers to be installed in zones.

What is an Inverter AC?

An air conditioner or heat pump fitted with an inverter-controlled compressor is one that varies its output from about 40% to 120% of rated capacity.

That explanation needs unpacking.

There are three compressor options for central air conditioners and heat pumps. A compressor regulates the flow of refrigerant. The faster the flow, the more heat is moved from inside to outside in summer.

Single-stage Compressors (standard split systems only):

The single stage is 100% power. The refrigerant is cycling as fast as it can. 100% on, then off. 100% on, then off. Up to 18 times a day.

If you drove your car as fast as it would go for 5 miles, then turned off the ignition and coasted to a stop, and then repeated that cycle, your gas mileage would be terrible.

When your AC runs that way, it uses a lot of electricity. Plus, it cools a few degrees past the thermostat point because it’s running at 100% when it hits it. All the cold air still in the system is blown into your home. These are reasons most single-stage models are 13 SEER to 15 SEER, though a few are more efficient.

Two-stage Compressors (standard split systems only):

The two stages or refrigerant flow speeds are 65% (stage one) and 100% (stage two) in most models. They run on stage one most of the time. This is more efficient and maintains better temperature balance than a single-stage model.

Two-stage compressors kick into stage two when a significant boost in cooling is required by quickly rising outdoor temperatures that heat up your home or when you drop the thermostat setting by 4+ degrees.

65%, 100%, off. Repeat. Using the driving analogy: Drive fast. Drive very fast. Coast to a stop. Repeat. Usually 8-12 times a day. Cycles are a little longer, but still quite frequent.

While more efficient than single-stage compressors, there’s still a more efficient way: Inverter technology.

Inverter-driven Compressors:

Power is sent directly to the compressor in single-stage (100% power) and two-stage (65% or 100% power) models.

When inverter technology is used, electricity is sent to an inverter first. The inverter regulates its frequency – how much of the electric power reaches the compressor. In turn, that regulates the speed of the refrigerant flow.

In most units, it can be as low as 35% or 40%. When there is heavy demand for cooling, it can deliver as much as 120% of its rating. For example, an 18,000 BTU (1.5 ton) mini split outdoor unit is capable of running at about 6,500 BTUs indefinitely and at about 22,000 BTUs for a short time.

Arizona contractor Magic Touch Mechanical explains the advantage of variable capacity cooling in a very warm climate, “In Phoenix, Arizona you may need a 5-ton unit to handle the load in mid-July when it’s 110F outside, but what about in September when it’s only 90 degrees? You still need to run your cooling system but maybe the load is now only 3.2 tons. With an inverter driven variable speed compressor you only use what you need, thereby saving you money by not consuming energy you don’t need.”

The inverter difference: An inverter-driven compressor rarely shuts off, so it is always ready to speed up or down to match heating or cooling demand. It adjusts in increments of 1% or less to exactly match cooling requirement. Cycles are slow, vary slightly as needed and are continuous.

This makes them very efficient. And temperatures are very balanced. Other advantages are discussed in the pros and cons below.

To summarize:

  • Single-stage compressors: Cycles are short but frequent. Temperature swings are noticeable.
  • Two-stage compressors: Cycles are a little longer, but still quite frequent. Temperature swings might be noticeable
  • Inverter-driven compressors: Cycles are slow, vary slightly as needed and are continuous. It’s unlikely you’ll notice temperature changes.

If you’d like more details on the technology behind inverter compressors, this post by industry leader Daikin has plenty.

Inverter Air Conditioner Pros and Cons

Here’s a review of those we’ve covered and a few more advantages and disadvantages of inverter-driven compressors.

Pros:

  • Higher efficiency: Efficiency is two to three times higher for converter-driven ACs and heat pumps than for single-stage and two-stage standard split system models. Details below.
  • Comparable to geothermal: With efficiency rising with each generation, inverter-driven systems rival geothermal heat pumps in reducing energy consumption.
  • Value: Since they cost far less, inverter split systems, especially mini split heat pumps, are a better value than most geothermal systems.
  • Indoor heat and humidity control: Inverter-driven heating and cooling delivers temperature balance that is superior to other types. When cooling, the longer, consistent cycles remove more humidity from warm-weather air. This means you’ll feel comfortable at a higher temperature, which leads to further energy savings.
  • System options: Inverter technology is available in mini split and standard split, so you have size and installation options.

Cons:

  • Cost of standard split systems: Standard split system air conditioners and heat pumps with inverter-controlled compressors cost 25% to 55% more than single-stage and two-stage systems of the same size.
  • Cost of multi-zone mini split systems: Multi-zone systems require 2 to 8 indoor units with separate installation charges for each zone. The cost can rise to 100% higher than a single-stage or two-stage split system. See the table below for details
  • Communicating technology: Some installers push communicating technology on standard split systems with inverter technology. At this time, we are not recommending this technology. The issue is discussed here. If you don’t have time to read the link, here’s a quote from one of our readers regarding his communicating system. Paul says, “I have a York system with an Affinity thermostat. I get frequent communication fault messages. The AC techs don’t have any good tools to troubleshoot communication problems and York isn’t any help…I am an electronics engineer who works with instrumentation and controls. My advice is to stay away from systems with communicating thermostats until the bugs are worked out of the designs.

Efficiency Comparison

ACs and heat pumps with converter-controlled compressors have efficiency ranges from about 16 SEER to 38 SEER compared to 13 to 18 SEER for non-converter units. This chart sorts it out.

System TypeCompressorEfficiency Range
Standard SplitSingle-stage13-18 SEER
Standard SplitTwo-stage16-20 SEER
Standard SplitConverter-driven19-26 SEER
Mini SplitConverter-driven17-38 SEER

Cost Comparison

We’ve said there’s a cost premium for the efficiency and indoor comfort that comes with inverter technology. But how much more does it cost?

Here’s a comparison of common HVAC systems, both inverter and non-inverter types. The mini split systems use converter technology. We priced the mini split systems as single-zone and as multi-zone options.

All prices in the table are for a 3-ton/36,000 BTU system.

Heat Pump System TypeAverage Installed Cost
Single-stage with air handler$5,850 - $6,435
Two-stage standard with air handler$6,330 - $7,550
Inverter-driven standard with air handler$7,385 - $9,665
Mini split with one indoor unit$6,700 - $8,135
Mini split with 2-4 indoor units$7,580 - $10,650

Payback Period – Is it Worth It?

The payback period is the time required to recover higher equipment costs through lower energy bills.

For example, if you spend $100 per month on electricity for your 15 SEER AC, the cost will be about $63 with a 24 SEER unit and just $50 with a 30 SEER AC.

That translates into $37 savings for the 24 SEER system and $50 savings for the 30 SEER unit.

15 SEER vs. 24 SEER: If the 24 SEER system cost you $2,500 more than a 15 SEER system, it would take approximately 5 years, 7 months to recoup the money at $37 per month savings. Go for it!

15 SEER vs. 30 SEER – Single Zone: If you spent $1,750 more on the 30 SEER single-zone mini split, your payback period would be just 35 months – a month short of 3 years. But there is one important issue here. A 3-ton single-zone system would be suitable for one large room. If your home has a very open floor plan, it might work for all but the bedrooms. Most homes require multi-zone mini split systems to maximize comfort throughout the house.

15 SEER vs. 30 SEER – with 4 Zones: You might spend $4,000 more on a high-SEER multizone mini split system. At $50 savings per month year-round, the payback period would be more than 6 years, 8 months. That’s still a relatively short time when you consider a heat pump lasts up to 20 years.

When It’s Not Worth It: If you live in a very moderate climate, your savings might be less than $15 per month. This will make your payback period much longer, so might not be worth it. For example, you won’t recoup $4,000 in the life of the system at $10 per month.

Which Inverter System is Best for You?

There isn’t an answer that works for every situation.

Here are several common scenarios that can help you decide.

New Construction:

Your options are wide open. A zoned mini split system will cost significantly more for the equipment and installation. Having an indoor unit (air handler) in each of the four zones is slightly louder too.

However, you won’t have the cost of ductwork installation ($1,000 to $2,500 depending on the size of the home and number of levels). And you won’t have the cost of wasted heating and air conditioning that escapes ducts that can get leaky with age.

Secondly, if you choose a high-SEER mini split, you might cut heating and air conditioning costs by as much as 33% compared to a standard split system with inverter technology.

Our suggestion is to get several estimates on each type system so you can compare them head to head for cost, efficiency, sound levels and other features. If price isn’t the biggest concern, choose an installer you believe will properly install the system, a vital aspect of how it will run in the years ahead.

Existing Home with Ductwork:

A standard heat pump system attached to your ductwork will cost significantly less than a zoned mini split system with 4-8 indoor units, each $650-$900+ to install. Your efficiency might be lower, based on the systems you’re comparing. That’s something to consider. If you decide you want maximum efficiency, consider a ducted mini split system. The technology is the same, but the indoor units are hidden in existing ductwork near the vents.

Existing Home Without Ductwork:

Homes built with boiler systems don’t have ducts. If the boiler system is being replaced with a heat pump system, a mini split ductless system is your best option. Adding ducts to an existing house is very costly, invasive, messy and time-consuming.

Large single zone without ductwork:

Finishing a basement or attic? Turning a garage into living space? Adding a homeowner bedroom (aka master suite)? A single-zone mini split system is a great choice.

1 comment… add one
  • Very informative! I’m interested in the statement about “inverter driven” AC units exceeding stated BTU nominal listings when demand indicates…for periods of time. “When inverter technology is used, electricity is sent to an inverter first. The inverter regulates its frequency – how much of the electric power reaches the compressor. In turn, that regulates the speed of the refrigerant flow.
    In most units, it can be as low as 35% or 40%. When there is heavy demand for cooling, it can deliver as much as 120% of its rating. For example, an 18,000 BTU (1.5 ton) mini split outdoor unit is capable of running at about 6,500 BTUs indefinitely and at about 22,000 BTUs for a short time.”

    Is this normal for inverter type compressors…are Trane units equipped to do this. I’m borderline between a 2.5 and 3.0 ton unit. Maybe I can stay with 2.5 ton if on very hot days inverter exceeds 30K BTU’s. Your thoughts are appreciated.

    Reply

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