Richard Hayes asks: is that light OFF?

“What does off mean?”

In the good/bad old days you turned off your lights with a physical switch.  This device broke the circuit between the load and the device so no current could flow.  As a result no more light was emitted.  “Simples” as the Meerkat would say.

These days things are not quite so simple, you can do away with physical switches by using smart systems to control your lights.  But what does it mean when you use an application or a voice activated device to control a hub which then sends signals to a smart device to switch off your lights.  Is it really “OFF”?

You might wonder why I became interested in this, well to cut a long story short last year I moved to a very rural location, the house is old and to make life easier we installed some smart lights.  When your nearest streetlight is 1.5 miles away and your nearest neighbour is a few hundred yards away; on a moonless night it is dark.  I have seen the milky way more times in the past 18 months than in my entire life before we moved.

Waking up on a quiet night I noticed that the smart bulb in the opal globe in my bedroom was softly glowing.  Not much but enough to see that it was on.  Once you notice something like this you can’t “un-notice” it.  Pretty soon my wife noticed it too, at that point I was obliged (for the good of my health) to do something.  I simply put the physical switch (which I had bypassed) back into circuit, so last thing at night I physically switch off the offending lamp so that I can sleep in peace.

This pretty much negates having the damn lamp on a smart circuit.

Interestingly other types of smart lamps from this manufacturer do not emit any light (that I can see) when switched “OFF” by the software.  Is this particular lamp faulty?  Luckily we have 2 other examples of this type in a reception room, checking on a dark night showed that these 2 lamps both emit light when turned “OFF”.  Having an enquiring mind and access to some reasonable equipment left over from when we ran a Photometric laboratory.  I was able to measure the “OFF” state or parasitic or quiescent or Standby (whatever you want to call it) power of these devices.  See Table below.  (For the Tech minded I used a Voltech PM1000 power analyser).

On Power (Watts) Quiescent Power (Watts) % full load
10.04 0.36 3.58
10.13 0.40 3.93
10.08 0.39 3.83

I did some digging on the manufacturers website and found that this type of lamp indeed has a maximum standby power of 0.5 W.  Interestingly on the website all of the other types of lamp do not have the same information but most of the newer types list a standby power of 0.5 W Max.  As far as I can tell only the E27 White lamp emits light when off.  I am sure other smart systems will have other levels of standby power; I have heard from of smart lamps that glow when “OFF” , they are out there.  I hope in the near future I will be able to measure the lumen output of this particular lamp when “OFF”.

I did some calculations to see what this means.

  • The average UK 3-bedroom house is approx. 110 m2
  • Presume we light it to an average of 125 Lux.
  • At 80 Lumen per watt (presumed average efficacy for a smart lamp) this will require about 350 watts of smart lamps.

If all of these lamps are in standby they might be drawing 13 watts.  If you need a Wi-Fi router or hub to make the system work this will consume another 0.1 Watts in standby.  Add in 3 or 4 smart voice activated devices at 1.5 Watts each standby and I allocate 0.5 W standby to the lighting, your power consumption when everything is “OFF” could be 15 Watts.

15 Watts doesn’t sound like a lot but remember this is 24 hours a day 365 days a year or 135 kWh per annum or about £20.00 per year or about 45kg of CO2 per year for average UK grid electricity.

If we assume that the average smart lamp in the average house is ON for 1.5 hours a day then the lighting consumes about 190 kWh per annum costing about £28.00.

Oi – Signify! Turn that light off!

So, we can conclude that using Smart lighting could very nearly double the running cost of the lighting system!  You are never going to justify a domestic smart system on power saving by load shedding, the only rational for using a smart system is convenience.

Again, LED technology has changed the industry, when we had control systems for GLS and Halogen lamps the power draw of the controls was an insignificant proportion of the lighting load.  With LEDs the control system power is very significant, but the total power consumed is still well below the levels associated with the old technology, but not as good as the headline figures.

We are very proud in our industry that we have gone from 10 Lumen/Watt GLS lamps to 80 Lumen/Watt Smart lamps in a few years, BUT if we add in the control system the effective efficacy of a Smart system drops to less than 50 Lumen/Watt.  This pushes us close to the minimum energy efficiency for lighting recommended in the Domestic Building Services Compliance Guide to the Building Regulations Part L1A.

This is a very complex area, if I use a smart system to dim my lights then the lighting load reduces still further and the effect of the standby power increases and the overall efficacy of the system reduces even more.  I have tried to make reasonable assumptions, controls are generally seen as good, but in reality, the situation is far from simple.  For instance the percentage of load attributed from the smart speaker to the lighting is arguable. 

As far as I can tell there is no guidance

What does “OFF” really mean?




Editor’s note: Richard is absolutely right to say that this is a complex area, which is why I found myself on a rainy Sunday morning watching a hilarious YouTube video from Mehdi Sadaghar, Iranian Canadian comedian and electrical engineer (whut?), explaining why LEDs glow even when they’re switched off.

This may, or may not, have something to do with what Richard is writing about . . .

Don’t try this at home!
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