Induction hobs, the question of standby and the “power factor”

inductionhobMy blog post of the 11th November, where I talked about the apparently huge standby consumption of my “efficient” induction hob (click here to read it), has generated much discussion and debate about the relative merits of this type of electric cooking.

Since I published the post I have been in discussion with some technical expert colleagues of mine who work in a testing house that deals with measuring appliances on a daily basis so are pretty well informed on such matters.  They have been telling me all about the rather complex issue of the ‘power factor’.  Now, I have to admit that it has taken me a while to get my head around the concept, but I think I finally get it and ghence this is my semi non-technical understanding of the issue taken from resident techie expert Steve’s explanation (Steve’s explanation is copied in full at the bottom of this post for the more technically able amongst you):

Power factors are a result of the difference between ‘true power’ which is the actual, real power (measured in Watts) consumed by an appliance and the ‘apparent power’ (the product of Voltage multiplied by Current (Amp), and represented by VA) that appears to be consumed.  With most household level appliances these two types of power: ‘true’ and ‘apparent’ are identical, BUT with certain products this is not the case and the ‘apparent power’ can appear much higher than the actual power that is being consumed – still with me?

The reason for the existence of ‘apparent power’ is the power factor.  The power factor exists when there is a phase shift between the Voltage and Current waveforms, and this phase shift can happen when the appliance being powered has inductive or capacitive properties – ie such as an induction hob, and, apparently, low energy light bulbs such as CFLs.

The typical type of domestic ‘Real Time Display’ unit which displays overall electricity consumption in your home, such as my Wattson or others on the market, are of a type that cannot cope with the differentiation of true versus apparent power, and so, when measuring the consumption of, say, my induction hob, is actually displaying the ‘Apparent power’ rather than actually what my hob is consuming.  Still with me?

The main implication of such a ‘power factor’ does not lie with the householder or their electricity bill, as they only pay for the true power they consume, but the power station has to work harder than normal to deal with this issue – so for the supply side and grid, such ‘power factors’ are an issue as they require a bigger drain on the energy production side.

To test out if this was actually the case with my induction hob, and because you cannot measure a hob’s consumption on a single device electricity meter as it is wired in to the mains, I carried out an experiment overnight.  I turned everything powered in the house off at the plug, so my electricity consumption reading on Wattson was zero, and then turned my hob back on.  The Wattson told me I was consuming 150 Watts or thereabouts. I took a meter reading, went to bed and ten hours later took another reading.

The actual consumption of my induction hob over that ten hour time period was 260 Wh, so an average standby consumption of ~25 Watts.  That is a big difference to the 150 Watts my Wattson was recording; a difference of six times larger in fact!  I downloaded the recorded data from my Wattson for that time period and sure enough, it had recorded apparent consumption of 1560 Wh over the ten hour period.

So, what does that tell me?  Well, my induction hob is not as bad as I thought for starters which is a good thing, as I do love cooking with it.  A standby consumption of 25 Watts is not great by any means, but isn’t anywhere as bad as 150 Watts.  So its costing me around £30/year to do nothing, that is about 12% of my total annual consumption, rather than costing me about £180 and being over 50% of my total.

What is does mean though, is I cannot trust my Wattson to give me any sort of accurate reading on my total electricity use. As a result of this power factor issue (I also have a load of CFLs, so it is probably overestimating their consumption too)  I cannot, with any confidence, work out how much of my PV generated power I am actually using as I haven’t an accurate way of measuring total consumption, which is annoying.

 

 

 

 

28 comments… add one
  • sue December 7, 2012, 5:18 pm

    Thanks for such thorough research. Am wondering if halogen, ceramic or ' normal' electric offer any better alternatives for those of us not on the gas?

    Reply
  • Richard December 10, 2012, 7:14 pm

    This reminds me of the various gripes that went about as CFLs became more prominent. A nice, short, debunking can be found here: http://www.alteraeon.com/~soda/dsp/cfl-power-factor.html
     
    R

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  • Martin March 8, 2013, 1:27 pm

    Interesting article, but your use of units like Watt/hours is very confusing.  I suspect you meant that it used 260Wh (Watt hours) over night and the hence the standby consumption is ~25 Watts.

    Reply
    • Paula March 12, 2013, 8:45 pm

      Thanks martin

      You are, of course, correct, I will amend.

      Reply
  • Molly May 3, 2013, 1:06 pm

    Hi Paula Very worrying, especially as I have just purchased a set of induction pans for my new induction hob (not yet purchased, but investigated!). There is a Zanussi model which boasts 95% energy efficiency, as opposed to 60-70% from other induction hobs.  However I am now worried that the Zanussi could use more energy on standby. Any thoughts on that?    

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  • Ben May 7, 2013, 7:07 pm

    Glad to find this blog post, I recently did a check on power consumption of various appliances and was rather surprised to find my Electrolux induction hob drawing approx 130W according to my Efergy energy monitor. I also checked the draw with an inline ammeter which showed 0.52A which at 220V is not a million miles away.

    I did suspect this reading may not be correct as if dissipating 130W I would expect to feel that as heat on the casing of the hob, but there is no sign of this – the power factor issue explains it, thanks!

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  • Steve June 15, 2013, 1:30 pm

    I've got a 12 year old Baumatic induction hob & it pulls 80 milliamps at standby which roughly equates to 20 watts & it still works like new too, it should do for what it cost though! Overall it's a fantastic cooker & the quickest at boiling something that I've seen so far & at 20 watts on standby I reckon it's ok from reading about the running costs of others.

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  • Carol June 30, 2013, 3:16 pm

    Really helpful clarification on cost of use. I am considering an induction hob at the moment and I am concerned about the reviews I read on the web.  They seem to give a mixed review.  Can you advise on their use.  I want to be able to seal meat quickly and at high temperature and I am used to cooking on gas which allows me to heat up the pan before use giving a good quick brown.   Research seems to indicate this is not as good on an induction hob please can you confirm one way or the other.  You have allayed my doubts about running costs would love to hear some honest info on their use.  Thanks.

     

    Carol

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  • Alan Holmes July 12, 2013, 4:08 pm

    What your "wattson" measures is VA, (volts x amps) which is only watts,  (power) in purely resistive loads, such as conventional electric hobs and cookers and tungsten light bulbs. In ac circuits which have a mix of resistive, inductive and / or capacitive loads the power used is volts x amps x the power factor (watts) = VA x cos Ø, (power factor). The power factor can vary from a value of zero to one. Zero is for a purely inductive or capacitive load (zero power) and value of One is for a purely resistive load (100% power).

    Your induction hob will have a small power supply unit built in to provide low power to its electronics and control functions, the touch controls, indicator lights timers and on / off functions which will no doubt be fed from a small transformer (inductive)  to reduce the voltage to supply the electronics and with capcitors to smooth the volts and amps used.

    So on standby it will take the 26 va quoted but probably with a low power factor which will result in a very few watts of power, 5 ~ 10 watt at a guess.

    When it is in use the power taken will be as quoted in it's technical specificaction.

     So, I hope the above will help you to sleep at night.

    Alan Holmes. Electrical Engineer.

     

    Reply
  • Craig McLeod August 9, 2013, 2:48 pm

    Another issue you may or may not have discovered is that these Wattson type meters cannot discern which direction the current is flowing in. This means that with your PV system it will look like you are using a ton of power on a sunny day whereas actually the power is being exported not consumed. There are some useful articles about this in the MSE forums. Hope that's useful.

    Reply
  • russell August 19, 2013, 1:42 pm

    Needless to say, this 'green' application of an induction hob is not so green. Neither is the need for new pans and a new product creating any saving whatsoever as the offset of the new products ( hob & pans) alone is significantly greater than staying with traditional electric or gas hobs. Simply because of the production overhead.

     

    Thereafter the figures kind of justify a 'NEUTRAL' acquisition, in that the gain would appear to be hygiene and not energy based for the total lifecycle 

     

    Induction hobs are by far more convenient but 

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  • Andy September 6, 2013, 9:49 am

    I have read both of your posts (as I am thinking of getting an induction hob myself).

    I have a background in electrical engineering and thought it worth mentioning that most of the power usage in the UK comes from devices with very poor power factors. This is mostly motors and pumps used in industry. It is possible to implement power factor correction, this usually involves adding a capacitive load to the network to balance out the, usually vastly lerger, inductive load.

    This would bring the apparent usage closer into line with the real usage of the appliance. It is probable that the electricity board already does this at substation level, as it is not cost effective to implement power factor correction in a home environment.

    Reply
  • Gabriele Nadin September 10, 2013, 5:36 pm

    am thinking of buying induction hob for new kitchen. Is there any truth in possibility of getting electric shocks by touching pots? Also, can you use pots with a much larger base than standard size – ie pots for jam/marmalade making? would appreciate feedback.

    Reply
  • bob October 4, 2013, 10:42 am

    It would help to decide whether to buy induction or not if you gave info on the comparitive running consumption of induction to 'normal' electric hobs. And also explain why induction has to be left in standby mode

    Reply
  • Natalie November 5, 2013, 2:23 pm

    Thank you SO much for this article.  I've been planning a new kitchen for a house that we're in the process of buying and an induction hob is an appliance that we'd definitely like to have.  Reading your article prompted me to contact Fagor (they're part of the same company that make De Deitrich induction hobs) and they have a 5 zone 90cm hob that I'd found at a good price.  Their Technical Manager told me that regulations in October 2010 came in that meant that an appliance couldn't use more than half a watt whilst on standby and that he'd recently tested their 90cm model that I was asking about that that it did conform.  I also asked him if regularly turning the hob off could cause problems and he explained that as long as you wait until the cooling fans underneath the appliance had stopped working and had cooled the electronics after cooking, it wouldn't do the appliance any damage or harm to be turned off at the isolation switch.  (The main thing would be remembering to do that, if the switch is located in a cupboard and not immediately visible.)

    Thank you again though for highlighting this possible major problem with an induction hob!

    Reply
  • Liz S November 6, 2013, 10:08 am

    Thanks for this. My power meter was showing 150 W for my new whirlpool induction hob in standby, so it's good to know I'm not actually burning through that much power!

     

     

    Reply
    • Paula November 6, 2013, 11:07 am

      Hi Liz

      Thanks for your post.  Can I ask if you bought the Whirlpool hob brand new?  If you have then I suggest you might want to challenge Whirlpool about the factor they are not adhering to the 1 watt standby regulation.

      If you can, do the experiment I did to find the true standby consumption of your hob. Turn every electrical appliance off in your home overnight (if you can of course, I’m lucky that nothing is hard wired in) but leave your induction hob on.  Take a meter reading before you go to bed, then take another when you get up in the morning.  That will give you the true standby consumption of your hob.  As your display device is reading 150 watts for the hob’s standby, just like mine did, then I’m guessing your standby consumption might really be about 25-30 Watts.  This is much higher than the regulations dictate and you should challenge Whirlpool about it (my hob is 9 years old and was bought before the regulation on standby consumpion came in, so i cannot challenge my manufacturer)

      I’d be really interested in how you get on, so please keep my informed if you do manage to find your true standby consumption and challenge Whirlpool

      Best Paula

       

      Reply
  • Susan November 9, 2013, 1:20 pm

    Thanks for the info we are hoping to get an induction hob for a new kitchen and this has been great and funny may I add. I am no longer confused.

     

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  • Anne O'nmoyous December 2, 2013, 12:42 am

    You can get power consumption tools that can cope with power factor.

    Reply
  • Mark December 4, 2013, 10:01 am

    Paula,

    OK, update on my previous post.  It turns out that the actual energy use of the Lamona induction hob is only 5W in stand-by (from my measurements of actual consumption on the electricity meter), but the clip-on energy monitor is confused by the AC current going into/out of the induction hob and reads it as 100W.  Apparently it's to do with 'power phase' and capacitors in the induction hob, and all clip-on power meters will have this inaccuracy in them.  The only the energy montitors which will accuratly measure this are the ones which read the flashing red LED on the front of the electricity meter.

    Kind regards, Mark.

    Reply
  • Margaret December 4, 2013, 12:59 pm

    I have come across your comments by accident while googling if induction cookers use more electricity and find all the remarks very helpful, haven't digested it all yet but now I know why my electricity bill is higher than my gas bill for the last year !!  My induction hob was installed last Sept '12 and is an AEG cooker.

    Reply
  • Tommy December 11, 2013, 12:28 am

    Do you have any feedback as to reliability. I am considering purchase of an induction hob but wonder at the high charge for insurance cover as to expected working life and replacement costs

    Reply
  • Riccardo Grillo December 15, 2013, 10:23 am

    Good old power factor caused me confusion as well until I got my head around it!  Those power meters should really come with a warning!

    I noticed that high frequency electronic fluorescent fittings in my home had a power factor of nearly 1, so would measure accurately enough using a clamp on type power meter, but older switch start fluorescent fittings might have a power factor as bad as 0.49 so this made a huge difference.  What surprises me is that you mention CFLs – all the modern ones I have seen incorporate electronic ballasts, and do not have a bad power factor, you must be unlucky.

    Another gripe I have with the power meter I once used (from Essex County Libraries) is that it asks for the mains voltage as part of the set-up but the default is 230V.  Officially, UK mains is now 230V, but, in practice, 240-250V is more likely in many areas.  If the power meter is simply using the given voltage to calculate VA and show the result as W, this will cause it to underestimate power consumption.

    Luckily we all have (hopefully) accurate meters provided by the electricity board.  The analogue ones with a disc are particularly good as the disc is usually marked from the top, and, once you work out what each mark represents, this can be useful.  Sadly this type seems to be becoming less common.

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  • mag December 15, 2013, 4:39 pm

    Paula, You suggest that regular unplugging may damage an induction hob. I am wondering why?  I have a cheap (£30 SilverCrest)  portable 1 ring  induction hob that is unplugged and moved from worktop to storage trolley at least 5 times a day.  It is almost 2 years old.

    Regards, Mag

    Reply
  • simone December 18, 2013, 7:49 pm

    Thank you for your thorough research, and the other experts for their comments – although, I'll need to read it again for it all to sink in! I am looking into getting a free-standing induction cooker and have been googling for several months and I've used the Which? site…now I'm research burnt and still unsure as to the best make/model to get due to the mix of reviews. So glad I found your site to be able to ask the question…which is the best over-all free-standing induction cooker?  

    Reply
  • fred richardson January 10, 2014, 12:08 am

    This power factor problem in most induction cookers is caused by a radio interference suppression circuit. The circuit is connected directly across the mains and is designed to eliminate radio interference (to meet compliance). It is usually a capacitor that causes this deliberate action. The only side effect is, as a few people have already pointed out, a 'virtual' current that ebbs and flows every 50 Hz cycle of the mains. This ebbing and flowing of current doesn't use real power but does cause hassle at the power station. You could say that the current in the power transmission line is pushed and pulled all the time (even when the cooker is operating) when it doesn't need to be.

    Reply
  • Miro March 24, 2016, 10:40 pm

    So how did you solve the problem with power consumption ??

    My Siemens hob runs 50W max in standby mode, that is about 50 Euro a year.

    I’m thinking to buy 380V switch and power it off if not in use.

    Do you know if 380V radio controlled switch exists?

    and thank you for the info.

    Reply

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