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Smart meters 0, Induction hobs 1
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John Wood
Posted 2012-07-28 15:46 (#1093)
Subject: Smart meters 0, Induction hobs 1


Posts: 79
Location: Cromarty
Transition Black Isle has encouraged the use of smart meters and on the whole they have been helpful, but I thought I should share the following as it shows they can have their limitations.

I installed an induction hob as part of my house refurbishment, partly on the advice of my brother who has one. Later he said his seemed to use a lot of power when it was apparently off with only the LED clock working, and he had decided to turn his off at the wall when not in use. I assumed this was just something to do with the model he had, but recently I found a surprisingly high load recorded on my smart meter when nothing was apparently switched on, and by turning off all the circuits one by one and checking I tracked this down to ... my induction hob, which was apparently using 100-120W when on 'standby'. There was no doubt about it - I turned it off at the wall, and the meter registered a similar drop in current being used each time.

So I started turning my hob off at the wall when not in use. It's a bit inconvenient to have to restart the clock every time you want to use the cooker, and over 100W on standby seemed a lot, and I started making enquiries. Is this something inherent in the technology, in which case why aren't manufacturers owning up? Or is it a fualt in some of them, in which case what can be done about it?
Online I found several other enquiries from people on different forums with the same issue, but no answers. So I contacted the Energy Saving Trust, who were mystified and said they had never had this enquiry before. I was advised to go to Which? The Consumers Association.

I then contacted my brother, who happens to have a degree in electronic engineering. Here is his reply - I'm sure he won't mind me sharing it:

"Yes – we have been switching our hob off too, because the energy meter suggests it uses 64 watts on standby, so I decided to investigate this morning. I strongly suspected that the problem lay with the design of the energy meter rather than the induction hob, so I have done a controlled experiment this morning. I switched off every circuit in the house apart from the hob/oven and watched the supply meter. After one hour thirty nine minutes, the revolving disk on the meter was in exactly the same position as it started. In theory, with a consumption of 64 watts, it should have gone round 17.6 times.

There is a good reason why they may differ. The electricity meter is a genuine power meter – that is it measures the power being consumed (in KWh). The “energy meter” actually measures the current (number of amps) going into the house and then does a calculation to derive the power (current x voltage = power). Normally this is fine, but AC power has its complications. The current can slosh in and out of the house without actually doing anything - ie without actually providing any energy – particularly if there are big magnetic coils such as are found in transformers. This fools the energy meter into thinking that there is energy being consumed, whereas in fact there is none. What is happening is the current runs into the coils (of your appliance) – and builds up a magnetic field – but then the current stops and reverses (because it is AC) – at this point, the magnetic field collapses and generates a current back into the coils which itself runs out of the house – although there is a time lag. In energy terms, there’s no net flow, although there is a current flow.

Ah well, this is good, from now on, no need to turn the cooker off."

I hope this is helpful to others who may be mystified by smart meter readings - the answer seems to be: if in doubt, check against your electricity meter!

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We are part of the rapidly expanding worldwide Transition Towns movement. The Black Isle is a peninsula of about 100 sq miles ENE of Inverness in Scotland, UK.

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