User Guides "Kill A Watt" to measure power consumption for 220v !

Device to measure power consumption for 220v devices!

There seems to be a lot of interest in tech enclave for measuring the amount of power that is drawn by the various devices in the house. So Im reproducing a blog that I had written last year on how to make your own "kill-a-watt" device. Im still using the device that I had and it is a great help.

Here is the original blog :
Random musings...: "Kill A Watt" for 220v !

And the contents reproduced below for those who cannot access blogs from work :

I have been thinking a lot about how to conserve power. Part of this is driven by the ever increasing number of gadgets in the house which are leading to larger and larger electricity bills !

I did a lot on searching on the net to figure out if a device to measure the amount of current being drawn by any device is available on the net. Sure it was, "Kill a watt" is a much used, much loved device to do this exactly. Unfortunately, like most things American, it uses 120v and there is nothing for the 220v side of the world. And one 220v option available for Europe was just ridiculously expensive.

So that set me thinking that this should not be too difficult to build. With the help of a few friends who were good at circuits, I finally built it for just Rs 750 ($15).

View attachment 11427

Here is the circuit diagram of the same. Remember that the current has to flow in series through the multimeter. The current has to pass through the multimeter and the selected component in series to measure the current flowing through the circuit.

Now for some live photos and DIY steps of how I to made it.

Disclaimer : Remember that we are talking of live current here, so the standard disclaimers apply. If you electrocute yourself or burn down your house, it is all your fault !!! Be careful with the circuits and do not connect anything till you are reasonably confident it is all going to work.

What do you need ? A digital multimeter that can measure current, these are available for Rs 600 in any good electrical shop. Remember that the multimeter's available for Rs 200 cannot measure current. They only work on DC and measure current voltages. (If there is a doubt, check if there is a "A~" option among the options).

This is the one I used:

View attachment 11423

You also need a ready made switch box, again available in any electrical shop for Rs 50. Make sure you get a 5amps model as I would not be comfortable connecting a 15amps component to the multimeter.

Now, you are ready to connect the multimeter to the switch box. Chop off one end of the multimeter cable so that you can connect it directly to the switch box. Remember to cut the cable from the side which does not connect to the mutimeter (It will be a special molex cable on one end to connect to the multimeter and a sharp pin on the other side. Cut the side with the sharp pins). Current flows from the mains to the box and the multimeter wires are connected in series.

Here is the snap of the same.

View attachment 11425

Close the box and bring the cables from one side of the box so that no live wires are hanging out. This is how the completed box looks like:

View attachment 11424

Now connect the multimeter molex cables to the multimeter and leave it there. This is because current will be flowing through the molex ends and you do not want them to touch the ground.

That is it basically. Set the multimeter to the 20amps mode to make sure the fuse does not blow, connect ANY device to the box and plug the box to the mains. Switch on the current and the device and you can start seeing live data on how many amps are being drawn by the device. If you multiply the amps reading with 235, you get the number of watts being drawn by the device. Note that even though India claims to run on 220v, we actually get anywhere between 220v and 240v in the mains. I seem to be getting 235v almost constantly, hence I multiply by 235. You can measure the voltage in your house using the same multimeter at the precise moment you are taking the reading if you want perfect figures, otherwise 235 is a close approximation.

See the picture here of my old 17" TFT monitor monitor connected to the circuit.

View attachment 11426

The 17" TFT monitor is drawing 0.21 amps, which translates to 50 watts of current being used. Note that my new 22" LCD draws a similar amount of current which shows how electronic components are getting more and more power efficient !!!

That is it folks, connect any 5amps device to the circuit and measure the current being used. Not precise, but a pretty close approximation of power usage.

A homemade DIY "Kill a watt" for 220v at a cost of Rs 750 !!!
 

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First off, many thanks for putting this guide up in TE - it is accessible from work. Not the blog! :p

Few quick queries -

1. What is the maximum current that can be measured by the DMM? This should limit the devices that can be plugged into the socket. I am ssuming this is a limitation of a DMM - I have an old and a pretty basic DMM lying around. Don't think it can be used to measure current.

2. What is the sensor resolution? 0.01A? Since that would determine the accuracy of the reading. For instance, your LCD draws 0.21A which equates to 48.3W.

3. Can the DMM be replaced by a simple ammeter instead? Would be a lot cheaper. :ashamed:

--- Updated Post - Automerged ---

Vinayaga, Kill A Watt is a registered trademark of P3 btw. The thread title may have to be edited. :p
 
Desecrator said:
1. What is the maximum current that can be measured by the DMM? This should limit the devices that can be plugged into the socket. I am ssuming this is a limitation of a DMM - I have an old and a pretty basic DMM lying around. Don't think it can be used to measure current.

2. What is the sensor resolution? 0.01A? Since that would determine the accuracy of the reading. For instance, your LCD draws 0.21A which equates to 48.3W.

3. Can the DMM be replaced by a simple ammeter instead? Would be a lot cheaper. :ashamed:

Vinayaga, Kill A Watt is a registered trademark of P3 btw. The thread title may have to be edited. :p
Click to expand...

Thread title edited, but the title in the forum does not seem to change.

You seem to have answered all your question yourself. The max power is limited by how much power can pass through the DMM. My DMM shows a max of 20A, but I think that is for short bursts of power to measure and not sustained power passing through it. That is why I mentioned that you should stick to 5amp devices so that there is no chance of a blow up. A ammeter might work, but Im not sure how it can be hacked to pass current through it safely without any live contacts outside. The DMM has a nice layout with plastic covers for all connectors that makes it safe.
 
vinayaga said:
The DMM has a nice layout with plastic covers for all connectors that makes it safe.
Click to expand...
True that. :)

OK I just asked a few queries relating to this to my boss and he seems to know all the electrical fundas as well. :S

He said a digital clamp multimeter is also useful to measure the current flowing through the device and some of the advanced clamps can as well measure power by rotating the dial to select that option. :)

And I just found that there are cheap digital clamps available in eBay! Digital clamp, Tools Hardware, Everything Else, Consumer Electronics items at low prices on eBay India

And here's an interesting article to use a clamp - How to Use a Digital Clamp Multimeter | eHow.com

Vinayaga, you deserve reps! :)
 
Well the device is perfect but there is little flaw...you are assuming your load i.e. the device which is consuming power is purely resistive...but it's not in your case...they must be having capacitive and inductive components...the actual formula for calculating power i.e.

Power = Current (I) x Potential Difference (V)

which works perfectly in case of DC translates to

Power = Current (I) x Potential Difference (V) x cos φ

where cos φ is power factor in case of AC. If you want to use this device...use it with resistive load only...like room heater, water heater or geyser...or even an electrical bulb will work...but not with PC Monitor or CPU. Rest is fine.

PS: I'm an Electrical Engineering graduate so am telling you this ;)
 
^How big the difference would be otherwise if the load is not purely resisitive, taking into account the capacitive or inductive loads? :)
 
Thanks for the information on the clamp multimeter. I seem to keep learning stuff as I spend more time on TE. I will read it up.

Desecrator said:
True that. :)

OK I just asked a few queries relating to this to my boss and he seems to know all the electrical fundas as well. :S

He said a digital clamp multimeter is also useful to measure the current flowing through the device and some of the advanced clamps can as well measure power by rotating the dial to select that option. :)

And I just found that there are cheap digital clamps available in eBay! Digital clamp, Tools Hardware, Everything Else, Consumer Electronics items at low prices on eBay India

And here's an interesting article to use a clamp - How to Use a Digital Clamp Multimeter | eHow.com

Vinayaga, you deserve reps! :)
Click to expand...
 
Nice howto, but a couple of things to keep in mind:

Most cheap meters are not accurate at the low end of the range. So for example, a 20A range on cheap DMM (which usually is accompanied only by a low-range measurement like 200mA) is not accurate below 2A or so. Since 2A is ~500W, most PCs except the most extreme and at full load, will suffer inaccurate measurements.

Clamp meters are definitely a better and safer way to check current flow through a wire. One does not need to cut the wire or expose anything to AC, and no switchbox is required.

However both of these methods would result in a RMS power measurement, and as post #7 correctly states, this is not always reflective of real consumed power in devices that behave as reactive loads. It may be used as an approximation at best, as the actual current through the device is not constant through the entire AC cycle.
 
Nice writeup. Brings out the inner-geek in me to assemble one. Luckily I found a 220V Kill-a-Watt on sale for about 10 pounds at Maplins when I was in London last and picked one up. It sure is useful if you can get your hands on one as it gives you the wattage, VA, PF, frequency, Amps etc..
 
cranky said:
Nice howto, but a couple of things to keep in mind:

Most cheap meters are not accurate at the low end of the range. So for example, a 20A range on cheap DMM (which usually is accompanied only by a low-range measurement like 200mA) is not accurate below 2A or so. Since 2A is ~500W, most PCs except the most extreme and at full load, will suffer inaccurate measurements.

Clamp meters are definitely a better and safer way to check current flow through a wire. One does not need to cut the wire or expose anything to AC, and no switchbox is required.

However both of these methods would result in a RMS power measurement, and as post #7 correctly states, this is not always reflective of real consumed power in devices that behave as reactive loads. It may be used as an approximation at best, as the actual current through the device is not constant through the entire AC cycle.
Click to expand...
Hmm, I agree with this, so, there is no home made recipe?

+LT
 
Lol @ vinay, i found your blog the last time this question popped up a month or so back but did not link it here as it did not address the problem of power factor.

cranky said:
Most cheap meters are not accurate at the low end of the range. So for example, a 20A range on cheap DMM (which usually is accompanied only by a low-range measurement like 200mA) is not accurate below 2A or so. Since 2A is ~500W, most PCs except the most extreme and at full load, will suffer inaccurate measurements.

Clamp meters are definitely a better and safer way to check current flow through a wire. One does not need to cut the wire or expose anything to AC, and no switchbox is required
Click to expand...
This is the other problem -- inaccuracy of the equipment used.

After i found vinay's blog, i chanced upon an aussie blog who went into more detail about this. Thing is he was using clamp meters that go for $350+.
 
I got myself a simple digital clamp meter a while back. :p

Precision Mastech Enterprises Co., LTD.

Costed 650 INR but worth it IMO. Build quality is pretty good. Bundled a cheap Chinese 9V cell along with it. The banana probes bundled are good. And it does the job as advertised. :)

And measuring is very very easy - just have the live wire between the clamp and it measures the amps. Voltage can either be measured or assumed to be 220V and multiply for the wattage. :p

Many thanks to Sangram aka Cranky for the inputs. :D
 
Yeah but you still can't reliably resolve the main question that ppl have -- how much power is the rig (or any other electronic equipment for that matter) consuming ?

I'm coming to the conclusion the only way to do it and it
- does not require any special equipment.
- costs nothing
- 100% safe
- no tech skills required cept maybe 5th std math.

Switch off everything in the house, run the rig for a few minutes and watch the elec meter then extrapolate :D
 
^ remember @cranky's post in that home wiring thread where he has mentioned that even the wires have a part in adding to the load? What if the wires from meter to the house are adding say 5-10W or so minimum to the load, there's no way you could find that out is there? All you will get is an approximate amount. And that amount will vary with the device drawing power from the meter if i'm not wrong. In the end you might end up having a sum about +- 15-20% off from what could be the real load.
 
Then we have to normalise for it.

- Connect a light bulb to the same power outlet as the rig, purely resisitive, run it for a few minutes with everything else off and watch the meter.
- Then use Vinay's method with this light bulb alone.

The difference between the two should be the power lost in the wire to the meter.

The remaining part is how much load on the PC ?

Are there benchmarks that can be used to say run the PC at 20%, 50% and 100% load ?

But each PC is different so how to tell whether the PC really is running at the loads desired ?
 
The flaw with that is that the light bulb and the PC have to form the exact same load.

The wire is part of the load if the load is proportionately large. At 5000 watts load (the example used earlier), the wire uses about 25 watts. So at about 200 watts, it is 1 watt - negligible for small loads (though not at full load).
 
BTW MECO has this month launched 220v power consumption meter with a max load of 10amp or around 2200watt.

One also see the power consumption if you have one of those digital power meters,which show everything from voltage to PF ,costs around 1300 for L & T Single phase ,these can handle upto 30A or around 6600watt.
 
Will the same problem with accuracy of equipment ie this L&T meter come into it ?

Hmm, this is something the electricty board approves of as its what they can charge for.

--- Updated Post - Automerged ---
cranky said:
The flaw with that is that the light bulb and the PC have to form the exact same load.

The wire is part of the load if the load is proportionately large. At 5000 watts load (the example used earlier), the wire uses about 25 watts. So at about 200 watts, it is 1 watt - negligible for small loads (though not at full load).
Click to expand...
Scratch light bulb idea and use geyser instead, nice big resisitive load that :eek:hyeah:
 
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