Electric Meter readings with volt drop

[mattysupra]

Hi, can anyone tell me if you had a large voltage drop (down to 200 volts), would the electric meter read more?

As I understand, if you have appliances running and the volts drop, the amps increases? so would this cause the meters to read higher due to the increase of amps caused by the low voltage?

 

[BS3036]

Resistive appliances would take less current (so less meter reading).
Switch mode power supply appliances would typically take more current, but the same Watts (so same meter reading).

 

[EFLImpudence]

No, Amps go down and up with the voltage.

 

[AdrianUK]

As others have said, for a resistive load, like a heater, if the voltage drops, then so does the current and hence the power.

Simple loads do not behave in a 'constant power' manner which is what would be needed if the current were to increase in response to a fall in voltage.

As an aside, an electric meter measures power - the product of volts & amps. (Lets ignore power factor for the moment!).

So, to answer your question, a volt drop as you describe would cause the meter to read less not more.

 

[ericmark]

The big question is does an electric meter measure amp/hour or watt/hour, I am told it measures watt hours so would compensate for volt drop, but I have never stripped one down to find out.

I would have thought it would either measure watts or VA, think there would be complaints if it measured only amps.

As to power used, likely very little change, heating items are normally thermostatically controlled so would run longer, and switch mode supplies would also compensate, there may be a drop in power used by lights where a simple capacitor is used as current limiting or it is tungsten.

 

[AdrianUK]

Eric,

A 'proper' domestic tarriff charging leccy meter is a true real power meter. It measures both the voltage & the current and integrates the in-phase product of the voltage & current giving a real power reading.

The older, mechanical meter has two coils - a potential coil which was connected across the mains & a series, current coil. The fields from both the these acted upon the rotating disk.

Commercial meters are a little different in that they often measure kwh & kVArh. Thus allowing a commercial customer to be penalised for a poor powerfactor.

 

[davelx]

The big question is does an electric meter measure amp/hour or watt/hour, I am told it measures watt hours so would compensate for volt drop, but I have never stripped one down to find out.

I would have thought it would either measure watts or VA, think there would be complaints if it measured only amps.

As to power used, likely very little change, heating items are normally thermostatically controlled so would run longer, and switch mode supplies would also compensate, there may be a drop in power used by lights where a simple capacitor is used as current limiting or it is tungsten.

They measure kilowatt-hours - true energy consumed.

 

[ericmark]

Thank you @AdrianUK and @davelx this is what I expected, but nice to be confirmed. So answer to @mattysupra is no the electric meter will not read more.

As to:-

As I understand, if you have appliances running and the volts drop, the amps increases?

Amps only increase where some electronic device compensates for volt drop, be it a switch mode power supply, pulse width modulated power supply, and an auto transformer with automatic switching of tapping, although not seen the latter for years, they were used with old fluorescent lights to save energy.

As to fluorescent lighting new systems are controlled with a switch mode device, but with the old wire wound ballast the current increase with voltage was not linear, as I found out the hard way, I put 25 x 60 watt fluorescent on a 110 volt supply, should be around 14 amp, but due to voltage being slightly high, found clamp-on meter showed nearly 25 amp, the lamps were 230 volt really with an auto transformer which gave option of 110 or 127 volts input, moving first 20 lamps to 127 volt dropped the total to around 15 amp so was OK on a 16 amp MCB.

So with fluorescent lights with a wire wound ballast least power used at the voltage where they will just strike, and years ago there was a problem with 220 volt ballast from the EU, they would work, but used more power. After harmonisation to 230 volt, the problem actually go worse, as lamps made for 230 volt but in real terms the supply still 240 volt.

As the role out of solar panels started it was found they would lock out on over voltage, so the DNO started to drop the voltage to 230 volt, and with the old 5 foot fluorescent the fat 65 watt tube had stopped being made, but with 240 volt the thin 58 watt tube still worked OK, but once the volts dropped to 230 volt the 58 watt tubes would only last around 2 months before they failed to strike, where old 65 watt would last around 7 years in the same fitting. So I fitted an LED tube which clearly has some switch mode driver built in, as you could leave the wire wound ballast in place, which would cause a volt drop, but watts went down to 22 watt and also the lumen dropped to around half that of the fluorescent tube.

So in real terms no answer, as some items use more/less power as voltage alters and some auto compensate and use same power.

 

[SUNRAY]

I've found running 230V power tools on 110V have reduced performance and speed but the drop in current is only minimal. Not much of a surprise really with the reduction of back EMF at the lower speed.

 

[JohnW2]

Commercial meters are a little different in that they often measure kwh & kVArh. Thus allowing a commercial customer to be penalised for a poor powerfactor.

Not just commercial ones - my ('dumb') electronic domestic meter measures both, but it is the kWh that is used for billing (regardless of my power factor).

Kind Regards, John