[Eustacian]

Hello everybody,

Real simple question here - you'd think I'd know this or have found the answer by now. Anyway, here goes.

If a stereo is rated for "500 watts audio output" does that mean it consumes 500 watts of electricity? Are audio watts the same as electrical watts?

Just a little shocked to realize that my surround sound system may be consuming as much as 1kw while watching a movie, that is a frightening amount of waste.

[lowem]

PMPO or RMS?

I would guess that rating of yours is PMPO - which is the max instantaneous "peak music power output". Home amplifiers can't maintain this for much more than a split second and then only under lab conditions. From that point on, the marketing folks take over and splash this useless number all over the place

The *other* number is the more realistic one, it will be much smaller than PMPO, usually it might be something like 85W, 57W, 117W or something of the sort even though PMPO for the same amplifier may be rated at crazy numbers like 400W, 600W, or 1000W ...

You're moving the paper cones (usually) of your speakers back and forth, not trying to melt them ...

[lowem]

... and most of the time you will be using much less than RMS (and at least 100x less than PMPO, doh).

From my own practical experience, pumping in just 1 or 2 W into a small (PC beep-size) speaker is already pretty loud. To move (drive) larger speakers, you will need a little bit more power, but nowhere near PMPO ...

[smallpoxgirl]

$this->bbcode_second_pass_quote('Eustacian', 'I')f a stereo is rated for "500 watts audio output" does that mean it consumes 500 watts of electricity? Are audio watts the same as electrical watts?.

[Carmiac]

A watt is a watt is a watt is a measure of energy over time. Specificaly a joule per second. A joule is the amount of energy required to raise one gram of water by one degree Centigrade.

And then what the others have said.

[mindfarkk]

great question!!!

[Cool Hand Linc]

RMS wattage is what higher end equipment use to tell what their amps produce. Cheap stuff will use a peak number. Jenson, Xtreme, lightning, Roadmaster, This practice being worse with car stereo's. Generally home systems don’t do this but it still happens with some. High end equipment such as Onkyo give RMS ratings only.

Wattage being produced at the speaker varies at all times. Constantly changing with sound output. A watt is a watt but to produce a listening level at 5 watts continuous produces allot of sound. With peaks of much higher wattages giving the dynamic changes heard.

It isn't really possible to say watts in equal’s watts out because of the nature of a sound reproduction device. The "phantom load" or stand by requirements are present all the time and uses some of the power being consumed. Most (All) electronic equipment has a power supply. The nature of these power supplies means they will use small amounts of power at all times. A small amount until one looks at how many of the devices there are in the world and then add the total amount of power up. Then it becomes a hugh amount of enegy wasted.

So to your stereo. If you really want to know how much juice it uses. You will need to measure its consumption. Use an amp probe. To use an amp probe correctly. It must be placed over a single wire. Most 120 volt power cords have 2 wires or 3 wires in them. A power, Neutral and ground or a power and neutral. So you will need to separate them. Care to not expose the wires inside. Exposed wires can result in electric shock. Once the wires are separated. The amp probe is placed around the power lead preferably and a reading is taken. It will vary with different volume levels. I haven't taken a reading on one my equipment but I believe with a lower volume level. You will see a point it doesn't drop anymore. But at extreme high volume levels you will see an increase. I would read the lower amount and consider this what it consumes all the time.

Take the reading from the amp probe and apply Ohms Law to determine the wattage of power feeding the system.

As a rule of thumb 120 volts and a 20 amp circuit breaker means the max wattage possible for the circuit is 2400. In countries that use 220 power you will need to use the 220 and I am not sure what size breakers (fuses) are used.

[Axiom]

$this->bbcode_second_pass_quote('Carmiac', 'A') watt is a watt is a watt is a measure of energy over time. Specificaly a joule per second. A joule is the amount of energy required to raise one gram of water by one degree Centigrade.

[Cool Hand Linc]

Axiom, Not a nitpick but a correction.

Thanks for helping to correct the error. I didn't really pay attention. But now that you mention it. I did some checking.

Calorie [n] unit of heat defined as the quantity of heat required to raise the temperature of 1 gram of water by 1 degree centigrade at atmospheric pressure.

Joule [n] a unit of electrical energy equal to the work done when a current of one ampere passes through a resistance of one ohm for one second.

Calories = joules x 4.1868

watts = joules/seconds

I think this correct.

[MarkR]

Using a current meter (amp probe) to work out the power consumption of an appliance is unfortunately not very accurate - you'll get a rough idea, but not much more than that.

If you want an accurate reading there is really no alternative to a true power meter (watt meter). It's also more convenient because there is no cutting, or calculation - just plug in.

It's also interesting to note that there are several different types of amplifier which have different power consumption properties:

Most are 'class AB' amps which use a modest amount of power at idle, but which rises rapidly the more power is sent to the speakers (often managing about 40% efficiency)

Very high quality 'audiophile' amps are 'class A' - these take their maximum amount of power at all times. That not sent to the speakers is converterd to heat in the amp. These have the rather odd property that the harder you work them, the lower their operating temperature.

The latest technology is 'class D' or digital amps. Extremeley efficient, but historically of low quality, so often used for powerful subwoofers, but improvements in technology are now making them practical for normal use.

[Cool Hand Linc]

Mark, what you say is true. Both about the amps and using a power meter.

The class A amps send the power to the speakers at a high volume rather than dealing with it internally as they do at a lower volume. So they do run cooler at a high volume.

At work we have a meter. It is hooked up the same as an amp probe. With industrial applications. It is easy to place it over one with. This meter is just a little larger than a standard sized digital volt meter. It has the ability to take an RMS reading. We need this for our AC drives. So this type meter can be used and it does the converting internally. Gives a print out too.

I'm not familiar with the power meter you described but I have no doubt there are many ways to determine the amount of power being used by a piece of equipment. I just gave one option.

[Carmiac]

$this->bbcode_second_pass_quote('Axiom', '')$this->bbcode_second_pass_quote('Carmiac', 'A') watt is a watt is a watt is a measure of energy over time. Specificaly a joule per second. A joule is the amount of energy required to raise one gram of water by one degree Centigrade.

[Kingcoal]

$this->bbcode_second_pass_quote('Eustacian', 'H')ello everybody,

Real simple question here - you'd think I'd know this or have found the answer by now. Anyway, here goes.

If a stereo is rated for "500 watts audio output" does that mean it consumes 500 watts of electricity? Are audio watts the same as electrical watts?

Just a little shocked to realize that my surround sound system may be consuming as much as kW while watching a movie, that is a frightening amount of waste.

[Cool Hand Linc]

So what method do you recommend for determining the power consumption of a 350 hp ac electric motor running 3 phase?

Or the power consumption for all the lights in an entire bay of a large inductrial plant?

Or the power cumsumption for a 3 phase 7.5 hp ac motor that is being run by an AC drive?

[MarkR]

$this->bbcode_second_pass_quote('', 'S')o what method do you recommend for determining the power consumption of a 350 hp ac electric motor running 3 phase?

[Cool Hand Linc]

Mark can you be more specific?

$this->bbcode_second_pass_quote('', 'T')he use of a suitable power meter on the circuit in use. Most industrial monitors will have suitable adaptors for connection to virtually any power system.
Here is an example of a high-end portable device, but less sophisticated devices are available.

[Kingcoal]

$this->bbcode_second_pass_quote('MissingLink', 'S')o what method do you recommend for determining the power consumption of a 350 hp ac electric motor running 3 phase?

[Cool Hand Linc]

The formulas were not what I was questioning. I failed to notice your link to the meter you were referring to. Sorry for that. Talk about over kill! God!

I generally use my companies RMS amp probe on everything because my old analogue doesn’t always give an accurate reading on every piece of equipment such as an AC drive. Our Fluke RMS amp probe “basicallyâ€

[MarkR]

No, merely pointing out that a typical hand-held clamp meter won't measure Pf, or phase balance on a 3 phase system.

As your scenarios included AC motors and industrial lighting, unity power factor would be a poor assumption.

Power meters are a commodity item - I just happened to post a link I had to hand.

[Cool Hand Linc]

I'm not sure that anything that has been covered is critical. Phase imbalance is important especially when setting up a plant and keeping all the phases balanced. Unbalanced phases are also a sign of a motor or wiring problem. You are correct that if you desire a reading of power consumption that is down to a nats ass that a reading that in real time takes an RMS reading from all three phases at the same time is required. But I was unaware that anybody wanted such an accurate number. I mean really.

But in the original scenario of a stereo. You said that my method of using a hand held wasn't accurate enough. How important was the accuracy for a quick reading anyway? It's not perfect down to a nats ass but a Fluke RMS amp probe would be pretty dam close. I gave a quick way of being pretty accurate in a single phase circuit. Not a perfect method but there is no such thing as perfection anyway. There will always be a small margin of error. Using a hand held amp probe that gives an RMS reading is not perfect but I was unaware we required perfection. You quickly pointed out that the method I gave wasn't accurate enough. There was no mention of a requirement for accuracy.

We were originally talking with a person who wanted to know how to tell how much power was being used. Not an engineer that wanted a detailed study performed on the power consumption of a stereo. Just something basic.

You were correct about my method not being as accurate but I ask you to look at the who, what and why. Then consider what level of accuracy is required. Are we talking moped or Formula 1 here. I think I was talking moped when you were talking at least a Formula Ford or Formula GT.