How to work out power consumption

Hi,

I'm intrested to know the power consumption of some of my household items but with my limited knowledge could someone help me with the following.

I have brought one of the 2000MU Plug in mains power and energy monitors UK and currently testing for usage in KWH of a fishtank filter.

I plugged it in yesterday and after a couple of hours it read point 9 of a KWH, then after five hours went down to point 5 and after 12 hours plus it says point 74.

The rating of the pump is 48 watts.

Does anyone know why the reading would fluctuate?
How do I work the cost?
Would the meter go up to .999 before 1 KWH was displayed?

I know the price of a KWH with my elctric company is 12.72 per KWH,

Any help would be most appreciated,
Thank you in advance,
Rod.

The rating of the fish tank heater will be at a given voltage - nominally 230. If the actual mains voltage is higher, the consumption of the heater will increase, if the voltage drops, the consumption will be less. Mains voltage does vary during the day.

A plug in power meter may monitor voltage and current, giving a true power consumption - others that clip round the incoming power cable may not monitor voltage and are less accurate.

As for the readings.

your 48W heater is consuming power at 48 watts. In an hour it will have consumed .048 KWh (0.048 of a Kilowatt in one hour) of energy. In ten hours it will have consumed .48KWh (0.048 for ten hours) and will have cost you about 6.5p.

Is it on a thermostat? If so, the power consumption will drop to zero. So, if it was heating the water for 7 hours in a ten hour period, the energy consumption would be 7*0.048 = .336KWh.

The absolute accuracy of these meters is not that good - especially at low power levels, but they are useful for monitoring trends. If you want to accurately monitor your energy use, just read your electricity meter every day or week (at the same time of the day) and build up a long term picture. If you put the readings into a spreadsheet, you can turn them into a graph showing daily or weekly trends over time.

Quote:

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Originally Posted by peterb

The rating of the fish tank heater will be at a given voltage - nominally 230. If the actual mains voltage is higher, the consumption of the heater will increase, if the voltage drops, the consumption will be less. Mains voltage does vary during the day.

A plug in power meter may monitor voltage and current, giving a true power consumption - others that clip round the incoming power cable may not monitor voltage and are less accurate.




As for the readings.

your 48W heater is consuming power at 48 watts. In an hour it will have consumed .048 KWh (0.048 of a Kilowatt in one hour) of energy. In ten hours it will have consumed .48KWh (0.048 for ten hours) and will have cost you about 6.5p.

Is it on a thermostat? If so, the power consumption will drop to zero. So, if it was heating the water for 7 hours in a ten hour period, the energy consumption would be 7*0.048 = .336KWh.

The absolute accuracy of these meters is not that good - especially at low power levels, but they are useful for monitoring trends. If you want to accurately monitor your energy use, just read your electricity meter every day or week (at the same time of the day) and build up a long term picture. If you put the readings into a spreadsheet, you can turn them into a graph showing daily or weekly trends over time.

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Hi Peter,
Thank you for explaining the fluctuation, just to clarify this is a pump/filter not a heater so would be on for most of the day apart from a two minute period where it switches of and bleeds any air accumalated.
The monitoring at the main meter won't help as I have many fish tanks and a large pond and couldn't switch of for trending overall usage, hence plugging a meter in at the device points.
My aim is to work my way through the devices to gain a rough estimate of cost, I do appreciate I could go of the device ratings but wanted to test first hand.

One of the things confusing me was the readings quoted at different times, point 9 or point 5 for instance. Should this not be .009 or .090 etc... My logic is in that it is working out the average but now thinking about it it may be working out the total consumption over the given time period of monitoring.
But then why would the usage drop I'm confused.
I can provide a link to the PDF document which may explain but to me I don't understand.

Sorry for the confusion and my questions, hence any help is most appreciative.

Quote:

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Originally Posted by peterb

The absolute accuracy of these meters is not that good - especially at low power levels, but they are useful for monitoring trends.

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Which reminds me that ComputerBase.de did a group test of meters:
http://www.computerbase.de/2014-05/s...rblick-test/2/ (Google Translate)

Now of course being German they are all Schoko socket but an adapter should sort that. While I do have one unbranded meter, I have no idea about its accuracy (or lack thereof), and since I haven't come across a UK group test if I wanted more accurate results I would take these into account (assuming the everything can be mail-ordered).

This would be for people wanting to be accurate , so it's probably rather OTT for the OP's needs (and peterb has pretty much covered everything above), but maybe for Hexus users wanting to do some (somewhat) accurate mini-reviews it might be ideal. The "Brennenstuhl Primera-Line PM231E" there certainly seems very accurate for €11+. Amazon also have some by the same manufacturer but there's no guarantee that all their models are that accurate.

EDIT: @OP. From my experience, some of these can indeed trend things but I though it would be more a commutative total rather than an estimate into the future (as yours seems to be doing). One problem may be that your meter cannot cope with such a low load and a trick (somewhat wasteful) is to plug in something else with a steady load (incandescent light bulbs with a nice accurate 100W or 150W load should be ideal as once warmed up (seconds really) they should be very accurate). This moves the load you want to measure (your pump) from ~40W where your meter may be skewing the readings to a more measurable 150W(ish) load.

My 'el-cheapo' meter does allow me to select different things like Watts, Amps and (I think) Power Factor (which AFAIK domestic users do not have to worry about), so make sure you have it set to the correct setting.

Could be due to low load giving confused readings not sure, and can hook up a lead light to boost for a possible more accurate reading.

I've just checked and the reading is now .84 - grrrr is this the avearge per hour or total usage.

The following link is the PDF - please could some one look and see if they can advise, i think section 7 is relevant but can't decipher the detail. I can't post a link until I have completed 5 posts (forum rules) but a search with 2000mu-uk manual in google gives the maual on the first result...

Quote:

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Originally Posted by stango

I've just checked and the reading is now .84 - grrrr is this the avearge per hour or total usage.

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Depends on what display you have selected! The KWH switch has a toggle function, but shows the total energy use since the meter was powered up or the averag power per hour since power up (eg, if it was powered up for 10 hours and used 10 KW/h, the average is 1KWH per hour.

Quote:

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Originally Posted by peterb

Depends on what display you have selected! The KWH switch has a toggle function, but shows the total energy use since the meter was powered up or the averag power per hour since power up (eg, if it was powered up for 10 hours and used 10 KW/h, the average is 1KWH per hour.

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Had me going then Peter - thought you had cracked it but sadly not.....

There is only two displays when pressing the KWH button, totally hours running and usage.

It is currently showing 23 hours at .99 KWH

I wish I could find the contact details for the manufacturer as could ask them via mail but they are in Japan I think.

Does anyone think that maybe by process of elimination that the fluctuation could mean this is an avearge?

But then I guess .99 KWH would be close on 1 KWH which would far surpass the rated .048 KWH of the pump.

Hope I don't get told off, am posting this response so that I can get to five posts to post the link :)

Now this is my fifth post I can link the PDF for anyone who would,'t mind checking out the user manual http://www.prodigit.com/pdf/2000mu-01.pdf

Quote:

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Originally Posted by stango

Had me going then Peter - thought you had cracked it but sadly not.....

There is only two displays when pressing the KWH button, totally hours running and usage.

It is currently showing 23 hours at .99 KWH

I wish I could find the contact details for the manufacturer as could ask them via mail but they are in Japan I think.

Does anyone think that maybe by process of elimination that the fluctuation could mean this is an avearge?

But then I guess .99 KWH would be close on 1 KWH which would far surpass the rated .048 KWH of the pump.

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But surely 0.99 / 23 = 0.043 KWH aka 43W per hour, or am I missing something? That is over 23 hours a 48W load should have a cumulative total of 1.1 KWH. In other words (if that is a cumulative total), pretty much close to expected.

Quote:

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Originally Posted by kompukare

But surely 0.99 / 23 = 0.043 KWH aka 43W per hour, or am I missing something? That is over 23 hours a 48W load should have a cumulative total of 1.1 KWH. In other words (if that is a cumulative total), pretty much close to expected.

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Sounds promising kompukare - maths and electrics are not my strong points.....

would you mind explaining in lay man terms what is meant by the cumulative total at 1.1 KWH?

Also from all the readings over 24 hours and as a re-cap, it has shown the following readings - .9 - .5 - .74 - .84 - .99 Any ideas?
As I guess if I had made the calculation you suggested say at .5 which was taken at 5 hours then the reading would be different. My calculation .5 / 5 = .1 KWH.

I do think though you have cracked it, but some understanding on my last two questions would be appreciated if someone wouldn't mind takling the time for me.

Thank you so much. Rod.

Quote:

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Originally Posted by stango

Does anyone think that maybe by process of elimination that the fluctuation could mean this is an avearge?

But then I guess .99 KWH would be close on 1 KWH which would far surpass the rated .048 KWH of the pump.

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The pump isn't rated at .048 KWh - it is rated at 0.48KW. In one hour it will consume .048 KWh, in ten hours it will consume .48KWh, in 100 hours it will consume 4.8KWh - you are confusing pwoer with energy. Power is the rate at which is consumes energy measured in watts

The energy consumed is power x time, measured in watt/hours - ie watts per hour.

If you have a 3KW heater, that tells you its power rating. Run it for 6 minutes and your power meter will show .1 for time, and .3KWh for energy consumed. The KW button will show 3KW because that is the power being used, but not the energy consumed. After 12 minutes, the KWh reading will be .6, after 18minutes it will be .9, 24 it will be 1.2, and after 30 minutes it will be 1.5. (assuming you haven't disconnected the heater from the power meter)

Thank you Peter for the explanation and the time to help me understand, much appreciated.

I will apply this to the workings to understand the energy consumed, it's all starting to make sense now thank you...