Individual monitoring of fridge does not match meter reading

northernsoul

I have been monitoring the consumption of my fridge (with icebox). I decided to check the reading against my electricity meter reading during the night.
I turned off all the circuit fuses except my kitchen.
On the first time I switched off every socket except the one supplying the fridge (single socket). Over 6 hours my tester said I had used 0.21kWh, my electricity meter 0.26kWh, 24% more. I did the same test last night but unplugged everything except the fridge. Over 8 hours my tester said I had used 0.32kWh, my meter 0.37kWh, 15% more.
Is this normal?
I don't really want to test longer than 8 hours because I had to turn my freezer off. I suppose I could test my freezer for 12 hours or more and turn the fridge off.
Which would you trust most, the Brennenstuhl PM230 tester I have or the electricity meter?
If I measure an electrical appliance over time (best I have got is the kettle or fan heater in bathroom) how do I know the kettle, for example, is pulling exactly 3 kWh ( so say 3 minutes /20 to come to a reading). Think on my tester it said 2970 but can I trust it?
Would you think the electricity meter is over reading? It did not seem to move when I had every fuse turned off for about 2 minutes.

matelodave

Are you absolutely sure that nothing else was on - phone chargers, routers, clocks etc.

It's hard to check your meter unless what you are checking it against has been calibrated and any inaccuracies are known and taken into account. Have you got a calibration certificate with it? If not then you can't assume that it's accurate enough. I note that the accuracy is specified at +/-3% and +/- 10watts so a 200w reading could be anywhere between 194 and 206watt without taking into account the +/- 10watt uncertainty

Trying to check it against a kettle doesn't work either.
A 3kw kettle will take 12 amps at 250v so would have a resistance of approx 20.83 ohms but unless you know for certain what the actual value is (which probably changes as the kettle heats up anyway) you can't make any assumptions about it's accuracy either.

If you assume that it is 20.83 ohms then if your voltage is 235v then you'll only be drawing 11.28amps which is 2650watts. So you need to know exactly what your kettle element resistance is and the exact voltage during the whole time you are measuring to be able to check your monitor.(my voltage is any random number you like between 244 and 249 which equates to a wattage somewhere between 2858 to 2940 for your 3kw kettle without adding in the +/-3%)
There will also be some resistance and consequently voltage drop between your incoming mains and the point at which you are measuring which might add a bit more uncertainly.

It's a lot harder with stuff that has motors (like fridges, freezers, vacuum cleaners, fan heaters etc) or fluorescent lighting as something called power factor comes into play so the current and voltage aren't synchronised so it's not so easy for a cheap monitor to get an accurate reading.

You'll just have to accept that unless you get your supplier to put in a check meter or to get your electricity meter tested and calibrated then your monitor is only going to give you a rough idea of what each appliance takes..

Kitchen_Sink

northernsoul wrote: »

I have been monitoring the consumption of my fridge (with icebox). I decided to check the reading against my electricity meter reading during the night.
I turned off all the circuit fuses except my kitchen.
On the first time I switched off every socket except the one supplying the fridge (single socket). Over 6 hours my tester said I had used 0.21kWh, my electricity meter 0.26kWh, 24% more. I did the same test last night but unplugged everything except the fridge. Over 8 hours my tester said I had used 0.32kWh, my meter 0.37kWh, 15% more.
Is this normal?
I don't really want to test longer than 8 hours because I had to turn my freezer off. I suppose I could test my freezer for 12 hours or more and turn the fridge off.
Which would you trust most, the Brennenstuhl PM230 tester I have or the electricity meter?
If I measure an electrical appliance over time (best I have got is the kettle or fan heater in bathroom) how do I know the kettle, for example, is pulling exactly 3 kWh ( so say 3 minutes /20 to come to a reading). Think on my tester it said 2970 but can I trust it?
Would you think the electricity meter is over reading? It did not seem to move when I had every fuse turned off for about 2 minutes.

Normally I would say the official meter is more accurate.

But I've never heard of the monitor you are using so I did a bit of reseach. It appears to be a simple plug in monitor that sells for about £10

More importantly, the instructions state it's accuracy as +/- 5% or +/- 0.1kWh (whichever is larger)

... and you are looking at differences of some 1/100ths of a kWh. :cool:

I usually find my gut instinct to be correct, and it has proven the case again today

northernsoul

Thanks very much for your replies
Metelodave. There was only the fuse on supplying the kitchen and I am 99% sure everything was turned off in the kitchen apart from fridge. I have plugs in cupboards and one above the wall units and I am 99.9% sure I found everyone and I am 100% sure all the sockets above the worktop were turned off and also my boiler was off. Also when the fridge was off the meter did not go round. My tester has not got a calibration certificate so I will accept that its probably the tester that is wrong.
Kitchen Sink. Thanks for taking the time to research my monitor. I should have thought about doing that. Its more than 5% out but also less than +/- 0.1kwh out. So on a, admitably, short test, with very little electricity used, its within spec and out of spec (the +/- 5%).
Maybe I should test a device longer, preferably one without a motor! But I will accept that the tester is wrong.
Been round testing most of my electrical stuff and I still cannot figure out where my £700 / 5000kWh a year electricity usage is coming from!
For anybody's interest, my 10 year old fridge uses at least twice as much electricity as a new one but an integrated fridge costs £250 upwards and at £30 a year extra running costs will take 8 years to recoup!

Andy_WSM

These plug in testers are notoriously inaccurate on low power devices.

Your bill may partly as a result of lots of small loads, making for a large background load 24/7. Chargers, clocks, automatic lighting, doorbell, plug in air freshners, clock on oven / microwave.

It's surprising when you look around a house at the things that run 24/7 that we don't give a second thought to.

matelodave

I don't think that its a good idea to chuck out something that's working in the hope that buying a new one will save enough money in energy costs to give you a reasonable payback. By all means look for an energy efficient item when replacement time comes round but even then paying well over the odds for a super energy efficient one may not be cost effective, especially if it dies before it's paid for itself.

Robin9

From your own figures 0.21 kWh over 6 hours at 11p (inc VAT) a unit your fridge is costing £35 a year. Assuming a Standing Charge of 23p a day then that is £75 a year.

Count up all the other appliances, lighting, chargers, then you may well find that an annual bill of £700 really is quite good deal.

northernsoul

Thanks Andy_WSM I know what you mean. I see some people with much much lower than me and I wonder how?! One recent poster said he was using about (from memory- cant find it now) £100 a year in a flat. How!
Nevermind

Andy_WSM

northernsoul wrote: »

One recent poster said he was using about (from memory- cant find it now) £100 a year in a flat. How!
Nevermind

Neanderthal man used less, I'm sure

I've often wondered how it's even possible, but I don't want to be paranoid about putting the TV on or having lights on or concerned about how many times I run the washing machine to keep myself at a decent level of hygiene.

There's been a spate of burglaries around here of late, so I've got CCTV on 24/7 and lighting on PIRs coming on & off all night - there goes my hopes of lower bills any time soon

northernsoul

Metelodave - I agree. Not worth changing it.
Robin9 Thanks for replying. It may well be that £700 is okay. Would just like to get down to an average usage of 3500kWh/year! Highest usage is my Pioneer Kuro 40inch plasma TV at about £80 a year. That though, is something I will not compromise on. Even if it breaks down (its 6 years old), I will still get the best quality TV regardless of running costs.

matelodave

I can't get mine down to 3500kwh a year (well I could if we weren't here during the day) it's nearer 4500kwh but that does include heating the hot water tank as well.
Heating the place uses another 4000kwh a year so our total is about 8000-8500 depending on how cold it is