Measuring power consumption

Try

I found the SAME item (listed there at about 15 quid plus VAT, total of about 24 quid including shipping) for 39.95 plus 10.00 shipping plus VAT (i.e. 57 quid) somewhere else!

resistive loads like light bulbs.

I got mine from Machinemart about 18 months ago - when I think it was slightly dearer than the current price.

My power/consumption meter cost £6.99 from Lidls a year or two ago. Works fine, enter your electicity cost per unit and it tells you the total cost in £ and p. Currently on fish tank to see the running costs. One complication is it tells you the power factor of the load, but not how, if it does at all, affect the reading, because obviously there are two powers now when power factor is not one, but instructions don't tell you which. I suspect you have to multiply the meter read power by power factor to work out your power in a domestic environment, but I may be wrong.

Hi,

How about a recon electric meter, something like:

or maybe available from a boot sale or similar.

Do you still have 12 PCs? :)

cheers, Pete.

You are. In an industrial environment, it's important, as the power company may charge you extra. But in a domestic one, (at least in the UK, and I believe everywhere else) it's not a factor.

Maybe. But, on some loads, some of these can be very inaccurate. Both the Lidl ones, and the one from maplin drastically over-read (double) on many PCs, for example. Fine on heaters, motors, lights, ...

Power factors of less than unity apply to reactive loads - like some type of electric motors. [It's actually the cosine of the phase angle when volts and amps are not directly in phase with each other]. If your device does the same as mine, you can get separate reading for volts, amps, watts and power factor. The power consumed in watts (and what you are paying for) is volts x amps x PF. For a purely resistive load - like a light bulb, PF is 1 - so it's just volts x amps.

I was aware of that. In my case, I'm more concerned with relative load of the same appliance under differing conditions....!

Can't you just use an ampmeter and measure the current?

P=IV so for 5 Amps P= 5*240 P= 1200W

now to convert that into Kwh , I dunno maybe someone can help out?

-- wig

That's only part of the story for power factor, non-sinusoidal waveforms (i.e. harmonic currents) being the other. Any electronic power supply working from a single-phase supply and using a rectifier with a capacitor-input filter (reservoir capacitor) - and that's most of them - will only draw current from the mains near the peaks of the voltage waveform. This gives a peaky or spiky current waveform which also leads to poor power factor, despite it only having a small phase shift.

The more general definition of PF is true_mean_power / apparent_power, the latter term meaning the product of the RMS voltage and RMS current. Your meter or monitor instrument (& the Maplin one seems to be the best) will display true power as W (or kW) and apparent power as VA (or kVA). PF = W / VA. (The electricity company's meter reads kWh, so you don't pay for the apparent energy you're not actually using - large users excepted).

There's an idea!

Bit more than that now...just got a bigger IP block!

OK, who are you?

Yes, I just about remember that from my electronics degree! But I want to integrate a variable current over time...

Lidl did just a gadget a few months back - not sure how much it was but it did integrate power over time - It was a 13 Amp plug one side and socket the other, with a display and a few buttons, a bit like a timer. Perhaps someone here got one and doesn't want it ?

Nick

It was about £6.49 and it appears to work well. I use it to monitor how much power my various incarnations of media centre PCs use.

maplin do

It's OK, the Machine Mart one appears to do it.

To bang on about this again - it may well be overreading by up to double. The Lidl one I got does, on many PCs. (Well, 2/3 power supplies I've tested) (both of 2 Lidl units do, indeed)

The maplin one I have misreads in a similar way to the Lidl ones. Indeed, I bought the Lidl one in the hopes that it'd be more accurate than the maplin one.

What way is that - and how much in error? (Genuine questions.) I have a Maplin one and whilst I haven't rigorously assessed its accuracy it does give quite plausible readings on low consumption low PF loads (such as CFLs).