Measuring wattage of appliances with a multimeter

Hi there,

I want to measure the watts of appliances, like my computer and see if I can increase efficiency and reduce consumption.

I see there are devices like Kill-a-watt, which you can just plug the socket into and this gives out a reading. From what I've read, these are accurate to 5%, then give or take 10w. So if a reading is 180w, it could actually be 161-199w, which isn't very accurate. That is my understanding anyways of the accuracy of these simple devices and as a complete novice I could have misunderstood what I've read.

What I'd like to know is if it is possible to do this sort of measurement with a Multimeter, perhaps just by wiring the multimeter to the head of the power chord, then plugging it in and measuring the current of Amps. Then amps * voltage = watts?

Is this how its done? I tried finding some things on youtube but I saw people using an additional 'amp clamp'. Some people said you need a really good RMS meter.

If it can be done, is this method more accurate then using a Kill-a-watt?

I need to buy a multimeter as well, so I thought rather than get 2 devices, if I can get away with getting a single device I would prefer to do that.

Cheers,
Rob

It can be done, but for safety and convenience it's best to stick with a Kill-a-watt or similar device -- check the specs of competing devices to see if some are more accurate. The longer left plugged in, the more accurate it should be.

Eric Thompson wrote:It can be done, but for safety and convenience it's best to stick with a Kill-a-watt or similar device -- check the specs of competing devices to see if some are more accurate. The longer left plugged in, the more accurate it should be.

Thanks Eric. I think I may have misunderstood the accuracy as I found a device that says accurate to .5%, which is accurate enough for me.

But assuming I am aware of the safety of dealing with electricity from power points, how is it done with a multimeter?

Rob, a lot of multimeters have the ability to measure amperage. They are more expensive than the more basic ones. The easier to use ones have a little clamp looking extension that goes around the wire in question so you don't have to have electrical contact with the circuit. I haven't looked for a while, but decent ones tend to be pretty expensive. And of course that leaves you to measure voltage and calculate wattage. Some of them may do that for you too, I don't know.

If you looked in the electronic maker forums, it might be possible to cobble together some electronics yourself just for that one function requiring, of course, understanding of electricity and electronics.

Luke has the right idea in suggesting you look for an "amp-clamp". The hard part of getting the wattage is not knowing the voltage, that's standard out of the socket. The hard part is determining how much current the device is drawing at any particular time and then averaging it over the time in use.

The cheapest amp-clamp you are going to find is probably this one from Harbor Freight.

On edit: I see you are in Estonia, so no going to Harbor Freight. But there has to be someone there importing the same Chinese made meter if you look around.

You can't put an amp clamp around a standard cord, you will get a reading pretty close to zero--as the power goes out on the white wire, it is coming back on the black and cancel each other out. They work on a single wire at a time only. So you either have to open up the breaker box or make a split extension cord--neither of which are that safe unless you know what you are doing.

R Scott wrote:You can't put an amp clamp around a standard cord, you will get a reading pretty close to zero--as the power goes out on the white wire, it is coming back on the black and cancel each other out. They work on a single wire at a time only. So you either have to open up the breaker box or make a split extension cord--neither of which are that safe unless you know what you are doing.

True, but easier to do in Estonia with their Soviet style 220v outlets. It's like the old days here in the States with ungrounded zip cords. All you have to do is slit the cord so that you can get the amp-clamp around just one wire.

John Elliott wrote:

R Scott wrote:You can't put an amp clamp around a standard cord, you will get a reading pretty close to zero--as the power goes out on the white wire, it is coming back on the black and cancel each other out. They work on a single wire at a time only. So you either have to open up the breaker box or make a split extension cord--neither of which are that safe unless you know what you are doing.

True, but easier to do in Estonia with their Soviet style 220v outlets. It's like the old days here in the States with ungrounded zip cords. All you have to do is slit the cord so that you can get the amp-clamp around just one wire.

This could be true. We didn't even have grounding in this house so it was a very old system.

Many thanks for explaining all that to me everyone.

Thank you Rob for making an effort to reduce your energy usage! As your downwind neighbor, I appreciate it. In an effort to reduce my downwind pollution headed in your direction, Iam about to implement some water heater conservation strategies. The first step in any conservation strategy is measurements, which as you point out can be tricky. The number of devices for this task continues to grow and I think the Kill-a-watt device is perfect for any appliance that plugs into a standard receptacle. The biggest energy users are hard wired however, and require more invasive strategies. Measuring the amperage or wattage at any one time can be very misleading as John points out, it can vary greatly over time. The measurement we are after in these exercises is kilawatts (electricity usage over time).

After buying a Hobo data logger (which I plan to use for other measurements as well) to measure the current draw of my water heater, getting a useful Kilawatt measurement proved difficult even with 4 second intervals throughout 48 hours. http://www.onsetcomp.com/products/data-loggers/U12-data-loggers

Sooo I took the old fashioned route and bought a dedicated electric meter. Pretty simple setup and now I have a very easy way of measuring my water heating energy use. I guess its time for a dedicated thread..

Said thread: https://permies.com/t/33022//Measuring-electricity-usage-water-heaters

Rob Irish wrote:Hi there,

I want to measure the watts of appliances, like my computer and see if I can increase efficiency and reduce consumption.

I see there are devices like Kill-a-watt, which you can just plug the socket into and this gives out a reading. From what I've read, these are accurate to 5%, then give or take 10w. So if a reading is 180w, it could actually be 161-199w, which isn't very accurate. That is my understanding anyways of the accuracy of these simple devices and as a complete novice I could have misunderstood what I've read.

What I'd like to know is if it is possible to do this sort of measurement with a Multimeter, perhaps just by wiring the multimeter to the head of the power chord, then plugging it in and measuring the current of Amps. Then amps * voltage = watts?

Ya the kill-a-watt meters are not the greatest. If it is off though, it will always be off the same way, so it can be useful. I have also found they do not measure anything under 1 watt, not even over time. This may not seem like much, but it seems that it is often these low watt bits that run all the time. Anyway, back to multi meters, yes voltage times current will give instantaneous power... sort of. It depends on the load. Most multimeters do not measure true rms voltage or current, but rather peak and then calculate the rms from that. Loads that are bad for this are CFLs, LED lamps... anything after a dimmer... computer power supplies. This may have something to do with kill-a-watt errors too. Purely resistive loads should be fine, AC motors should be ok, but not dc motors like hand drills (anything with brushes) Most multimeters are made to measure voltage.... and if there is current

Try to get the old analog meters if you can or get a meter that actually reads true rms. There are also watt meters (three terminals or four... or six or current loops ).