Measuring power consumption of immersion heater?

Just checked a 2 year old Panasonic LCD, and it's 0.5W standby. It doesn't have a TCO'03 sticker on it, but this brings it within the TCO'03 requirements (1W max standby) and thus able to be sold in some countries where that's a requirement.

That seems an awful lot, or the power meter is not handling loads with power factor < 1 correctly.

Is this a trick question? The supply doesn't have a power factor; only the load has a power factor.

It wasn't supposed to be a trick question. The supply has a power factor by definition. The question is whether it equals unity or not. Are you implying that a domestic supply is guaranteed to have a power factor of 1?

Its an odd question granted, although you could argue that the supply may have a distorted voltage waveform as a result of local high current non-power factor corrected loads. This (and other things) will introduce harmonic noise to the supply. The result would be lowering of the effective power factor seen at the load.

In which case you'll have to give your definition, because it is meaningless with the standard definition of power factor.

Well, it's not _my_ definition it's _the_ definition. Perhaps you could Google for one?

I am very familiar with the standard definition which applies to loads, having designed and built a true power meter myself some 25 years ago, and having explained power factor many times here over the years.

You'll have to point out a definition of power factor of a supply, as that's not covered by the standard definition. Google draws a blank too. The only related thing I could think of would be a measure of the worse power factor load a particular supply can drive, but even that's meaningless as supplies are rated in [k]VA and in many cases can handle worst case loads with a PF of zero.

They are charging me per kwHr so they had better have the correct power factor.

If they are charging per kWh, power factor is irrelevant.

Tom, the issue is *how* the meter makes a measurement that can be accurately displayed as "real power" - Watts - what the domestic customer is billed for.

The device can measure voltage of the supply. The device can measure current drawn.

That's VA.

To arrive at Watts, it has to calculate (guess, fiddle) what the power factor is and this is where the over reading inaccuracies result, especially with low current devices, and cheapo gadgets of this sort.

Power factor of the supply? What's that then?

?????

Not so!

The numerical power factor eg 1, 0.9 etc - is a measure of the phase angle between the LOAD current and the applied voltage. It is also the measurement of the ratio of real power and apparent power or the ratio of watts (W) to voltamperes (VA).

Look at Google:

I'm clearly trying to discover whether the power factor I measure is affected in any way by the electricity network. If you put a purely resistive load across the supply coming into your house, are you guaranteed to measure no phase difference between the voltage and the current? If the phase difference is zero, then the power factor of your supply is 1.

There's a whole load of stuff on the web about the lengths the National Grid go to power factor correct their supply. Perhaps you should ring them to tell them they are wasting their time?

If the load is resistive there is never any phase difference between current and voltage it is governed by ohms law..

It's something the National Grid are obliged to keep within certain limits: 0.85 lagging - 0.95 leading.

....

You have just contradicted yourself.

IIUC, it is not effected by the network as such, although it may be affected by other users of it.

With purely resistive loads then it is not an issue. For loads with a reactive component then the reactive component will typically be the major influence on the power factor. However the quality of the waveform that you are supplied with can further influence it.

They go to some effort to mitigate the effects their users have on the supply. A big industrial user (or the cumulative effect of many smaller ones) pulling large currents from the supply that are not phase aligned with the voltage, can end up distorting the supply waveform. This leaves the waveform non sinusoidal and hence introduces other frequency components into it. These will interact with the reactive elements of any load differently than would a plain 50Hz supply.

Well sort of...

The concept of power factor only really has any meaning when you know the characteristics of the load. With a resistive one, the power factor is 1 regardless of the supply.

What the grid are obliged to do is provide a stable supply, and ensure that the required true power output of the generating plant is available to the grid. Obviously this would not be the case if capacity on the network is going to be absorbed by large reactive currents flowing around it. They also need to ensure local voltage stabilisation.

The limits you specify above are limits for the *load*. i.e. they must meet certain performance criteria when the connected load lies within those limits. With applied loads outside of these limits they are not required to achieve the same standards.

A distribution system itself will also have a number of reactive components within it, like the cables (transmission line effects), and transformers. The effects of these will also need to be compensated for.

I think he was just being a bit sloppy with terminology. However it is worth remembering that the distribution grid is also part of the load.

AFAICS, a supply doesn't have a power factor. It is determined entirely by the load downstream.

In the simplest terms, the generator is responsible for the alternating voltage, but your load is responsible for drawing the current. Given the available voltage at the point of connection, your load is the sole determinant of the magnitude of the current that it will draw, and its phase angle relative to the applied voltage. Therefore the PF is solely a property of your load.

I see your point, but let's build this up one step at a time.

The simplest case is a single generator and a single load, connected by a loss-free line. As the consumer, you measure the power factor of whatever load you choose to apply. Meanwhile, the supplier measures the power factor of the load that *it* sees, back at the generator output. In this very simplest case, the consumer's and the supplier's measurements of PF are obviously the same.

Now add another consumer and another load. Each individual consumer measures a PF that is determined by the current flowing into his own individual load. The other consumer's load may affect your supply voltage, but it does *not* affect the PF of your load (unless the load happens to be non-linear - but that is still entirely a property of your local load, not the supply).

Meanwhile, back at the generator, the supplier can measures the PF of

The complications arise when we have a distributed network with several generators, a very large number of loads, propagation delays and line losses. As you know, this is a hugely complex situation which includes the possibility of generators not being synchronised, and not necessarily contributing power to the network. This blurs the boundaries between "generator" and "load", as the network has some characteristics of both.

But even then, the same basic principles apply. Each individual generator has only one set of output terminals, and sees the rest of the network as its load. So it is still the *load* that has a PF - not the generator. I'm not sure how the "network PF" is defined... but it still doesn't affect you.

When you connect a load somewhere on the network, all the complexities drop away - we're right back to basics with a single connection point. Your load is the sole determinant of the current that it draws, and the phase angle relative to the applied voltage is still determined entirely by the characteristics of your load.

Whatever may be happening in the network upstream of your connection point, it does not concern you. When you connect a load, it will affect the voltage/current relationships somewhere back upstream in the supply network (at least to some small extent) but it still doesn't affect the PF of your load, as measured by you at your single point of connection.

Hope this makes sense, because the coffee mug's empty now.