Thread: Is a Watt a Watt or Not?

A watt's a watt, but you're not quite getting the full picture. (To be more accurate a watt = voltage x current. IE; 240volt x 8amps, gives 1920 watt.)

The power running to your house is 240v. Low voltage down lights are generally 12v.

Here's where it gets pear-shaped;

To run 12v lighting, you've got to step down the voltage from 240v. This involves a transformer, which in itself sucks a little bit of power on top of the lights. So 2x50watt halogen bulbs + transformer will actually suck down a bit more power than a single 100w incandescent.

Just to confuse the topic even more, when you first flick the switch you get what's called a surge current. An easy way to explain it is the dimming off all the lights in a workshop when you arc up a lathe at high-speed. The same thing happens with a light bulb turning on, but nowhere near as dramatic.

So... long story short, halogen ain't any cheaper. They also produce a shiteload of heat, too. Much more than a standard bulb. So, if you run an air conditioner on a thermostat, that's going to be sucking more juice to offset the extra heat from your halogen globes.

Want some food for thought? If you were to use a 12v transformer with LED globes, rather than halogens, they'll run cooler than a normal bulb and consume much much less electricity. Catch 22 is that the globes themselves are quite dear compared to normal ones.

(Note; I'm not a sparky. I worked at Jaycar many moons ago, hence the small degree of familiarity with the subject at hand. Hence, it shouldn't be taken as gospel. I could be wrong.)

This is one of those subjects like sharpening, rust removal, holden v ford, festool &/or triton v the rest of the world.
Everybody has an opinion & they have a higher opinion of their own opinion that everybody else's opinion.

The argument is all going to be void when they finally phase out incandescent bulbs 'cos they are too inefficient.

lol.....led's....I've actually opted myself to go with energy saving fluoro downlights with lighstar bulbs (the funny looking ones encased in a reflector bulb shell)...they are only 15 watts each and are VERY bright. Supposudly equivelent to a 75 watt ordinary bulb.And no heat issue such as halogen either.
Just wanted the argument over 12v lighting being cheaper on the bill to run, so from the reply I have so far it appears I was right? They arent because a watt is a watt!!

I shake my head at how many people put in lots of downlights and wonder why their electricity bill goes up.

The transformer produces heat and heat is where the inefficiency of them mainly is.

As for the flouro downlights, I have seen them and bought one from Bunnies for $16 to see what light output they produce. I haven't put it in anywhere yet as when it comes to wiring at home rather than work, u get lazy. They have a pack of 4 for I think around $50. They are made by Crompton.

[quote=Article99;469824]A watt's a watt, but you're not quite getting the full picture. (To be more accurate a watt = voltage x current. IE; 240volt x 8amps, gives 1920 watt.)

This is not strictly true. The Volts x Amps = Watts only applies to DC. For AC you have to take into account the power factor (which is calculated from the angle between the voltage and the current) This is where we start to get into real technicalities, but if you say that you have 1 consumer taking 10 amp on for one hour at 240 volts, therefore you have used 2400 Watt hours, your meter will disagree with you.
Bill

While on the subject I have a walk in scullary where I have a movement sensor and light bulb. The question is do the new fluoro light bulbs take more power to start up than incandescent? In other words if the light is turning on and off frequently is it cheaper to run incandescent. I have always believed that fluoro is only cheaper if it is on for a considerable time

So does the same argument hold true for 240 volt downlights? I know they push out a lot of heat, but then so does a standard incandescent. Excuse my stupidity, but aren't the 240v downlights in my kitchen likely to be halogen? I guess so, seeing as they are 50W but bright as hell......

Yes! A watt is a watt. Power consumed = volts times curret(amps) times power factor.
A flouro has a lagging power factor becasue of the inductive ballast(maybe around 0.6 to .8 roughly so it needs a capacitor to raise the power factor to near unity(1). Same for any inductive load (motors, transformers etc). The ballast is needed becasue the flouro tube is a short circuit when it strikes and the ballast acts as a current limiting device.

Incandescent lights are resistive loads so power factor is 1. I have yet to see flouro replacements for incandescents which give compare in brightness to incandescents. I have flouro lights in incandescent fittings claiming to equal "60 watt" but do not give the same bright light for easy reading etc.

As mentioned low voltage lights (12v) need a transformer to run them so you introduce another loss and point of failure into the system. May be fashionable but I wouldnt bother with them let alone the heat issue.

As far as replacing incandescents with flouro to save the planet, yes they may last longer (note may, not guaranteed) but they use a whole lot of other resouces to manufacture and toxic ones at that and you will probably need more of them for the same light output. All you need for an incandescent is a bit of tungsten wire and glass basically.
Have I mentioned the strobe effect with moving machinery under flouro light? Not good with you lathe types, when turning at a particular speed the work can appear to "stand" still.
Has happened where a worker has grabbed a work piece thinking it was stationary in the flouro light.
The starting current of flouros and cold incandescents is VERY short duration and is not something I would be worried about over the billing period.

Mythbusters
Lights On or Off

Myth: You save on energy bills by leaving lights on. Some people believe that the energy to turn on lights exceeds savings of turning lights off.
They talked to Mark Reisfelt, manager of the Independent Electric Supply where they purchased their light bulbs. He felt that it was best to turn the lights off.
To test the myth, they needed to measure energy usage during startup, maintenance (steady state), and shutdown.
For steady state energy consumption, they turned on several different types of bulbs for 60 minutes and measured their consumption using a Kill A Watt:

• Incandescent 90 Wh
• Compact Fluorescent (CFL): 10 Wh
• Halogen: 70 Wh
• Metal halide 60 Wh
• LED: 1 Wh
• Fluorescent: 10 Wh

For startup energy consumption, Grant hooked up an inductive current loop to a computer and measured the amount of energy used when the turned on the bulbs. With an inductive current loop, you run a wire through the center, which induces a current in the loop. This current is then measured by a digital sampling oscilloscope.
Based on the amount of energy consumed turning on the bulb, they were able calculated how long the bulb would have to be turned off in order to make it worth the energy savings, i.e. "It's best to turn off the bulb if you are leaving the room for":

• Incandescent: 0.36 seconds
• CFL: 0.015 seconds
• Halogen: .51 seconds
• LED: 1.28 seconds
• Fluorescent: 23.3 seconds

In other words, its almost always best to turn the bulb off. Even the 23 seconds for the fluorescent lights isn't very long, and the rest of the times are pretty much blinks of an eye.

Normal house meters only work on in phase V & I
so what the meter reads is in watts (or KWH to be more exact)

Peter SM,
I think you may be misinterpreting the results. A fluro will consume as much on start up as it does in 23 seconds running. Therefore, unless you're leaving the room for less than 23 seconds, it's not worth turning off.

Mick

Let's work this backwards...

50 watts at 12 volts requires 4.17 amps or 8.33 amps for two bulbs.

A normal 100 watt bulb requires .417 amps at 240 volts.

To get 12 volts from a 240 volt source requires a 20:1 step down transformer. The transformer will draw about .417 amps at 240 volts and provide (20 x .417 = 8.34 amps) the current for the low voltage lamps. (In a transformer, the current increases when the voltate is lowered.)

So 12 volts at 8.33 amps or 240 volts at .417 amps, it all works out to about .1 kW. At the high residential rate, here, the bulb(s) burning all day would cost $0.36. The meter doesn't care what you do with the power after it passes through the meter.

A Watt is a Watt is a Watt regardless of how it is used.

Originally Posted by jagfromaus

lol.....led's....I've actually opted myself to go with energy saving fluoro downlights with lighstar bulbs (the funny looking ones encased in a reflector bulb shell)...they are only 15 watts each and are VERY bright. Supposudly equivelent to a 75 watt ordinary bulb.And no heat issue such as halogen either.
Just wanted the argument over 12v lighting being cheaper on the bill to run, so from the reply I have so far it appears I was right? They arent because a watt is a watt!!

LED's will take over, the latest XR-E device from Cree is capable of 80 lumens/watt and available now.
In terms of comparing light output, these are now near the exact same efficiency of flouro lighting. The advantage they offer is a smaller source of light and much more robust than a fragile tube of glass.

And yes, a watt is a measure of power.
As others have stated, the lower voltage units simply draw more current, ending up with a similar power consumption. Add the transformer, and yes you will probably use more power than the naked 100W 240V bulb.
The major difference will be your colour temperature as the halogen runs much hotter giving you get a much whiter light.

- Quite the powder keg we've lit here...