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 Max P 07-20-2007 09:38 AM

Watts vs. Volt-Amp Billing?

I'm confused on how watts and volt-amps work especially when it comes to my electric bill.

Let's say I have a refrigerator in my house and it's using 75 Watts/sec (just making some round numbers up), and the volt amps are 100 Volt-Amps/sec (so a power factor of .75). What happens to the other 25 volt-amps? Is it just wasted electricity?

- And am I being billed for 100 'watts' vs. 75 watts? My utility company charges residences by Kilowatt hours.

It would seem to me that I would be charged for 100 watts. 75 Watts used and 25 wasted.

And my second question would be, then why do utility companies charge by KVar if they can measure Volt-Amps as well?

Thank you very much,
Max

 SecretSquirrel 07-20-2007 10:19 AM

The reason you're being billed for the power factor corrected wattage is because you're the one introducing the electrical losses into the system and they (the electrical distributor) have to make up for it. The good news is this descrepancy is *generally* not a big issue in a residential environment. Unless you have massive inductive loads; motors, ballasts, etc. then you should be in pretty good shape. When I say massive I mean something on an industrial scale. If you're counting pennies then it may seem like you're getting robbed. The big question is what can you do about it. You would have to perform an analysys with the appropriate instruments (power factor meter), calculate the amount of correcting capacitance and install the hardware as required. Since most residential inductive loads are cyclical I'm really not sure what you would be gaining with respect to the capital costs involved with implementing a solution vs. just paying for the corrected power usage.

Just my \$0.02

Authors Note: I worked in an industrial environment as an Electrical Project Engineer and power factor correction was one of my responsibilities.

 Max P 07-20-2007 10:35 AM

Thank you very much. Just so I'm not confused on what you said, do I have this correct?

100 Watts to my house for my refrigerator
75 Watts to my refrigerator
25 Watts wasted
--------
Electric company bills me for 100 Watts?

Is that right?

Thanks.

 SecretSquirrel 07-20-2007 12:38 PM

I apologize... I skipped over one of your key questions and concerns. The short answer to your question is "no". I think where you're getting confused is the notion that you're incurring a 25 watt penalty (loss) because of the power factor. Your power bill is based on KWh and a power factor adjustment is made based on Kvar hours or KVA demand (depends on the utility). It's not a watt for watt penalty.

They could reflect this on your bill as a surcharge percentage on kW demand based upon the power factor or a direct charge for kVAR-hrs or kVA demand... once again it depends on the utility. I don't know why they would choose one method over the other. The penalty rate gets exponentially worse based on a poorer power factor. Not knowing your utility's rate structure or method for performance I can't comment on how that would relate to the numbers that you presented without knowing more about their specific rate structure.

Max, I would suggest you get in touch with your utility so that they can explain how the billing for your specific electric usage is affected by power factor. Perhaps they have a case type scenario that they can present, in terms easily understood by the average homeowner detailing any cost penalties. It is mind boggling and I'm hoping that what I attempted to explain doesn't look like a bunch of gibberish. You may have to find someone in their engineering department as I'm sure the average customer service agent won't have a clue.

 Max P 07-20-2007 06:51 PM

I've been working hard researching this and I found this on wikipedia about power factor: http://en.wikipedia.org/wiki/Power_factor

"The significance of power factor lies in the fact that utility companies supply customers with volt-amperes, but bill them for watts. Power factors below 1.0 require a utility to generate more than the minimum volt-amperes necessary to supply the real power (watts). This increases generation and transmission costs. Good power factor is considered to be greater than 90 to 95%. Utilities typically charge additional costs to customers who have a power factor below some limit, which is typically 90 to 95%."

So now I'm confused about my original example.

So is the utility actually billing me for volt-amperes while calling them watts? I don't get it.

 SecretSquirrel 07-21-2007 06:57 AM

Quote:
 Originally Posted by Max P (Post 53812) So now I'm confused about my original example. So is the utility actually billing me for volt-amperes while calling them watts? I don't get it.
That's not what Wiki said;
Quote:
 The significance of power factor lies in the fact that utility companies supply customers with volt-amperes, but bill them for watts.
I know that my last post was a lot to chew on but what I said was; You are billed on Kwh and surcharged based on power factor (VA).

Now let me confuse you more; A watt is calculated by multiplying volts x amps. Soooo in a non-reactive system (no power factor loss) 1 watt is the same as 1 volt x 1 amp. This happens when the voltage is in phase with the current. After introducing an inductive load a phase shift between current and voltage begins. This is the 'Apparant' power that you've been reading about and this is where the power factor is derived.

Kwh is measured by a wattmeter and VA or KVA or KVar is measured with some other instrumentation. That is how they (the utility) can discern between the two.

I don't feel like I'm helping you. :(

 Max P 07-21-2007 07:59 AM

I think you're helping, I just have all the wrong assumptions, which is confusing me.

Let's see if I have everything correct now.

So basically, residences have both a watt meter and a VA meter. They charge for watts, but they also check the VA meter. And in some cases if the power factor isn't good they charge your some 'penalty'.

So assmuing I don't get charged that penalty - I will get charged the same from my electric company if I have a refigerator that has a power factor of .1 vs. a power factor of .99. Does that sound right?

Now taking it one step further (again assuming I don't get charged that penalty), the lost VA is electricity just gone. Could I theoretically invent some crazy futuristic gizmo that stored that lost VA and supplied/recycled it for use by my refrigerator? Then I would see a drop in watt usage right?

Thanks SecretSquirrel I appreciate you helping me out.

 SecretSquirrel 07-21-2007 08:51 AM

I believe you're getting the concept. :thumbsup:

There are some "energy saving" plugs out there made specifically for plugging the fridge in to. I quite honestly don't know how they work or what they do to supposedly save you electricity and money. One would hope that they do a little power factor correction but unless the product literature specifically states that, I wouldn't bank on it. The biggest buzz out there is for the Energy Star Appliances. You'll have to investigate to see what this certification does for you.

There's really no hocus pocus (crazy futuristic gizmo) to this... you just have to figure out the power factor and calculate the amount of capacitance to remedy the situation. It is always best to make power factor correction at each specific device. But, I don't want to over simplify this either as it takes instrumentation and know how to implement a solution... something the average homeowner isn't capable or qualified to do.

Max, before you pursue any sort of remediation why don't you contact your utility and get them to give you some real numbers on how much you're being penalized for power factor. If you feel that it is a worthy endeavor to perform some correction then at least you'll have some basis to go back to to see if you actually made an improvement.

Addendum: I'm not sure what your utility does but I've looked over my electric bill with a microscope and can't find ANYTHING that refers to any power factor penalty. Having gone to Duke Energy's web site there is no mention of this in the rate schedules. Can you verify that your utility indeed poses additional charges for power factor? If not, then where did all this concern originate from?

 Max P 07-21-2007 01:34 PM

Quote:
 Originally Posted by SecretSquirrel (Post 53874) If not, then where did all this concern originate from?
It's because numerous reputable electricians and electrical engineers in my area have been signing up to sell this thing (link), so I did too because it looked like a good opportunity. But some skeptical friends brought up questions I couldn't answer - such as the watts/volt-amp billing. I just wanted to understand for myself if this thing would actually make a difference. One electrical engineer quickly explained it to me, but I didn't quite understand it all. Ultimately I want to know if the theory behind this thing is for real and I hadn't been duped.

So at first I thought this thing wouldn't make a difference if I'm billed by watts - so I thought I'd been duped. But then you explained that because utilities are providing volt-amps but billing for watts so I guess that's where this device comes in. The wasted energy (volt-amps minus watts) is supposed to be stored by the device and then it provides it to the loads, thus pulling in less watts from the utility. I needed to make sure that I wasn't in turn selling a bogus product. I can't go around recommending this to my friends and family if I don't know the theory behind this is for real.

Thanks.

 dmaceld 07-22-2007 12:32 AM

Quote:
 Originally Posted by Max P (Post 53899) Ultimately I want to know if the theory behind this thing is for real and I hadn't been duped. So at first I thought this thing wouldn't make a difference if I'm billed by watts - so I thought I'd been duped.
Check out the discussion about the same subject here: http://forums.jlconline.com/forums/s...t=power+factor

The technology and theory is real. The dollar savings is not. Don't expend your money on one of these until your local power company starts selling them at a subsidized price.

 Max P 07-22-2007 06:14 AM

Thanks dmaceld,

I understand that just a power correcter won't lower my bills since I'm being billed in watts. So my uneducated understanding of it would be: the 'lost' energy is stored by these things, then almost immediately supplies it to the loads as watts, thus pulling in less energy/watts from the utility.

Thanks.

 SecretSquirrel 07-22-2007 09:20 AM

Quote:
 Originally Posted by Max P (Post 53969) Thanks dmaceld, So my uneducated understanding of it would be: the 'lost' energy is stored by these things, then almost immediately supplies it to the loads as watts, thus pulling in less energy/watts from the utility. Does that sound about right?
By definition, a capacitor is an electrical storage device. The concept of it storing "lost" current baffels me. Remember where we discussed how inductive loads create a phase shift between voltage and current? The capacitance introduced into a power factor correcting application merely makes the load look more resistive... thus shortening the "phase lag" between voltage and current, and improving your power factor. IMO the capacitor has no way of knowing about lost current... it's installed ahead of the load. And all this "correction" is happening 60 times a second.

I'd be wary of slick marketing claims and lingo.

Max, I really want to keep my nose out of your business and not give you advice on what you should or shouldn't buy... but there's some key things that I have said during the course of this discussion that I feel strongly about;

Quote:
 Since most residential inductive loads are cyclical I'm really not sure what you would be gaining with respect to the capital costs involved with implementing a solution vs. just paying for the corrected power usage.

Quote:
 Max, I would suggest you get in touch with your utility so that they can explain how the billing for your specific electric usage is affected by power factor.

Quote:
 Max, before you pursue any sort of remediation why don't you contact your utility and get them to give you some real numbers on how much you're being penalized for power factor. If you feel that it is a worthy endeavor to perform some correction then at least you'll have some basis to go back to to see if you actually made an improvement
.

These are the things that I would do before spending hundreds of dollars on some device that claims to "save you up to 25%" on your power bill. Just my \$0.02

 Max P 07-23-2007 08:44 AM

I talked to my utility and they told me to ask an electrical engineer. :) And since I don't know any, that's why I hoping to get info from the internet. (This isn't about a dispute with my utility, it's to see if that device can do what it says.)

So even though a capacitor is an "electrical storage device", it doesn't give the energy right back to the load, thus pulling in less power (watts) from the utility?

Thanks.

 SecretSquirrel 07-23-2007 10:28 AM

Quote:
 Originally Posted by Max P (Post 54131) I talked to my utility and they told me to ask an electrical engineer. :) And since I don't know any, that's why I hoping to get info from the internet. (This isn't about a dispute with my utility, it's to see if that device can do what it says.) So even though a capacitor is an "electrical storage device", it doesn't give the energy right back to the load, thus pulling in less power (watts) from the utility? Thanks.
Sorry that you didn't get any better response than that. I would assume they could tell you about performance and penalties with respect to your electrical consumption. I can understand that they wouldn't want to be involved in any discussion about end user devices.

A capacitor "stores" a charge. They are generally used as filters to smooth out electrical wave forms (spikes and so forth). You could argue that it behaves like a battery. Well it does, sort of, but there are limits and a duration to the amount of charge that it can contain. In using a capacitor for power factor correction it counteracts the inductive loading... helping to get the current wave form closer to the voltage phase. For a lack of better words, it makes your inductive load perform more efficiently by making it look a little more "resistive" (less power factor). Think of it as a bucket brigade... it's stepping up (advancing - not increasing) the delivery of buckets to keep the fire under control whereas before the fire was burning a little hotter. As long as you have the load turned on (fuel for the fire) you'll never put the fire out.. you just keep it under control.

I am not going to state that a capacitor "gives energy back to the load". It's more of a "pass through" device. I've done the best I could do at explaining its' function.

 Max P 07-23-2007 01:53 PM

That bucket analogy is great.

So the gallons of water is like watts. When I turn on a light, we get the water (electricity) from the tank to the fire with no bucket brigade. The gallons of water are poured directly on the fire.

But with a motor, there is a lag between the tank of water and the fire. Low PF would be like a slow bucket brigade. PFC is taking that slow brigade and making them work faster, so the lag time between the water in the tank to get it to the fire is reduced. The same amount of gallons/watts are used.

So a residential customer is billed on gallons used, not on the bucket brigade's work.

Hopefully I have that right.

Thanks. I know I've been a little slow with this, but it's just because I have such specific answers for questions I knew nothing about.

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