Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

This is the box of a LED spot I just bought. I'm wondering why the power consumption is measured in kWh/1000h and not simply in Watts.

Photograph of box

Edit:

The labeling standard can be found here. (Guide for the application of the commision regulation (EU) No. 874/2012 with regard to energy labelling of electrical lamps and luminaries).

share|improve this question
9  
Because they think you're a dumb consumer. – Olin Lathrop Jan 17 at 13:34
    
There are no other technical details on the box? – jippie Jan 17 at 14:57
    
I wonder where I can find "The Commission Regulation (EU) No. 874/2012 states in Annex VII, part 2.", No Annex VII in the document. – jippie Jan 17 at 15:10
    
Do ah look like ah know what a killer what is? – immibis Jan 17 at 21:20
    
Monthly electricity bills are in terms of kWh (e.g. $0.30/kWh), so a consumer can easily see the cost of running this light bulb in terms of money per hour (in this case 7*0.03 cents per hour). – Vortico Jan 17 at 22:20
up vote 33 down vote accepted

Anyone who has a clue about how physical units works will of course realize that kWh/1000h means "1000 watt-hours per 1000 hours" which can be shortened to just W.

But when it comes to lamps, the unit "W" is already used for the light output. Light bulbs which use more energy-efficient technologies than the classical incandescent light bulb often state their light output in equivalency to an incandescent bulb with a specific power consumption. Until 2010 you could often find LED light bulbs stating to be "equivalent to a 40W bulb". So the consumer knows that if they want to replace an old 40W incandescent bulb with an equally bright LED bulb, they need to look for a 40W LED bulb. A consumer buying an LED lamp with an input power of 40W might be surprised by how bright it is.

Also, the average consumer doesn't know much about how electricity works. They know they need to pay for their electricity consumption in a unit called "kWh", so they want to know how much they need to pay when they run the device for x hours.

So from the point of view of the average consumer, the unit "Watt" means "light-intensity" and "kWh per hour" means "energy consumption". A physicist will of course inject that the unit for visible light radiated by a source is "Lumen" and "Watt" is the unit energy consumption should be measured in, so that's what should be printed on light bulb boxes. But physicists aren't average consumers.

Using different units for each - even if both of them are misleading from a physicist's point of view - is what's the least misleading way to communicate it to the end-user.

share|improve this answer
1  
+1 I hate your logic. ;-) – Cort Ammon Jan 18 at 5:09
4  
That's the "least misleading" until marketing people figure out a light bulb is lit 4 hours a day on average, and invent a new clever unit 1kWh/year==0.68kWh/1000h – Dmitry Grigoryev Jan 18 at 10:27
    
@DmitryGrigoryev ...and then there is a lawsuit and they need to add a fine-print *Yearly consumption may vary depending on how you use your light bulb. And then another marketing company comes along and realizes they can make their exactly identical product look better when they manage to find a study which says a bulb is only lit about 3 hours a day. – Philipp Jan 18 at 10:36
    
@Philipp They can always start the fine print with "Does not contain asbestos. No animals were hurt during production..." so everyone stops reading before the sentence you mentioned comes up. So yeah, in the end that second company wins. – Dmitry Grigoryev Jan 18 at 10:42
3  
People, please, stop giving them ideas... – mikołak Jan 18 at 11:04

The energy rating covers all types of electrical appliances including fridges, washing machines, etc.

In the case of a fridge the instantaneous current could be zero or full on depending on the thermostat. It makes more sense to put the fridge into a 20°C room, power it up and read the kWh used in, say, 24 h and scale it up. This gives a better idea of the average power consumed by the device.

I agree that this could be quoted as X watts average. On the other hand if I know I pay €0.15 / kWh for electrical energy it is a very simple calculation for a non-technical user to figure out the cost of running the appliance.

Mind your units: 'K' is kelvin. 'k' is kilo. ;^)

share|improve this answer
4  
"On the other hand if I know I pay €0.15 / kWh for electrical energy it is a very simple calculation for a non-technical user to figure out the cost of running the appliance." Is it? Tell said non-technical user that it uses 7kWh/1000h and then ask them how much it'd cost a year. – NPSF3000 Jan 17 at 16:14
    
@NPSF3000 Let's just hope that price drops to 0.12 €/kWh and then we can simply state 1 W = 1 €/year :) – jpa Jan 17 at 17:33
4  
@NPSF3000 Knowing the number of hours in a year is irrelevant. How many hours is the light bulb on per day? 1000 hours of use could equate to 10 years of minimal use. The fact is that everything is rated 'over 1000 hours' and is therefore directly comparable to everything else. – CharlieHanson Jan 17 at 17:37
    
I'm not sure if it's just a coincidence, but the nominal lifespan of a traditional incandescent filament lamp is 1000 hours. So the kWh/1000h figure for an incandescent lamp is also the amount of power it would use over its lifespan. – Simon B Jan 17 at 21:17
    
"1000 hours of use could equate to 10 years of minimal use." It could also be 1 year or 100 years, as an average non-technical user, how do I figure this out? – NPSF3000 Jan 17 at 23:27

It allows you to do an easy comparison between different light bulbs, because it is a standardised way of presenting the power consumption of the item.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.