How can I figure out the amp + wattage draw if I don't know the efficiency rating?

[ZaO]

This is not strictly about computer Psu's. Infact, it's not really about them at all, as it's normally easy to tell what their efficiency rating is. But this seemed like the best forum section to post this in!

So, normally what I do when deciding which things to plug into which mains sockets, I work out the amperage of each device by looking at the stated wattage draw. Then I arrange things on different multiplugs and mains sockets in a way that doesn't push too close to the 16 amp maximum. I like to leave at least a couple amps headroom for each mains socket. Then of course, I have 13 amp fuses in the multiplugs to consider. I also work everything out with the full stated wattage, so that if everything were under full load at once, it wouldn't overload any of the mains sockets, in theory.

But, after reading about Psu efficiency tonight, and looking at an article by Hexus (http://hexus.net/tech/tech-explained...certification/), it seems that things actually draw more power than they state (or at least what I thought they were stating)! What they state, is the amount of power they use after they've drawn power from the mains, not the amount they're actually drawing from the mains.

So my question is - how the hell do I work out how to manage what's plugged in where, when I don't really know how much power things will actually be drawing from the mains!? I've never seen power efficiency ratings on anything other than my computer Psu's. Is it really just a guessing game!? This seems ridiculous to me, if so!

[herulach]

Kettles and such like are rated based on the draw at the wall, not output power.

Wall warts and stuff like that will be rated on output power, but then it obviously depends what you plug into them in terms of how much they'll draw.

I have to say other than not having a hairdryer a kettle and an oven all of the same socket I wouldn't sweat it too much.

[kalniel]

The way to tell is to measure it at the wall.

Computer stuff is an exception simply because it works on DC power, not AC. Therefore there are two parts to the actual power draw:
1) What the component draws
2) What the losses are in converting AC to DC.

The second part is where power supply efficiency comes in. As long as you've not gone overkill on specifying the supply then it's likely to be around 85% efficient, so just add 10% or so to the expected draw of a component to work out it's real draw from your mains (or read a review that tests at the wall). But this is more than offset by the fact modern computers don't run all their components at full tilt.

Because kettles run on AC, their power use is the same from the device as from the wall.

But computers use a fraction of the power of a kettle, so don't worry there!

[peterb]

You also need to know the power factor of an appliance. That is the current that is out of phase with the voltage.

For domestic appliances, that is rarely an issue. But let me cite an example.

You plug a (perfect) capacitor (suitably rated and of an appropriate value) into the main supply. It draws 12 amps. However, because the voltage and current are 90 degrees out of phase, the POWER consumption in the capacitor is zero. However, if you measure the current, that is real, and the wires have to be rated appropriately, and the wires have resistance (which consume energy) and they will get hot.

Power in an AC circuit is current (through the appliance) X voltage (at the appliance connections) X power factor (of the appliance).

An electric kettle is almost purely resistive, and has a power factor of 1. So power = voltage X current. A 3Kw kettle connected to a 250V supply will take 12 amps.

A 3Kw device with a power factor of .8 will take 15 amps from the supply, and that has to be taken into account with the wiring.

However, it would be rare for a high power domestic appliance to have power factor less than .95. It is things like electric motors that have lower power factors, and power factor correction devices can be used to increase it.

Computer power supplies also have power factor correction built in. The only time you might see reference to power factor correction is in UPS devices, which commonly quote output in VA (voltsxAmps) which refers to the maximum current it will supply. So a 1KVA UPS would only supply 800 wats to a .8 power factor load.

[EndlessWaves]

I would expect any label on the outside of a device to describe power draw. That goes for computer PSUs too. The typical side label giving their output capacity will end up inside the computer when assembled and not visible from the outside.

[ZaO]

Thanks so much for the info guys But, to be honest, I still don't know what I'm supposed to do to figure it out lol... It seems like it might not be a very straight forward process! I wish things would just state what they actually draw from the wall. Would make life so much easier!

[kalniel]

Thanks so much for the info guys But, to be honest, I still don't know what I'm supposed to do to figure it out lol... It seems like it might not be a very straight forward process! I wish things would just state what they actually draw from the wall. Would make life so much easier!

They can't because it depends on factors outside of their control.

But we can help. Tell us the system, what you're using it for, and we'll give an estimate.

[ZaO]

They can't because it depends on factors outside of their control.

But we can help. Tell us the system, what you're using it for, and we'll give an estimate.

Yeh I get that it's difficult to figure out.. But it would be cool to have a rough idea of what these things will be using if you can The one that's the main concern is a duplex outlet that has an electric heater in one socket, which is rated at 2000W on low, and 2500W when on high (so best to go by when it's on High). Then the other socket of the same duplex outlet has a multiplug (13A fuse) which has a TV, which on the back states "power consumption 180W" (does it actually only use 180W?), a hifi amplifier which states "150W max power consumption" (does that actually use just 150W?), a monitor which just says "1.5A" (so I assume it actually just consumes 1.5A?), and a simple lamp with a 60W bulb (would that just use 60W?).

I only have two duplex outlets in my living room, so I have to jam quite a few things onto each one. I've worked things out with the Wattages I thought things were actually using, but I'm concerned it's going to be a bit higher than I expected! Where do you reckon I'd be at in regards to Wattage and Amperage if everything were going full power at once?

Thanks!

[peterb]

Power is related to the voltage and resistance of the device. So strictly speaking, a power rating of 60 watts has to be at a specified voltage. In the UK mains voltage is nominally 230V, but can vary from around 220 to 245.

But at 230 volts, a 60 watt light bulb will take 60/230 amps = .26A

This is because as we said earlier, power = V*A (for a resistive load)

Your device that takes 1.5A at 230 V will be taking 345W.

Assuming all your devices are taking maximum power, you can calculate the current each one takes and add it all up.

Or for a rule of thumb, ensure that you do not connect more devices that consume 3Kw to one socket outlet.

[ZaO]

Power is related to the voltage and resistance of the device. So strictly speaking, a power rating of 60 watts has to be at a specified voltage. In the UK mains voltage is nominally 230V, but can vary from around 220 to 245.

But at 230 volts, a 60 watt light bulb will take 60/230 amps = .26A

This is because as we said earlier, power = V*A (for a resistive load)

Your device that takes 1.5A at 230 V will be taking 345W.

Assuming all your devices are taking maximum power, you can calculate the current each one takes and add it all up.

Or for a rule of thumb, ensure that you do not connect more devices that consume 3Kw to one socket outlet.

Yeh, I normally work stuff out for 230v to be safe because it gives a higher amperage outcome. I usually just divide watts by volts to get the amperage. But it's the power efficiency thing that's thrown a spanner into the works!

I can't believe my 24" monitor would take 345W. That seems crazy :/

By the sounds of it, I am pushing it a bit much on that socket! It's that damn electric heater. Good thing I only use it on Low!

And sorry if you guys have explained some stuff in more detail that I don't understand. I just get confused when it comes to numbers!

Thanks.

[kalniel]

1.5A at which voltage for your monitor? Typically you'd expect power draw in the 10s of W range. What is the monitor model?

[ZaO]

1.5A at which voltage for your monitor? Typically you'd expect power draw in the 10s of W range. What is the monitor model?

ViewSonic VX2433wm: http://www1.viewsonic.com/products/archive/vx2433wm.htm

I don't know if the supply here is 230v or 240v, or something around those numbers. But like I say, it's why I normally go with 230v, as that gives a higher amperage draw when calculated, which is the safer way to do it. Thanks.

[peterb]

I think you miss read the monitor value. It should be .15A.

However you need to be careful with wide voltage devices. They will take the current they need (they are non-linear, to be technical) so while a resistive load will take less power as the voltage is reduced, your monitor will take 37 watts regardless of the voltage, so as the voltage falls, the current will increase. So if you ran it in the US on a 115 volt supply, it would take .3A.

[herulach]

From the specs page you linked:
Voltage AC 100-240V (Universal); 50/60Hz (Switch)
Consumption 45 (typ.)
Eco Mode optimize 37W (typ.) / conserve 30W (typ.)
Save Mode (W) ≦2W

Which would equate to either 0.15A at 240V or 1.5A @ 30V, I assume the former as most monitors tend to run on between 12 & 24V IME.

[ZaO]

Yeh I probably messed up reading the numbers on it. Unfortunately, that's not uncommon for me to do Thanks guys.