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Old 12-26-2015, 02:41 AM
67 posts, read 95,098 times
Reputation: 32


I'm trying to decide if I should buy a new cable modem+switch+router, or use 3 devices I already have - based on power consumption.

For example, I have this typical wall wart here (for hard drives) whose specs read:
Input 100-240V AC at 0.6A (max)
Output 12V DC at 1.5A

From what I understand, that translates to 66W. That's a huge amount of power! As much as a laptop would consume. Furthermore, there's a notice in Spanish on the AC adapter: "power consumption: 22.4Wh. In standby mode: 0.1Wh".

So how come it's 22.4W and not 66W?

At 16¢/kWh (how much tier 1 electricty costs here in SoCal), 2 additional similar AC adapters that run year-round would cost me $168 in electricity! Something here doesn't add up at all...

Figures from a couple of other modern wall warts:
Input 100-240V AC at 0.4A
Output 5V DC at 2.6A

Input 100-240V AC 0.5A-0.3A
Output 10V DC at 2.0A
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Old 12-26-2015, 04:12 AM
40,169 posts, read 41,782,366 times
Reputation: 16740
Check the volts, watts or amps on the device. The transformer itself should draw little power unless something is connected. The output on the transformer doesn't necessarily mean that is what is being drawn.

volts * amps = watts

One thing to note, for a lightbulb that works because the power draw is constant. That may not be the case with electronics. They do have watt meters you can plug between the device and the outlet.
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Old 12-26-2015, 04:17 AM
1,292 posts, read 768,431 times
Reputation: 2285
That power consumption is not worth thinking about. Replacing one incandescent light bulb with a CFL can save as much as your modem/router uses.
The figures on the wall warts probably are for maximum power the wall wart can supply. The actual consumption depends on the needs of the device the wall wart is powering. If you want to be accurate in your assessment of this insignificant power use you should get a a way to measure it. One inexpensive meter for that purpose is a Kill A Watt. You can read voltage, amps, watts,volt amps, power factor and actual KWH used. I have 2 and find them pretty handy. Kill A Watt®*| P3

I used it determined the cost of using my energy hog coffee maker that keeps hot water ready so it makes a pot of coffee in 3 minutes. It can draw several amps but only when heating the water and maybe 40 watts continuously. It turned out to cost close to about $3.50 a month. I decided it was worth it cus I like the convenience of a the fast brew.
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Old 12-26-2015, 03:33 PM
Location: SF Bay Area
13,343 posts, read 17,768,488 times
Reputation: 19664
The adapters are not using the power. Check the power consumption of the device that's hooked up to it.
Here's a sample power consumption of some AC routers:

802.11ac routers review: 20 models tested - Test results: power consumption | Hardware.Info United States

Most are less than 10W.
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Old 12-26-2015, 07:04 PM
Location: (six-cent-dix-sept)
4,877 posts, read 2,497,770 times
Reputation: 3137
the output is 12 v * 1.5 a = 18 w.

the input rms value of 240 vac (worst case) ~ 158.4 v * .6 a = 95.04 w
(this seems high)

Last edited by stanley-88888888; 12-26-2015 at 07:05 PM.. Reason: this seems high... anyone else ?
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Old 12-26-2015, 09:52 PM
67 posts, read 95,098 times
Reputation: 32
Thanks for the responses. Yes I'm very much aware of the formula since junior high.

Yeah I'm familiar with the Kill A Watt, might be a fun/useful thing to get one day but not worth it for this small issue.

For me, it boils down to a simple factor of 1.4 to go from Wh to $$/year. (W / 1000 * 24 * 365 * 0.16) It seems the power savings from a new device won't be worth the investment.

I know that modern laptops are very smart at scaling power consumption, but wasn't sure how much that applies to an 8-year old GigE switch. Traditionally ICs just sat around idling and generating heat. Also, this isn't the same case as chargers that are either plugged in or not. Things like a modem, switch or AP are on 100% of the time and do highly variable amounts of work throughout the day.

As stanley pointed out, all this still doesn't explain the large discrepancy between the input and output power specs on wall warts.
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Old 12-27-2015, 02:58 AM
Location: Florida & Cebu, Philippines
2,808 posts, read 2,515,542 times
Reputation: 2877
Sometimes power ratings seem to be underrated, for instance Comcast changed our two spare cable boxes to HD cable boxes for free (the third was already HD) and while the numbers do not work out for me when I calculate them, just adding those two boxes has added around $15 a month to our electric bill, when compared to the kwh used the last two years of bills, how do I also know, I put on a power strip and shut them off when not in use and the bill dropped about that much. I have found that if it gets hot when off or on, as the HD boxes do, then we are paying for that amount of power times 24 hours a day 365 days a year and not just when in use like our old cable adapters, and even then, the ratings on the box have to be way underrated.
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Old 12-30-2015, 08:38 AM
Location: East of Seattle since 1992, originally from SF Bay Area
30,829 posts, read 56,210,459 times
Reputation: 32811
I have a Kill A Watt, and for fun, plugged my phone charger at work into it overnight (with no phone connected) and if it used anything at all it was too little to be measurable. I then charged my phone for an hour, and it used 5 watts, that comes out to 18 cents per year at 10 cents/kWh. At that rate, if charging 12 hours a day it's still only $2.19/year.
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