If you've been shopping for a new microwave, you may have seen the word "inverter" emblazoned on the front panel of many higher-end units. Manufacturers claim that an inverter microwave produces more evenly heated results, is more efficient, and is gentler with delicate foods. As part of our recently launched Microwave v1.0 Test Bench, we set out to find what exactly inverter technology is and if you should pay a premium for it. In short, our extensive testing has shown that although an inverter circuit is a real and distinct feature, its usefulness in this context is purely as a marketing tool in a product category that otherwise lacks significant differentiation.

Our test bench was designed from the outset to highlight performance differences between the seven models we purchased with inverters and the 19 models we purchased without inverters. The "deep dish heating uniformity" tests were specifically included for this reason. However, as we discuss in an accompanying article, all the microwaves we tested have equivalent heating performance; the evenness in heating is controlled by the food and physics, not by the appliance.

We did measure that inverter microwaves are slightly more efficient at the electrical circuit level, but you'll need to be an electrical engineer to appreciate it! The inverter models are marginally better at converting the total power they draw from an outlet (the apparent power) to useful work (the active power). However, your residential electricity bill is calculated based on active power use, so an inverter microwave won't save you any money.

We put in a lot of effort to test the claims that an inverter is necessary to effectively microwave "delicate" foods. We tried all the typical tasks manufacturers and users assert are sensitive to overheating by microwaves: cooking eggs, melting chocolate, softening butter, and defrosting meat. Ultimately, all microwaves can perform these tasks because of the physics of heating with microwaves. Regardless of the inclusion of an inverter circuit, a microwave oven only heats the outermost few centimeters of food, and each area is heated for short periods as the food is rotated relative to the standing wave.

We had to search very hard to find an exception where a microwave with an inverter performed a task that a non-inverter microwave couldn't! It turns out that inverter microwaves are better at tasks that require such little time that a non-inverter microwave can't effectively modulate power. The extreme example we found is the softening of 2 tsp of cold butter; an inverter microwave set to 30% power performs this task very well, while a non-inverter microwave yields melted butter.

We invite you to continue reading to learn more about microwaves and the testing we did to conclude that an inverter doesn't need to be a feature of your next microwave!

How Does a Microwave Oven Work? What Is an Inverter Microwave?

The microwave oven has become ubiquitous in kitchens across the world. And their basic design hasn't changed much since they were first introduced to consumers in the 1960s. As shown in the schematic on the left of the following figure, at its most basic, a traditional microwave consists of:

• Power Supply: TRIAC (switch), transformer, and rectifier, which provide high-voltage DC power
• Magnetron: vacuum tube that produces microwaves
• Cooking Chamber: metal box which reflects and contains the microwaves, leading to the formation of a stationary "standing wave"
• Turntable: moves food relative to the standing wave for more even heating

As food is exposed to the standing microwave within the cooking chamber, polar molecules (water, fat, sugar) align themselves to the electromagnetic field, and the friction of this movement creates heat. Energy (and, therefore, the ability to heat food) isn't evenly distributed within the three-dimensional space of the cooking chamber. This is because a wave has no energy at its nodes and max energy at anti-nodes.

The DC power supply and magnetron in a traditional microwave are only ever in one of two states: fully on or fully off. The power (i.e., energy over time) that's used to heat food is attenuated through "pulse width modulation" (PWM). In this control scheme, which is the basis of operation of all non-inverter microwaves (hereafter referred to as "PWM microwaves"), the circuit is pulsed on and off with a set period. The percentage of each period in which the magnetron is powered and microwaves are being produced is known as the duty cycle. It's interesting to note that the period of all our PWM microwaves is measured to be around 30 seconds and constant (i.e., it doesn't change with the input cook time).

As shown in the previous figure, an inverter microwave is like the traditional PWM microwave except for its upgraded power supply. The addition of MOSFET transistors in a high-frequency switching circuit allows for the generation of variable high-frequency AC power. This power is sent through a high-frequency transformer and rectifier to create variable high-voltage DC power. In the end, the power supply and magnetron of an inverter model can output microwaves of prescribed power (i.e., not just on/off).

Since Panasonic patented the inverter circuitry for microwaves, they and their licensees have marketed these appliances as being able to:

• More Evenly Heat Food: Always on microwaves of prescribed power
• Heat 'Delicate' Foods: Foods don't have to be exposed to 100% power like in a PWM microwave.
  • Melting chocolate
  • Cooking eggs
  • Softening butter
  • Defrosting meat
• Lower Power Consumption: Smaller, more efficient transformer

And indeed, these claims are often repeated by consumers in forums throughout the internet. However, we found it interesting that inverter microwaves also use pulsing of power in some circumstances. Namely, as shown in the following graph, we observed that an inverter microwave will cycle on and off when it's set to less than 30% power. We suspect this is because the magnetron can't operate at such low power inputs. We also noted that power pulsing features heavily in an inverter microwave's automatic defrosting routine; evidently, low steady power is insufficient for effective defrosting.

An Inverter Microwave Doesn't Yield More Evenly-Heated Food

From our description of how microwaves operate, it should be clear that, when set to 100% power, both PWM and inverter microwaves behave exactly alike. We, therefore, included the heating of food at lower power levels (chilled mashed potatoes at 50% power) in our test bench to elucidate any performance differences between the two types of microwaves. The theory/marketing behind including an inverter circuit in a microwave is that the constant, adjustable output of microwave power will result in more evenly heated food than the PWM power control mode.

The "deep dish" heating evenness test involved heating mashed potatoes chilled overnight in 1 L glass food storage containers. The containers of mashed potatoes were placed midway between the center and edge of the microwaves' turntables. We set the total microwave energy to heat the mashed potatoes at 350 kWs (so a lower-power microwave has a correspondingly longer cooking time, and the tests with 50% power took twice as long as the tests with 100% power in the same appliance). We then used an array of probes to measure the spatial temperature distribution at 26 points within the reheated mashed potatoes.

As shown in the following figure, our test results are remarkably similar when we control for total microwave energy. That is, an inverter circuit doesn't allow a microwave to produce mashed potatoes that are either more efficiently heated (i.e., with a higher mean temperature) or more evenly heated (i.e., with a smaller standard deviation in temperatures).