Wallwart energy use

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Wallwarts are little plug-in power supplies that power small appliances. They're often maligned for wasting power, due to often being left always on, and having some quiescent power use and less than perfect efficiency. A fact based assessment can clarify the truth of the situation.


Main Wallwart article


Search words

PSU, power supply, wall wart


Figures

I once had 5 minutes to wait and did a wallwart survey on an average 3 bed house with gas heating. The things I do.


Annual electricity spend: apx £600


Wallwart count:

  • Bedroom 1: mobile, laptop, mini fan
  • Bedroom 2: mobile
  • Bedroom 3: 2 mobiles, pc speakers
  • Lounge: 0
  • Kitchen: 0
  • Bathroom: 0
  • Other: mini tv, preamp, laptop, broadband modem


Bed 1:

  • mobile: 1 hr twice a week, apx 4w waste = 8wh/wk
  • laptop: 0.5hr/day, apx 10w waste = 35wh/wk
  • fan: 14 hrs/day for 6 months a year 3w = 21wh/wk


Bed 2:

  • mobile: 1 hr 2x a week, apx 4w waste = 8wh/wk


Bed 3:

  • 2 mobiles, 1 hr 2/week each, apx 4w waste = 16wh/wk
  • 1 pc speaker: 3hrs/day, apx 3w waste = 63wh/wk


Other:

  • mini tv: 24/7 2w waste off, 10w on. on 2hrs/day
  • = 64 wh/day = 448wh/wk
  • preamp: 24/7 apx 2w waste = 336/wk
  • cable modem 24/7 apx 5w = 868wh/wk
  • laptop 1hr/week @ 10w waste = 10wh/wk


So total estimated power losses:

  • Bed 1 106 wh/wk = 5.5 kWh pa
  • Bed 2 8 = 0.4
  • Bed 3 79 = 4.1
  • Other 1662 = 86.4 kWh pa


Annual total is 96.4 kWh pa.

  • At 10p/unit thats £9.64
  • And annual spend is £600 pa


so Percentage of electricity wasted by wallwarts is apx 1.6%


For electrically heated houses the percentage is much smaller, since annual electricity spend is much higher.



Switching off

What would happen if one went round the house turning them all off when not needed?

  • Bed 1 use would not be any less
  • Bed 2 use would not be any less
  • Bed 3 use would not be any less
  • mini tv saving: 22hrs 2w = 44wh/day = 308 wh/wk
  • preamp saving: 23hrs 2w = 46wh/day = 322 wh/wk
  • modem saving: 20hrs 5w = 100wh/day = 700 wh/wk

Total possible electricity saving 1.33kWh/week = 69kWh pa = £6.90 per annum.

However the heat this wasted power gives off has value in winter. In an electrically heated house (storage heaters) the annual saving will be halved for 8 months a year, and full for 4 months in summer. Saving is thus: 8/12 x .5 + 4/12 x 1 = 2/3 the above figure = £4.60 per year

In a gas heated house, saving is about 2/3 the above figure in winter, and 100% of it in summer 8/12 x 2/3 + 4/12 x 1 = 0.777 x above figure = £5.37 per annum

So by going round turning them off every time all year long you can save about £5 a year. Would it be worthwhile? Lets estimate 4 times a day, ave 30 secs each = 2 mins a day switching them off. That's 12 hours a year = 41p an hour payback for the labour of switching wallwarts off.


What if we got rid of warts

Getting rid of warts at the product design stage would mean incorporating the small power supply into the appliance itself, rather than it being external. The parts and function of that supply would be the same, in other words there would be no energy gain of any kind by eliminating external warts. The energy use would simply move from one location to another, from outside the appliance to inside.



Accuracy

The figures are for a fairly typical household, but of course changing patterns of use do cause some variation.


The power waste figures were typical inefficiency estimates rather than individually measured. However this is plenty good enough to demonstrate the nature of the situation.


Wart efficiency

The efficiency of warts varies a fair bit, but often they aren't marked with enough data to determine efficiency without resorting to testing.

As a general rule of thumb:

  • Modern lightweight switched mode warts are often very efficient
  • Lump of iron type warts have moderate efficiency
  • Some of the early switched mode warts (from 2000 or so) can have poor efficiency and run hot

Energy loss always shows up as heat. So relative power loss is the case temperature rise above ambient multiplied by the case's surface area. So in simple terms check how hot they are; stone cold warts are very efficient, hot warts are inefficient. If a wart doesn't even get warm, power loss is as good as zero.


Benefits of warts

Compared with using internal supplies in appliances, which is currently the only other mass workable option, wallwarts give the following benefits:

  • Smaller lighter appliances
  • No need to design, test or certify the appliance to mains safety requirements: cuts design time, costs, material use, and the legal & paperwork costs of certification
  • Removal of the power supply's heat from appliance extends the life expectancy of small appliances to a limited degree
  • Use of external power supplies makes it practical to use the appliance in another country with another mains voltage or frequency
  • An external supply makes one appliance model exportable to all countries, reducing business costs
  • Mass produced warts can be imported cheaply, offsetting design & build costs for British goods
  • A failed power supply is easy to replace, the more expensive appliance is still good.


Downside of warts

  • Use of the wrong wart can in some cases kill an appliance
  • Not known for their beauty


See Also