# Can capacitors save on your electric bill?

It has come to my attention that people are selling capacitors to lower electrical bills. Is such a thing possible? I would normally file this under 'too good to be true", but if it does work, I don’t want to miss out.

Only if you charged them at night (when the rates are cheaper) and dissipated them during the day. Seems like batteries would be more effective, but you’ll notice solar power uses batteries to store power at night, which works, because you’re not using as much electricity. You’re going to need some pretty darn big capacitors to work, plus a system to let them slowly drain without frying all your appliances.

“The world is inductive”, said the old prof that used to teach the power class in school.

It’s a bit mathematically complicated but in general, IIRC from 20-mumble years ago…

If you’re standing at the power line, looking into a house (or especially an industrial factory), you see a combination of resistive loads (stoves, light bulbs, etc) and inductive loads, usually electric motors. Inductors are created by coiling wires tightly - like how an electric motor is made.

The combination of the loads creates a offset in the sine waves for the voltage & the current used. This offset is called “power factor”.

The most efficient loads are pure resistance - zero angle between the waves, the least efficient are pure inductive (or capacitive) loads. (I put “capacitive” in parenthesis because there are almost no real capacitive loads).

Capacitors are, electrically, kind-of anti-inductors. A capacitor in parallel across an inductive load makes it look (from the street) as something more purely resistive.

Industry, which often has many, many electric motors in use, has used capacitor banks for years to reduce their power factor and lower their electric bills.

I didn’t know that they’re being marketed to home users but I can see it as possible. I suspect that they’re of little effect since most homes are just resistive loads. Between lighting, electric stoves, electric water heaters, coffee makers, etc, I suspect the few electric motors are a drop in the bucket.

Wiki article on Power Factor Correction.

Looks like I’ve got my “direction” wrong on power factor - 0 is worst - 1 is best. So you don’t want to “reduce” your power factor, you want to increase it.

Most modern home appliances that have inductive loads, such as air conditioners and refrigerators, come with power factor conditioning built in. Therefore, purchasing a power factor correction unit for a home is not really indicated. In fact, because the unit draws continuous power, even when no power is being called from the home, the unit can end up resulting in more power used, not less.

Power factor correction is usually used in industrial settings, where large inductive load exists; in which case tremendous power savings are realized, as well as greater longevity of the appliances.

The only time power factor correction is really helpful in a home power setting, is if there is an unusually high inductive load, such as from a pool pump, or a koi pond pump – unless the unit does not draw current during “idle” mode when no inductive load is present.

Otherwise, buying power conditioning units for home use is most likely to be a waste of money.

It is important to realize that billed energy is a function of amps x watts x power factor. So when talking about power factor increasing and amps dropping, be sure that the final tally is better than the original, by enough to make purchase of the unit worth while.

From Here.

I would like to add that it is my understanding that most residential power meters are measuring real power and not measuring RMS current times RMS voltage and as such they will measure the same power with and without power factor correction.

A side note:

In the 70’s they sold rectifiers for the incandescent bulb sockets, and those did reduce power usage. Today you should just buy energy efficiency type bulbs as they will save you more.

It might help the utility, but that’s their problem.

This sounds suspect.

If this were true, we could add huge inductors across our houses, really muck-up our power factor, and we’d be paying less for power than before.

Another article I read suggests that one way for Energy-star rated appliances to increase efficiency is to use built in power-factor correction. That would be unnecessary if the supplier only charged for “real” power.

I don’t think so. You would still pay the same for your real power, but you would create a huge reactive load, which would trip your breakers* and piss of the utility.

*Magnetic and thermally actuated breakers trip on current, not power

If you had a really, really, really old power meter it might make a difference, but I seriously doubt that any meters that old are still in service.

Residential loads are slightly inductive, but the power company generally doesn’t feel it’s worth worrying about too much. They do have capacitor banks installed in substations which compensates for the inductance of residential loads. These capacitors are switched in and out of service as needed. You would have to add a honking big inductor to affect the power factor of an entire branch line. You probably would tick off the power company folks if you tried it, though. And, beowulff is right. You wouldn’t decrease your “real” power consumption so you wouldn’t lower your own bill.

While residential customers aren’t charged based on power factor, business and industrial customers are charged extra if their power factor is too far out of whack. Capacitive power factor correction is very common in these types of facilities.

All the current that flows through wiring causes resisitive losses. Thus orthogonal component of the current DOES increase the bill, bot only slightly…not by the imaginary power, but due to the additional copper losses from the increased magnitude of the complex current, which is real power that is metered in all cases.

This applies on a much larger scale to the utility’s distribution wiring…which is why they tack on a surcharge for low power factor industrial consumers. Note that for maximum benifit the inductive reactance needs to be cancled as near the load as practical.