The mention in the hdd thread of "power saving" got me thinking about how much power my computers use. I know that they must use a fair bit and cost a significant amount to run, but I don't know how much or how to measure it.

Ideally what I want is something I can plug into the mains between the device I am measuring and the supply. Maplins at one point sold something like this, but I made the mistake of not buying one at the time. It was perfect because it could also calculate the cost of the power being used.

They don't appear to sell anything like it anymore, nor can I find anything similar. The Maplins device was made by a German company, whose name I have forgotten.

Is there such a device out there still and if not is there an easy way of measuring power use over time ?

Edit: oh yes, forgot to say it needs to work on the UK mains supply

I've seen a device that clamps onto power cables to measure the amount of juice the device is sucking - it was being used to quickly measure the load on a floor of a building so that the power in the new building they were moving to could be scaled correctly.

I've no idea what it was called or where you could get one though

Sorry, not the most helpful of posts

This is the one I see here a lot. Maybe they make a european version?

That is the sort of thing, though the one I saw had a few more features. According to their website they only do a 110V version http://www.p3international.com/products/special/P4400/P4400-CE.html

Maplin have this on special at the moment, in the FAQ for the product it says it can measure use over a time of up to 9999 hours.

Is that what you are after ??? Pretty cheap too, I may get a couple myself.

Cheers

Cris.

Maplins didn't have anything like that last time I searched their site.

That looks like it is a version of the US one posted. It definitely does most of what I want.

Placing my order now...

Thanks

Pretty good at £12.49 too!!!

Cheers

Cris.

Yes, I seem to remember that the all-singing-all-dancing model that they had before was more like £50.

Ah, I've been waiting for them to come on sale again, they are on permanent sales cycle at Maplin.

I asked my uncle about them and he said he just connects a meter to a blown fuse some wires soldered on. He's a bit like our Patrick.

Gareth

i wish these things were a lot cheaper

I use a handheld "clamp-on" AC ammeter for this purpose. To separate out the hot wire (cannot get a reading by clamping around the bundled cable), I also purchased a $15 accessory that simply has an AC plug on one end, and a jack at the other, and in between it routes the wires separately through a plastic cage thing for clamping purposes. One side of the cage thing can be clamped-to for direct reading, the other side gives a more sensitive 10X reading when clampled onto. Very useful that.

Cheers

Yeah, but if you're gonna do that, why not just install one of the $15 in-line meters rather than spend $15 on the "splitter" and then another few hundred on the clamp ammeter?

Naw.. less than US$100 total.

Cheers

The company I work for designs and makes 3 phase (single phase too now) energy meters so I have a fair bit of experience with this sort of thing.

We bought a couple of cheap meters that used a wireless link back to a monitor with an LCD screen display kWh or $ spent. They used a clamp for one line of the power cord. However they did not measure voltage at all. You "programmed" in the voltage from a few presets.

If you use a current clamp, you need to measure the voltage also. You also need to measure the phase angle since kWh = volts x current x power factor (cos phi) x time (hours))

Domestic power is charged in kWh so you need to know the phase angle of your load to know how much you'll really pay.

Those plug in ones might measure phase angle and voltage but I'm not sure. Cheapo ones possibly assume a power factor of 1. If it can display system voltage and phase angle you're a sure bet it will do things right.

I've got one of these measuring the amount of power used in my office (the whole lot runs off one socket). It's been going 3506 hours and I've used 1301KW/h. I've worked out my office costs £20 per month to run.

The cheap plugin one that we have ordered has a button labelled "PF", so I'm guess that you can either measure or set the power factor. Is this enough ?

The two meters, US and UK, do look like they are made by the same people:




The US one claims:

"Connect your appliances into the Kill A Watt™, and assess how efficient they are. A large LCD display counts consumption by the Kilowatt-hour just like utility companies. You can figure out your electrical expenses by the hour, day, week, month, even an entire year. Monitor the quality of your power by displaying Voltage, Line Frequency, and Power Factor."

So it looks like it will probably do everything that I want.

Edit: in case anyone is wondering what makes these devices tick, here is the inside of one http://cafeelectric.com/killawatt/

There are also instructions on its use here http://k0swi.microlnk.com/REVIEW/P3/P3%20International%20Kill%20A%20Watt%20REVIEW.htm

Yeah looks reasonable. I think the ones we got were particularly cheap and nasty.

Basically custom insides so nothing really to see there.

My Maplins meters arrived today, they do exactly what they say in their tin. It would be useful in the display was backlit, but you can't have everything.

Looks like I'm not using as much power as I though. My two servers (big Dell PowerEdge and old Dell desktop), routers and switches only draw about 180W.

A question about watts and kwh. Is the relationship between them a simple one ?

Is it just a case that if I am drawing 180W constantly that I am using 0.180 kwh every hour ?

Oh and a Linux kernel build draws an extra 15W...

Looks like it is about £170 a year here, which is fine.

The problem that I have had until now is that I have had no idea how much power we were using or even how much money we were paying a year.

It doesn't matter how many times they come to read the meter or we phone in the meter readings, or electricity supplier still manages to provide estimated bills most of the time. These bills are often 100% off of the actual usage.

Combine this with the fact that we are on their budget payment scheme where we yo-yo between them owing us a couple of hundred pound and us owing them a similar amount and it is a mess.

At least now I can see instantly how much power I am using and work out how much it is costing. I guess I could have done that with the electicity meter as well, but that isn't very conveniently located...

I forgot to post this when I first notice this thread but didn't have my bookmarks available at the time. The Wattsup-Pro does data logging and lets you connect it to your PC and download the data. It costs a lot more but the logging feature is very cool.