Does a 300w power supply draw 300watts AC?

[Rhonda85]

I have always wondered how much AC electricity my PC consumes. My monitor shuts off after 15min. but the PC runs 24/7. If I have a 300watt power supply which I assume is 300 DC watts. How much electricity is it consuming from my AC wall plug?

 

[xitshsif]

A watt is a measure of charge per second (Joule /sec). One Joule is one joule, whether it's drawn on an AC or DC circuit. A 300W power supply will provide at most a peak of 300W (in theory). Most power supplys are ~80% efficient, so in reality it may draw more than that if it actually is putting out 300W. Even most high end machines today barely draw 300W under load. Link

Anyway, your 300W power supply is probably not drawing anywhere near 300W from the wall.

 

[myocardia]

No, ATX power supplies are switching power supplies. They only output what is needed for the components they are powering. For instance, my system might use twice as much power from that 300 watt power supply as yours would use, or vice versa. Neither has anything to do with the rated output capacity of the power supply, assuming the power supply is capable of outputting what the components its powering require.

 

[potato28]

If it draws 300 watts from the wall, the circuit will break. It probably only uses about 60-80 watts at idle, depending on the system.

 

[Navid]

Originally posted by: Rhonda85
If I have a 300watt power supply which I assume is 300 DC watts. How much electricity is it consuming from my AC wall plug?

If you have a 300W power supply and it is a decent power supply from a reputable company, it can provide a maximum of 300W output guaranteed. But, how much power it really needs to output depends on what kind of hardware you have in your PC and how active the CPU/graphics card are at the moment.
At idle, a PC consumes much less than it does under load.

The power drawn from the wall is always going to be greater than the power output from the power supply. This is because there is no equipment with a 100% efficiency and your power supply is no exception.

 

[myocardia]

Originally posted by: potato28
If it draws 300 watts from the wall, the circuit will break.

Huh? Each outlet in a house that's up to code can supply 1800 watts of power, at 120v AC.

 

[soydios]

A 300-watt DC power supply outputs 300 watts of DC power. At 80% efficiency at maximum load (a very good PSU), it's drawing 375 watts of AC current from the wall (300 of which is going at as DC and 75 of which must be dissipated as heat).

 

[WackyDan]

Originally posted by: myocardia

Originally posted by: potato28
If it draws 300 watts from the wall, the circuit will break.

Huh? Each outlet in a house that's up to code can supply 1800 watts of power, at 120v AC.

No... A 15 amp circuit rated by code to 80% load can supply up to 1800 watts total for the circuit. There are typically up to several outlets on a circuit.

 

[myocardia]

Originally posted by: WackyDan

Originally posted by: myocardia

Originally posted by: potato28
If it draws 300 watts from the wall, the circuit will break.

Huh? Each outlet in a house that's up to code can supply 1800 watts of power, at 120v AC.

No... A 15 amp circuit rated by code to 80% load can supply up to 1800 watts total for the circuit. There are typically up to several outlets on a circuit.

And any of the outlets can supply all 1,800 watts, like I said the first time.

 

[Zepper]

You can get a Kill A Watt http://www.newegg.com/Product/Product.aspx?Item=N82E16882715001 meter from Newegg and find out for sure. they had them on sale a while back at a killer price - I'm sure it will happen again. Price right now isn't bad relative to other places at $22. shipped!

.bh.

 

[Puffnstuff]

It's not so much the wattage but how the amps are distributed across the different rails. Most modern system draw heavily from the 12v rail so having strong 12v rails is a must if you want reliable service from your pc. I have an enermax galaxy 850w ps on my system with is currently drawing about 234 watts from the recepticle per my apc powerchute software which monitors my apc xs1000 battery backup with LVR which has a 600 watt rating. It gives me around 20 minutes of battery runtime. I also have five 12v rails providing me with 120a total 12v capacity. Amperage is everything on a ps so a quality lower wattage ps will have high amperage on the rails. The amperage is what you need to look for.

 

[Rubycon]

The power supply rating is the capacity not the consumption. So if you have a 500W PS and upgrade to an 850 you're not automatically pulling 350 more watts when you turn it on.

 

[Muse]

Originally posted by: Zepper
You can get a Kill A Watt http://www.newegg.com/Product/Product.aspx?Item=N82E16882715001 meter from Newegg and find out for sure. they had them on sale a while back at a killer price - I'm sure it will happen again. Price right now isn't bad relative to other places at $22. shipped!

.bh.

Or you can do it cheaply, DIY. I have a simple device I made to measure power consumption. It consists of:

1. Digital multimeter which measures AC amps.

2. An extension cord.

3. Two male banana plugs that plug into the positive and ground females in the multimeter.

I took a length of pretty typical AC cord and spliced one end of it into one of the wires of the extension cord, taping it well for insulation. On the other end of that AC cord, I connected my two male banana plugs, one on one conductor and the other on the other conductor.

When I use this, I make sure to plug the banana plugs into the multimeter before plugging the extension cord into a power outlet. Failing to do so would probably cause a short circuit or worse, a potential for electrocution! My multimeter has two AC amps circuits:

1. Maximum 2 AMPs with a 2 AMP fuse

2. Maximum 20 AMPs, with no fuse. If 20 AMPs are exceeded, the multimeter would likely be fried.

I've burned out 3-4 fuses, but the meter's still going strong. I've used this to measure power consumption of anything I care to around the house, and generally jotted down the results. My laptop only uses around 20-25 watts when run on the AC adapter. My PC, more like 80 watts plus the monitor draw. My 22" CRT used to draw 80-100 watts, but my 19" LCD does more like 30-40 IIRC.

To determine the watts being used, you multiply the current by the voltage (i.e. x 120).

 

[zig3695]

Originally posted by: myocardia

Originally posted by: potato28
If it draws 300 watts from the wall, the circuit will break.

Huh? Each outlet in a house that's up to code can supply 1800 watts of power, at 120v AC.

more like 1440w in reality. circuit breakers are designed to trip after 80% load sustained for a certain duration. 20a circuit breakers will handle approx 2000w continuous. a 1800VA, size 14awg copper circuit can technically handle current for one size larger on the AWG scale, so in that sense a 14AWG wire can handle 2400w but code requires they handle no more then 1800w. the code then implies that breakers trip between 80 and 100% load, therefore 1440w is all you can really count on in a 15a circuit.

 

[Mark R]

Originally posted by: Muse

I've burned out 3-4 fuses, but the meter's still going strong. I've used this to measure power consumption of anything I care to around the house, and generally jotted down the results. My laptop only uses around 20-25 watts when run on the AC adapter. My PC, more like 80 watts plus the monitor draw. My 22" CRT used to draw 80-100 watts, but my 19" LCD does more like 30-40 IIRC.

To determine the watts being used, you multiply the current by the voltage (i.e. x 120).

Personally, as a true power meter like the kill-a-watt is so cheap, and is so much simpler and safer than splicing a multimeter into a live mains circuit - it's the only recommendation I could possibly make.

Not only this, but the multimeter solution only measures current, not power; rms current multiplied by rms voltage cannot be relied to on to give average power. In fact, for electronic devices like PCs it's likely to be significantly inaccurate.

 

[Navid]

Originally posted by: Muse

Originally posted by: Zepper
You can get a Kill A Watt http://www.newegg.com/Product/Product.aspx?Item=N82E16882715001 meter from Newegg and find out for sure. they had them on sale a while back at a killer price - I'm sure it will happen again. Price right now isn't bad relative to other places at $22. shipped!

.bh.

Or you can do it cheaply, DIY. I have a simple device I made to measure power consumption. It consists of:

1. Digital multimeter which measures AC amps.

2. An extension cord.

3. Two male banana plugs that plug into the positive and ground females in the multimeter.

I took a length of pretty typical AC cord and spliced one end of it into one of the wires of the extension cord, taping it well for insulation. On the other end of that AC cord, I connected my two male banana plugs, one on one conductor and the other on the other conductor.

When I use this, I make sure to plug the banana plugs into the multimeter before plugging the extension cord into a power outlet. Failing to do so would probably cause a short circuit or worse, a potential for electrocution! My multimeter has two AC amps circuits:

1. Maximum 2 AMPs with a 2 AMP fuse

2. Maximum 20 AMPs, with no fuse. If 20 AMPs are exceeded, the multimeter would likely be fried.

I've burned out 3-4 fuses, but the meter's still going strong. I've used this to measure power consumption of anything I care to around the house, and generally jotted down the results. My laptop only uses around 20-25 watts when run on the AC adapter. My PC, more like 80 watts plus the monitor draw. My 22" CRT used to draw 80-100 watts, but my 19" LCD does more like 30-40 IIRC.

To determine the watts being used, you multiply the current by the voltage (i.e. x 120).

This is very bad advice!

This could hurt you really bad, or even kill you! Even if you survive, as has already been mentioned, you will not be measuring real power. You will be measuring RMS current. Multiplying RMS current by the line RMS voltage gives you apparent power. From that, in order to calculate real power, you will need power factor, which you do not have.

 

[WackyDan]

Originally posted by: myocardia

Originally posted by: WackyDan

Originally posted by: myocardia

Originally posted by: potato28
If it draws 300 watts from the wall, the circuit will break.

Huh? Each outlet in a house that's up to code can supply 1800 watts of power, at 120v AC.

No... A 15 amp circuit rated by code to 80% load can supply up to 1800 watts total for the circuit. There are typically up to several outlets on a circuit.

And any of the outlets can supply all 1,800 watts, like I said the first time.

Yes.. any one outlet can... your original statement could be taken in a different context, as in each an every outlet can deliver the power (assuming 15amp) concurrently instead of collectively with the other outlets

 

[WackyDan]

Originally posted by: zig3695

Originally posted by: myocardia

Originally posted by: potato28
If it draws 300 watts from the wall, the circuit will break.

Huh? Each outlet in a house that's up to code can supply 1800 watts of power, at 120v AC.

more like 1440w in reality. circuit breakers are designed to trip after 80% load sustained for a certain duration. 20a circuit breakers will handle approx 2000w continuous. a 1800VA, size 14awg copper circuit can technically handle current for one size larger on the AWG scale, so in that sense a 14AWG wire can handle 2400w but code requires they handle no more then 1800w. the code then implies that breakers trip between 80 and 100% load, therefore 1440w is all you can really count on in a 15a circuit.

Oy! Even my math was wrong..... I knew that and am now an *sshat.

 

[Zepper]

Muse,
. Your rig measures Volt-Amps which isn't Power unless your PSU has active PFC with a power factor of near 1.0. The KAW measures both Volt-Amps and Power and Power Factor as well as accumulating kW Hours on intermittently-on devices.

.bh.