Definition
An inverter is an electronic device that turns DC (direct current, like battery power that flows in one direction) into AC (alternating current, like wall-outlet power whose direction flips back and forth). The inverters inside home appliances also adjust the strength and speed (frequency) of that electricity, so the motor spins with just the right force and uses less power.
A smooth gas pedal instead of on and off
Imagine driving a car and turning the engine off and on again and again just to hold your speed. You'd waste a ton of gas. A smart driver presses the gas pedal lightly or deeply and keeps the power smooth. That saves fuel and the car lasts longer.
Older fixed-speed air conditioners and refrigerators worked like that stop-and-start car. When the room got warm, the motor roared at 100% power. When the room got cool, it shut off completely. Running full blast and then stopping uses more electricity than running at low power steadily. Every restart also has a stretch where the machine runs inefficiently before it settles down. That's why those appliances ran up big electric bills.
Appliances with an inverter are different. Once the room is cool, the motor doesn't shut off. It just slows its spin speed to hold the temperature. Because it puts out only as much power as needed, it's much quieter and wastes far less electricity.
Making electric waves to control motor speed
Electricity from a battery is DC, which flows straight in one direction. The electricity from your wall outlet is AC, where the current reverses direction over and over, more than 100 times per second. (Korea's grid runs at 60 Hz, which means 60 full back-and-forth cycles per second. The U.S. grid is 60 Hz too.) It's like electricity flowing in waves.
The speed of an AC motor in an appliance depends on how fast those waves rise and fall, which is called the frequency. The more waves per second, the faster the motor spins. The slower the waves, the slower the motor turns.
An inverter opens and closes the path of DC electricity at lightning speed to build a brand-new AC wave at whatever speed you want. Since it can reshape the rhythm of the electric waves however it likes, it can control the motor's speed smoothly and precisely.
A little more precisely: how the inverter inside an appliance works
To be a bit more precise: the AC from your wall outlet already has a fixed frequency, so you can't change a motor's speed with it as is. That's why an inverter appliance has special conversion parts inside to reshape the electricity.
First, a rectifier called a converter smooths the outlet's AC into flat, steady DC. Then the actual inverter circuit chops that DC into tiny pieces and rebuilds it as AC at exactly the frequency the appliance needs right now.
So in appliances, people usually use the word inverter for the whole technology of turning AC into DC and then back into adjustable AC. This precise control is how air conditioners and refrigerators use energy smartly without waste. Meanwhile, electric cars and solar power start out with DC, so there's no converter step. The inverter turns DC straight into AC. That's how an electric car gets smooth motor speed control, and how solar power can connect to your home's wiring or the power grid.
Common misconceptions
With an inverter air conditioner, turning it off once the room is cool and back on when it gets warm is the way to save electricity.
Once an inverter AC reaches the target temperature, it slows the motor on its own and uses very little power. If you switch it off and back on, the motor has to run at high power to cool the room all over again, which can use even more electricity.
Where you meet it
It's used in modern air conditioners and refrigerators, which slow the motor instead of shutting it off once the set temperature is reached, saving electricity.
It's essential in electric cars, turning the battery's DC power into AC to control the motor's speed smoothly.
It converts the DC electricity made by solar panels into AC household power (220V in Korea; U.S. homes use 120V).
In one sentence
An inverter turns DC into AC and adjusts the frequency, so a motor's speed and power use are controlled to match exactly what's needed.