Ever seen the ads?

"Save 20-25% on your power bill with this device!"

"What the power company doesn't want you to know!"

There are several iterations of this; some plug in and some that electricians will try to sell you to go in your breaker box.

Don't.

Yes, there is a device in the box the person will install or that you plug in; its not an empty box.  And yes, there is a basis for the claim but it does not apply to a residential power customer in every case I've ever seen in the United States, and I've lived in multiple states and with multiple electrical companies providing my power.

What is being "sold" is otherwise known as a power-factor device.

Power factor is a measurement of the apparent amount of power consumed versus the actual amount of work done.  They're different measurements; "VA" for apparent power and Watts for amount of work done.  If the thing drawing power is a straight resistor (e.g. an electric heating element in a water heater, your stove, coffee maker and incandescent lights) then the power factor is 1.0.  That is, the phase or "alignment" if you will, between voltage and amperage is identical.

Motors, which are in all sorts of things (a washing machine, a dryer drum, your air conditioner, a fan and similar) typically use induction motors.  As the name implies there is an inductor in there.  House power is alternating current; when a changing voltage goes through an inductor the amount of current drawn lags the voltage. It is like a resistance but only to a changing voltage, not a constant one.  Thus a motor typically will have a power factor less than 1.0 because the rate of change of voltage and amperage as the power alternates up and down in voltage is not exactly lined up.

A "power factor correction" device is simply a capacitor.  A capacitor stores energy temporarily and thus "leads" the inductive load, smoothing its lag and "correcting" the power factor from the perspective of whatever is providing the energy.

It does not change what the consuming device uses or the device's power factor but it does change, by shifting the incoming amperage more in-line with the voltage, what the supply -- in this case the power company -- sees.

In other words it moves the perceived power factor from the supplying end's point of view toward 1.0.

It does not, because it cannot, change the actual power factor and thus efficiency of the supplied device.  That is a function of the device's design and engineering.

Here's the problem from a standpoint of someone being sold such a device for a residence: Residences are billed in kilowatt-hours, or kWh -- that is, actual work performed.

The power company is the one who winds up "eating" the differential because they have to supply the amps irrespective of the imbalance between voltage delivered and amps delivered across the alternating current cycle.  In terms of actual power delivered and consumed, which is measured in watts, you are billed in watts not "apparent power" or kVA and thus from a perspective of your bill it makes no difference whether the power factor at any given time in your house is 1.0 or 0.85.

Now an industrial customer does in some cases have a "kVAr" charge for apparent power, that is, the differential between the "null" or 1.0 power factor from the utility's point of view and the power factor at the panelboard consumed by the building.  Industrial and commercial users typically pay a base "energy charge" (for kWh consumed, like you do in a house), a demand charge (this is computed based on the maximum required power because the utility has to be able to deliver it when demanded, and thus must build out the capacity for it even if 90% of the time you don't use it) and, in some cases depending on the power factor of your facility, a KVAR charge, which is a reactive power charge for a power factor that is less than 1.0 because the power company has to be able to deliver the amps and your mismatch costs them money which they otherwise can't recover.  In that case correcting for power factor might be worth doing, depending on what is generating the imbalance in your facility and how large it is.  Large motors and things like arc lighting -- or even more-seriously an arc furnace used in some industrial processes, generate very large power factor dislocations and the power company, for an industrial user, charges you for that because they will have to build out additional infrastructure to be able to deliver energy into that.  If that's a significant amount then obviously installing something that smooths the power factor out is a win for you as then you don't pay said bill.  (In extreme cases utilities may demand you do it in order to provide power at all because it presents a seriously-destabilizing load on the grid.)

THIS DOES NOT APPLY TO RESIDENTIAL SERVICE BECAUSE YOU ARE BILLED FOR TRUE POWER CONSUMED -- THAT IS, WATTS.  GO LOOK AT YOUR BILL -- IT CONTAINS A 'kWh" CHARGE AND NOT A KVAR CHARGE.

There are some utilities that are trying to get demand charges into residential service.  They do this to provide an incentive for users to spread out their usage (e.g. don't both dry clothes and charge your Tesla at the same time) or, if consumers refuse to do that, force them to pay for the additional transformers, generation capacity and transmission lines to be able to get the power to them.  They're rare, but showing up in a few places.  (As an aside if they ever do that here you can bet I'll use HomeDaemon to deliberately shut off and enable loads to evade that demand charge to the extent I'm able -- I had to pay it when I ran MCSNet and there was no way around it either as you can't exactly shut the chillers off in the computer room in the middle of summer unless you want all your machines to melt!)

But a KVAR charge, which is not the same as demand billing, is something I've never seen proposed or implemented in residential service.

If you had a KVAR charge then you could reduce it by installing a power-factor correction device.  That does not change the actual power factor of the connected device; that can only be done by changing the device's design.  But it does change how the supply of the energy sees that load; that is, it shifts the alignment of voltage and current toward concurrence.

The problem is that the benefit is to the supplier of the energy and that's the power company.   You're not billed for being out of coherence as a residential customer so there is no benefit to you in the installation of such a device!

Contemplate this folks: Since it is the power company that eats the power factor imbalance in a residence, if there is one, if it was worth it they'd come out and on their own initiative install a power factor correction device in the meter box at no cost to you.  That would not change a thing in terms of the size of your bill because the number of kilowatt hours your meter reads would not change but it would reduce the amperage load on their infrastructure, which is of benefit to them.

The reason they don't do it to every house in your neighborhood is that the savings to them are not worth the cost.

There is one exception to this: If you have off-grid generation such as an inverter and battery bank.  You'll notice inverters are rated in "kVA" and not kW.  Indeed even your small battery backup UPS for your computer, which is nothing more than an inverter, charger and battery that takes over when the power fails, is rated in VA or kVA, not watts.

So if you have such a system powering your residence then there may (depending on your actual power factor of your house) be value to you because you are the energy supplier and you might be able to buy a smaller and cheaper inverter yet still produce the amount of power required.

Otherwise?

Don't waste your money and DO report any so-called "professional" soliciting you in this manner to your local consumer protection folks.  It is not possible for you to save money on a charge you're never billed for in the first place so in a residential environment, if you are "on grid" with a power company, unless you have a KVAR charge on your bill and I've never seen that on a residential power bill the total amount of money savings such a device will provide to you is ZERO.

Any percentage of zero, that is no billed amount at all, is zero.