30 amp variac instead of an autoformer?

For the self-educated informed user it can be a brick of gold

For the uninformed, it can be a grenade

It can not under any circumstances "auto regulate" voltage.

Technically, a variac is nothing more than a variable autotransformer.

Those sort of automatic variac voltage regulator devices, assuming that's what this is internally, have been around for quite awhile, and they do work. They do not respond to sudden changes in input voltage instantaneously (like a solid-state switching regulator typically would), but otherwise are OK. It's basically just a variac with a servo-controlled output adjustment; you typically can hear it motor up or down as needed. I saw them in use when growing up for the couple of years I spent overseas in areas with less than great electrical systems, typically to keep fridges happy and operating properly.

It's anybody's guess how effective the linked one in particular would be when used near its putative ratings for a long period of time. Based on some of the reviews, I have my doubts; some of the pictures looked downright scary for a piece of electrical equipment.

How about this one Only 3000 watts, but at a glance looks automatic.
Not sure if it will regulate without step up or down, though.

This is deserving of a separate post.

VARIAC plus a soldering iron, transformer manual charger, even a resistance space heater. Tempting as can be but glue a stop to the adjustment dial so device cannot do anything but buck-duty. I did this to a milk-house heater, dialed in sixty volts with 800-watt setting on the heater and placed it near but facing away from my bed (no possibility of falling bedding shrouding the heater). Ahhhhhh!

Don these devices boost or buck voltage solely by percentage. Forget the dial, it is utterly misleading.

Voltage can not vary to the point of exceeding the percentage allowable. Drooped AC line at 105 volts should be boosted to 120. This is a finite percentage (which I will allow you to calculate)

But if that 105 volts for any reason decide to rise to say 115 volts, multiply the percentage gain to apply to 115 volts base and what do you get?

Boost or buck, percentage calculation, and knowledge of line voltage trends and tendencies will give you the tools to decide if variac usage is rational.

"Been here eight months and line voltage has never gone over 109 volts" is tempting, isn't it?

Are variacs capable of increasing low voltage?