I work at a large residential site (imagine a hospital, but that's not
quite it), and our maintenance guy has been to a conference where he
saw a product which apparently down-converts incoming 240v supply to
the site to 220v. On the basis that most equipment these days can
operate at 220-240 anyway, we are being told that that is fine to do.
They are selling the "device" that does this as a means of saving
energy / money.
Can this possibly work?
I'm no engineer, but I'd have thought that for examply boiling a
kettle would draw the same amount of power whether it is fed 220 or
240v, its just that the current that would be drawn would be higher in
the 220v scenario. Similarly for any device that uses "power" at a
specific rate, reducing the voltage would just increase the current
I can see perhaps how normal incadescent light bulbs might not use as
much power, but they would simply burn slightly less brightly,
Does the device and its claims sound credible?
Wrong. If the voltage is lower the current will be lower and the power
lower. The *energy* consumed to boil the kettle *will* be the same (of
course). This means it will take longer to boil (by a factor
240**2/220**2 ) - you will save nothing anywhere where a certain amount of
energy is required.
Yes. Whether you would notice in practice is another matter.
Well yes but you would achieve bigger savings by changing lightbulbs to
energy saving ones (or lower wattage ones of the same type) and save the
money on the device.
(anti-spam is as easy as 1-2-3 - not)
Sounds like a complete scam to me.
to boil a kettle you would need the same amount of energy.
Watts is the thing, so a lower voltage would either have a higher current draw
for the same power output (same energy usage) or, where current was limited
then it would not work as well e.g. your incandescent bulb would glow with less
To save money look at _real_ techniques rather than rip-off gimmicks.
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On May 9, 3:25 pm, firstname.lastname@example.org wrote:
No. A kettle element is just a resistor designed to get hot. At lower
voltage, a *lower* current will pass through it. You need the same
energy input to boil the water so it will take longer. reduce the
voltage too much and it will never boil due to the heat loss being
greater than the input.
Something with say a switch mode power supply will take a greater
current at the lower voltage to supply a given power output.
Similar mechanism to the kettle example.
So, the only case where you save is by reducing light levels. Try
suggesting that to the surgical teams, or equivalent in your not quite
Why not go the hole hog and use candles, perhaps made from fat
rendered down from victims of the obesity epidemic.
Not for a kettle, its resistive so the power will drop as the voltage drops.
It will take longer to boil so someone will spend longer waiting around
being paid to do nothing.
It will lose more heat to the air over that longer time and so take more
energy to boil so waste more money.
Yes, but not many "domestic" devices draw constant power.
Yes, but how many don't use fluorescent tubes?
No, it sounds like it could cost you cash.
You would have to analyze things carefully to decide but I would say its
If they guarantee to save money in writing and to refund any costs incurred
as a result of it failing to do so then go for it.
email@example.com wrote on 09/05/2008 :
The kettle would draw less current on the lower voltage, but still use
the same Kwh to boil the same amount of water. Same principle applies
to water heaters and space heaters.
Incandescent lamps work at best efficiency at their rated voltage.
Reduce the voltage and less light is output, plus a greater proportion
of the wattage consumed will be turned into heat. Each lamp will
consume less, but more lamps will be needed to achieve the same
Snake oil, like so many of these gadgets. Initially with the unit first
installed, power will be saved on lamps but nothing else. Soon larger
wattage/more lamps will then need to be fitted to compensate for their
being so dim and because they will be less efficient, the net result
will be higher consumption.
Thanks to all that replied - my gut instinct that this was snake oil
seems borne out.
The device was on display at some green energy show (or something
similar) - if I get any specific details I'll post back so you can
Well sort of...
Lightbulbs will use less power and be noticeably dimmer. If you can cope
with that then fine, but if you switch more on or use more powerful
bulbs to compensate then no nett saving.
Some other things may well still work ok at reduced perfomance.
What you say would only be true for some inductive loads - e.g. certain
types of motor. Any resistive load will simply draw less current.
On Sat, 10 May 2008 00:37:37 +0100, John Rumm wrote:
Not quite. To raise water from room temp up to boiling requires the input
of a certain amount of energy (kW). Some of the energy will be lost. Now,
you can use a lot of energy for a short time or less energy for a longer
time, but the energy*time (kWh) will be about the same (with more lost as
the time becomes longer). So a water heater will draw less current but
take longer to reach the same temperature and waste more power in the
Reducing voltage to a site from 240 to 220 isn't going to make much
difference (remember that the "standard" for the UK is now 230, not 240).
You'll get less torque from all induction motors (e.g. fans in particular
will take longer to get up to speed). EFLs and many other flourescent
light fillings will have the same light output and run a bit cooler.
Incandescent lights will be dimmer and change colour (slightly more red/
orange). Electric heaters will be slightly cooler. Electronic stuff in
general won't see any difference at all.
There will be some losses in the conversion too! That thing had better be
a perfect transformer and be *really* cheap or it could take centuries to
repay its cost... IMHO you'ld probably get most gain in office
installations (provided that the air conditioning is happy at 220v).
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info http://mixpix.batcave.net
Just out of interest, I guess there must also be an upper limit to
this heat transfer rate (re efficiency)?
Assuming the element was so hot it boiled the water local to the
element instantly, would that waste energy as the general heat
transfer (convection?) throughout the kettle wouldn't be as good?
Maybe this new 'super kettle' would need a finned element to extend
the surface area?
All the best ..
T i m
I don't think anyone was suggesting otherwise - I certainly wasn't.
There are some appliances - say a vacuum cleaner where it will still
work and _may_ use slightly less energy overall if he performance is not
unduly affected by the reduced suction.
The standard is 230V (+/- a tolerance) , however the voltage is still
240V (which is within the tolerance).
There is another slightly more problematic issue here - in that some
motors will draw more current and hence dissipate more heat in their
windings. This could reduce the motor life. In extreme under voltage
situations it can cause fairly rapid motor failure. Fridge compressor
motors being a common victim. The worst case obviously being when a
motor stalls due to reduced startup torque and sits their quietly
It's possible that what the gizmo is actually doing is power factor
correction rather than voltage reduction. For customers on commercial
supply tariffs based on VA loading rather than straight kWhs consumed, a
corrector can result in reduced bills.
The trouble with asking these sort of questions here is this is not
an electronic engineering group, and you get as much noise as
signal. One of the sci.electronics ngs could explain more.
UK nominal voltage is still 240v, but it can also be described as
230v, and now regularly is. Its a complex and political story, but
IRL your light bulbs see 240.
Running filament lamps on 220 will make them less energy
efficient, not more. If you want better efficiency yet insist on
filament lighting, increasing the lighting circuit voltage can
that, albeit with issues.
CFL or fluorescent on 220 will simply reduce output a bit. You can
achieve the same result by choosing slightly lower total lighting
power. It would not affect life expectancy much. Linear fl can be
improved in efficiency by overdriving if required.
Voltages can be altered using transformers
On a large site you've got lots of appliances, all of which are
specced to run on 240v happily. A lot will also run on 220v, but all?
Sometimes all will, sometimes not. Many will work but not quite
meet specs (eg most motors). Compressors should work but may
stall at times, sometimes leading to overheating. Sometimes this
causes premature failure (eg fridges).
On any large site, appliances working correctly and within spec
matters. To toss that away for the sake of a possible minute saving
on some goods is hard to imagine being a good move. Yes it can
be done in some cases, but as a site-wide blanket measure, no,
its a mess of an idea.
In the context of this discussion, that statement is somewhat
misleading. It is true in the sense that you will be getting fewer
lumens per watt from the bulbs. However they *will* also consume fewer
kWh. The reduced efficiency will translate into a lower light output
that will be reduced by a greater factor than the power consumption.
I think thats an unrealistic comparison though. Changing from 240
to 220 will have a big impact on light output, around 30%. End
users choose their bulb wattages based on desire for a comfortable
lighting level, so with reduced voltage will change some of the bulbs
to higher wattage ones. The end result is definitely more energy
use, not less. IRL you dont reduce energy consumption by
choosing lower efficiency lighting.
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