Home Energy Monitor misreading SwitchedPowerSupply to Laptop

Whatdo

Hi Everyone,

Have been playing again with an NPower Home Enery Monitor and have noticed that my laptop with power supply attached is being indicated as drawing 0.235Kw.

Curious I read the back of the power supply unit and it indicates 19V @ 3.42 A which is of course 0.065Kw or 65W.

So A 65W switchedmodepower supply unit is being indicated by the Home energy Meter as drawing 235W.

I accept that the power supply loses energy as heat but not equivalent to a 170 W filament light bulb???? my slow cooker only uses 200W on Full Power.

Should I be placing my laptop power supply unit underneath my slow cooker.......:money:

Any thoughts anyone


Is there any point in a monitor that cannot differentiate PWM thesedays ?
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HappyMJ

Are you absolutely sure that nothing else is plugged in. Including the Npower energy monitor. LOL how do you check it without it being plugged in. Well you can't and that's the problem. A 65W power supply will probably only draw half that anyway at about 35W except when charging AND running an intensive power hungry application with every peripheral plugged in.

The point in the energy monitor is to see any changes from the norm. You have to figure out your own norm.

Whatdo

HappyMJ wrote: »

Are you absolutely sure that nothing else is plugged in. Including the Npower energy monitor. LOL

It was simple enough to isolate the laptop supply and the home energy meter from other peripherals by turning off all other circuits at the Consumer unit.

In any event

Turn the laptop supply on and the meter increases by a value
turn the laptop supply off and the meter decreases by a value

done several times to be sure that it was the introduction of the laptop as the responsible load and thereafter a simple arithmetic calculation using the principle of systematic error.

the home energy monitor is 5V 100ma probably has a bridge rectifier diode arrangement power supply unit which is 0.5 W and probably not within the effective range of the unit to effectively measure reliably as the readout scale is in KWs. to 3 decimal places.


I am hoping to introduce the discussion and awareness of the home energy monitor being able to measure a SWITCHED MODE POWER supply unit correctly.

sort of in a light hearted way.......


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HappyMJ

They are not accurate to 3 decimal places. Maybe 1 decimal place but certainly not 3.

Whatdo

HappyMJ wrote: »

They are not accurate to 3 decimal places. Maybe 1 decimal place but certainly not 3.

The readout currently says 0.290 kw which KW 10 to the -3.

I also referered to the abilty of the unit to effectively measure 5 watts. I would expect the unit to be able to display differences of say +/_ 10watts as it does in increments of 5.

The unit came with no calibration certificate unlike my other equipment, with the suitable probes my oscilloscope will happily measure current down to very small values (without hall effect).


I wrote the post not to troll or be trolled, but to point out to others who like my sister use their monitor to change their lifestyle, my point is, if she is not being given the correct information how can she choose effectively without possibly compromising herself. Is she turning the right equipment off.

I was maybe naive not expecting to be trolled by someone using the well established observation of physics namely the effect commonly referred to as Schroedingers' cat.

I also wished to seek general opinion on the new service meters being installed. If I am only using 40watts of power and being charged by Watts I would wish my service meter to be capable of measuring my actual power consumption. Not extrapolating the clipping of the phase, and billing me for using 170W.

Please no trolling..........

withabix

Whatdo wrote: »

I read the back of the power supply unit and it indicates 19V @ 3.42 A which is of course 0.065Kw or 65W.

You can't compare the output power with the input power.

19V @ 3.42A is the output power (65 watts)

The input will be 100-240V @ 1.3A typically

Some of this difference is lost as heat.

The fully current of 1.3A will only be drawn when your laptop battery is flat (and only the maximum on 100V supplies).

The lowest current will be drawn when your laptop battery is fully charged and the PSU is mainly using power to run itself.

Home energy monitors are not precise enough to provide accurate readings at lower currents, because they measure induced electric field and not actual current.

penrhyn

Don't believe any readings below around 200watts as pointed out they are not accurate.
If you want to know the consumption more accurately buy a plug in wattmeter, Lidl /Aldi often have them for around £8.

Whatdo

penrhyn wrote: »

Don't believe any readings below around 200watts as pointed out they are not accurate.
If you want to know the consumption more accurately buy a plug in wattmeter, Lidl /Aldi often have them for around £8.

I dug one out from the shed, a lidl unit listed as being manufactured by paget trading.

I let the battery run down on the unit and plgged the laptop power supply unit into the watt meter and then plugged the watt meter into the ring main. So as an idiots bit of kit not designed by me. the readings are as follows.

231V @50Hz 35/36 Watts fluctuating

This is the load being reported out of the watt meter.

therefore that being the case the Home energy Monitor displaying 235 Watts leaves 200Watts for heat to be dissipated from the SMPSU.

when I turn on a 50w gu10 filament reflector it is displayed reasonably accurately on the home energy monitor.

so for the time being either the SMPSU actually gives off 200watts (same as my slow cooker on full power) of heat or the Home energy monitor cannot cope with a switchmode power supply unit....

could the clue be that the sensor is wireless to the home energy monitor unit?

..

rogerblack

The problem is that the clip-on energy meters do not accurately measure power.
They measure current, which is sort-of-like measuring area by measuring width.
This works fine if the other side - voltage - is known well.

Multiply voltage and current, and it works fine, and you get watts.
But.
Some sort of loads draw power 'at an angle' - with much the same result as if you try to measure the area of a rectangle and have your ruler squint.

The extra power isn't 'going anywhere' - it never really existed, and is just an artifact of the measurement.

The 'plug in' energy meters tend to get this right - most of the time.
But even some of them can be confused by some loads.

grahamc2003

rogerblack wrote: »

The problem is that the clip-on energy meters do not accurately measure power.
They measure current, which is sort-of-like measuring area by measuring width.
This works fine if the other side - voltage - is known well.

Multiply voltage and current, and it works fine, and you get watts.
But.
Some sort of loads draw power 'at an angle' - with much the same result as if you try to measure the area of a rectangle and have your ruler squint.

The extra power isn't 'going anywhere' - it never really existed, and is just an artifact of the measurement.

The 'plug in' energy meters tend to get this right - most of the time.
But even some of them can be confused by some loads.

When dealing with AC, multiplying voltage by currect gives VA (VoltAmps). If the current and voltage are in phase, you have a power factor of 1, if they are out of phase (due to mainly inductive circuits), then you have a power factor less than 1. The problem with this is that you pay for more power than you use, the difference (the reactive power, which really does exist) being pumped back into the mains.

rogerblack

From a consumer perspective, reactive power - which does in fact exist - doesn't matter.
No retail customers in the UK are charged for the reactive component, only the real component.

I note my most reactive appliance is my induction cooker - it has several tens of uF on the front-end as a filter.
This causes a largish current, which registers about 60W IIRC on my clip-on whole-house energy meter.
A plug-in energy meter properly reads this as a load with a very high power factor, and only a couple of watts real draw.
The electricity companies meter also measures it as a couple of watts.