Why don't power meters measure volt-amps

Question

When large inductive or capacitive loads are applied, the power meter will not indicate and bill you as the electricity gets lost in the lines. Why don't power companies just charge for this?

The utilities company in my area (in Mumbai, India) has rolled out "intelligent" domestic power meters, which not only read power factor and its variation through the day, but also allow for remote readings directly from a central location - the "meter guy" no longer visits the building, except for a tampering audit every 3 to 6 months. — Anindo Ghosh, Sep 05 '13 at 10:16

score 9 · Answer 1 · answered Sep 05 '13 at 04:57

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Sometimes the power company will charge you. The issue isn't that the power companies need to make more "power", rather poor power factors mean that various equipment which transfers the power from the station to your house has to be bulkier, and thus more expensive.

In the past traditionally this was only a major concern for industrial customers who had very large loads and/or poor power factors. However, as our homes are filled with more and more switch mode power supplies (particularly those without active PFC) real and reactive loading becomes a larger issue for power companies. Traditionally households didn't use much electricity and had pretty high power factors to begin with so the companies don't bother.

As of late, though, it's become more of an issue with residential customers because we have more gadgets and are using more switch mode supplies which can have really low power factors. It's such a concern that in the EU they've passed regulation requiring larger SMPS's to have some sort of power factor correction to reduce the reactive loading.

Incidentally, there are some power companies with "smart meters" which charge higher peak usage rates, and offer free nights/weekends. They can do this because the actual amount of electricity you use is relatively cheap, but all of the "grid" part is quite expensive, especially if everyone wants to turn their gadgets on at the same time.

answered Sep 05 '13 at 04:57

helloworld922

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I thought switch mode power supplies (which are capacitive) tend to counter the large inductive appliances (e.g. microwave, refrigerator, vacuum cleaners) that most houses have had for the past half-century. – Random832 Sep 05 '13 at 12:47
Is poor power factor in consumer electronics actually a worsening problem? Except possibly at the lowest level virtually all desktop PSUs have some form of PFC because it's cheaper to have a global model that meets regulatory requirements everywhere. Beyond that desktops are being progressively replaced with laptops, mainstream laptops are falling in power consumption, and phones/tablets with even lower power consumption levels are displacing laptops. Even if(?) USB chargers for the latter have horrible power factors, their draw levels are so much lower I'd still expect overall improvement. – Dan Is Fiddling By Firelight Sep 05 '13 at 13:49
@DanNeely Suppose there are 1 million people in a city with 100W worth of electronics, and suppose the HV trans. lines operate at 135kV. With a PF of 1, the power company only needs to support ~741A, but with a PF of 0.5 they would need ~1481A. Now consider how many urban areas with multi-million people living in them there are, and typical peak usage patterns where there most likely is significantly more than 100W being used per individual. It all adds up really quickly. – helloworld922 Sep 05 '13 at 22:34
@helloworld922 you missed my point. Between our gadgets using less power than they used to and a lot of them now having PFC I suspect that the net residential effect has peaked and is now declining. – Dan Is Fiddling By Firelight Sep 06 '13 at 04:10
An individual device's power consumption is going down, but more and more people are getting gadgets and we have more of them per person. When I was a kid having a cell phone was almost unheard of, and very few people actually had PC's at home. Now nearly every one has a smart phone, a laptop/PC, etc. A 5W phone charger with a PF of 0.5 isn't so bad by itself, but it becomes slightly worry-some when millions are plugged into the grid. Then add in the ~70W laptop charger bricks. Luckily, many "quality" SMPS's do have active PFC so things aren't as bad as they could be. – helloworld922 Sep 06 '13 at 05:28

score 6 · Answer 2 · answered Sep 05 '13 at 06:29

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In Australia, New South Wales, if a consumer is drawing excessive power outside the normal power factors, they can be asked to pay. It is in the contract. Detection isn't by the meter though, but through occasional audits. Newer meters might increase the chances of an audit.

Poor power factor causes other problems though, so it isn't a good idea to use it as a means to avoid paying.

answered Sep 05 '13 at 06:29

James Cameron

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Interesting. How is this audit performed? Do they just use a current clamp on the main meter or something like that? – Rev Sep 05 '13 at 07:11
Guess so. My local retailer has dataloggers that are attached electrically or inductively as part of second level support, usually as part of ensuring standards are being complied with by their network. I last saw one during a persistent overvoltage investigation, which turned out to be solved by redistributing the loads on the phases. – James Cameron Sep 05 '13 at 07:34

score 3 · Answer 3 · answered Sep 05 '13 at 08:41

I'm going to say that question's assumption isn't universal enough!

One of my power meters does in fact measure both active and reactive power and I get charged for both kWh and kvarh measured by the meter. In addition to that, I also get charged for bad power factor and peak consumption. In my country, using this power charging scheme results in lower costs for consumed energy and additional charge for peak power consumption, making it more suitable for consumers which have constant loads with no large peaks.

I'm going to speculate a bit now, but my guess is that incentive to develop meters which measured reactive power wasn't great enough up until recently.

I don't have many devices in my home which are from this new high power factor era and despite that, my power factor is around 98%. In fact, only time it was lower than that was when the power company guy entered random numbers when reading power consumption.

Back in the era of electromechanical meters, measuring reactive power would have required a separate meter in addition to the meter for active power, which would result in increased costs for initial electricity installations, increased maintenance costs, since there would be one more meter to look after and calibrate from time to time, longer measurement times, since power company guy would have to read two meters per each consumer instead of just one and so on.

So I'm going to agree with helloworld922 here and say that households did have high power factors to begin with. In general, it would be cheaper to just assume a power factor for consumers and appropriately form the price of active power to take it into account.

Agreed. The per customer cost of power meters was kept to a minimum very early on for the network that I use, because the network was operated by government, and they could not justify, on the evidence, a more complex and costly meter. Now I think they have the evidence, and the cost of more functional meters is no longer such a constraint. The constraint now is labour cost for the guy who reads the meter, so optical and wireless link meters are the rage. That brings microcontrollers into the meter, so power factor measurement is an easy step. — James Cameron, Sep 05 '13 at 08:45

Why don't power meters measure volt-amps

3 Answers3