This is very controversial topic with only one inevitable result. If it sounds too good to be true, it usually is.
Have you seen this device advertised, Power-Saver 1200? For about $300 you can save upwards of 30% on your residential energy bill.

This product is based on the claim that it adjusts your energy usage to reduce I2R losses and a low power factor. The cost savings of managing power factor and I2R losses is perfectly legitimate but the truth is neither applies to the common US household. The average household doesn’t see or recognize a low power factor, so the residential customer isn't charged for it, so eliminating it will not save you any money.

Typically the companies that sell these devices use a video showing a motor drawing significantly less amperage with the device (capacitors) connected. All true but very misleading to the residential power consumer. This demo and angle is overdramatized and dramatically misleading. Truth is, you buy this thing, install it, improve the power factor by some amount (they don't state the KVAR value of the device) and not only save nothing but incur the cost of the unit, installation and the required fuse. You will never save 5%, 10%, or "up to 30%. Utility companies in the US do not charges residential customers for low power factors and don’t measure such usage.

In the commercial markets the utility company will charge the cost of delivering demand energy plus the adjusted profit the markets and the public utility commissions will normally apply to high demand. This is how Power Factor applies in determining your bill based on how much extra current must be carried by the transmission lines during high demand. But the residential market doesn’t measure power factor in the utility billing process unless you possibly live in a commercial market and are a business client. Businesses, however, benefit greatly from power factor correction. The utility has a separate device attached to the meter to measure it but the meter itself still measures only "true power".

As for I2R losses if this device is mounted at the panel, it will not reduce I2R losses because the stored energy oscillates through the house wires between the inductors and capacitors. As this happens, current flows, and losses occur regardless.
I personally have tested now 4 devices and I have purchased a total of 7. I have sent several to professional testing facilities to be tested – everyone with the same conclusion. Additionally several were the exact same device repackaged and relabeled. All of course with a 100% guarantee unless you open the box. Ironically you need to open the box to see what’s in it. For the curious who want to maintain their money back option it apparently only houses a bank of capacitors and one actually had a pilot light in it which appeared to most likely eat more energy than it could possibly save.

In my opinion, do not be sucked into this “go green” with a Power Saver type device. Use your $300 to plant a few more trees around your home, take your kids to the park a few more times a year and simply turn off that light when you leave the room.

I’d like to be proven wrong and will immediately post my apology. I get emails, spam mails and claims about this device. Here’s my challenge, my electric usage is very consistent month to month, I don’t have AC and I never use the heater. Send me a unit, I’ll test it for one month and share with you my billing for the past few months, the current month I install the unit, then the following month without the unit. If my bill is reduced by greater than 10% I’ll retract my statements pay full price for your unit and praise your product. All data during the test phase, including my retraction if I am wrong, will be made public on this and other blog sites along with press. DyoCore will additionally pay you $1,000 for your time If the product proves to actually work with a proven saving of over 20%. This amount being midpoint in most of these product’s marketing data. To participate in my challenge email me at dave@dyocore.com.

Facts about True, Reactive, and Apparent power - http://www.allaboutcircuits.com/vol_2/chpt_11/2.html