# Need some help with electrical sumsAugust 15, 2008 7:50 AM   Subscribe

Need some help working out electrical sums.

I've just bought a watt meter---the type that devices plug into in order to measure their wattage.

My broadband router consumes 9.3 watts. Because I'm asleep at night, I plug this into a mechanical timer, which itself consumes 1.4 watts. Thanks to the timer, the router is switched off for about eight hours during the night.

The question is: Am I actually saving power, bearing in mind that I'm powering the timer at 1.4 watts for 24 hours, but saving 9.3 watts for eight hours when the router is switched off...?

I'm having trouble converting wattage figures into time figures. I suspect the answer lies in joules, but I left school 20 years ago and did a liberal arts degree. I'm floundering.

Note that this ISN'T about saving money, and I know I'm likely only saving pennies, so don't give me a lecture on that topic. In fact, try and avoid giving me any kind of lecture, if possible (big ask here on AskMeFi, I know). This is about being environmentally friendly.
posted by deeper red to Science & Nature (12 answers total) 2 users marked this as a favorite

Response by poster: Mechanical timer, in case you don't know what I'm talking about. And I'd better provide a link to the power meter I bought too, to avoid inevitable confusion.
posted by deeper red at 7:53 AM on August 15, 2008

Energy=watts * time. (actually, watts=energy/time.) Energy is joules - good memory! YOu get charged by the electric company in kilowatt-hours, which is a direct conversion from joules.

So 9.3W*8=74.4 watt-hours (savings)
and 1.4W*24 =33.6 watt hours (extra spent)...you're saving a whole 40 watt-hours.
posted by notsnot at 7:57 AM on August 15, 2008

"Watt" is a rate of energy consumption, commonly called power. To compare the two devices, you just multiply by time.
```9.3 Watts x 8 hours = 74.4 Watt-hours
1.4 Watts x 24 hours = 33.6 Watt-hours```
I used Watt-hours instead of joules because your electric company charges you per kilowatt-hour. In summary, you save 50.1 Watt-hours by using your timer to turn off your router. Not much, but every little bit counts, I guess.

On preview, I agree with notsnot
posted by muddgirl at 7:58 AM on August 15, 2008

If it's a decent watt meter, it should have a mode that measures kilowatt hours. Run it in this mode for 24 hours or more and than compare with and without the mechanical timer. You should than be able to calculate how much you will save in a year by extrapolating.
posted by robofunk at 7:59 AM on August 15, 2008

Watts are joules per second, but that isn't important right now.

The router is a real pig compared to the timer so before I do the numbers, I'd say you're saving energy with the timer.

9.3 Watts x 24 hr = 223.2 Watt-hours or .02232 kwh (kilowatt-hour is the billing unit)

This is what running your router 24 hours costs.

Using the timer:

1.4 watts x 8 hours = 11.2 watt-hours (.00112 kwh)

Using the timer + router:

1.4 watts + 9.3 watts = 10.7 Watts x 16 hours = 171.2 watt-hours =(.0172 kwh)

.00112 (timer only for 8 hours) + .0172 (timer + router for 16 hours) = .0184 kwh

So using the timer reduces your total energy consumption by about 18%
posted by three blind mice at 8:04 AM on August 15, 2008

Run timer for one day: 1.4X24=33.6 watt hours

Not using router at night: 9.3*8=74.4 watt hours

savings per day: 74.4-33.6=40.8 watt hours, or about half a cent per day.

All of which has been said.

I know you're not worried about saving pennies, so I'll assume you're trying to be environmentally friendly. I'd guess that the effect of the switching on and off on the life of the router itself, and the potential savings (if router life is extended) or loss (if router life is shortened) of the energy to manufacture and ship a replacement may be a bigger consideration in the long run.
posted by jon1270 at 8:10 AM on August 15, 2008

I'd just like to point out that you don't specify if it is using 9.3 watts idle or 9.3 watts at full speed download. Measuring kilowatt hours over 24 hours or more will help average out power usage over different operating conditions.

To get better usage out of your mechanical timer, you should consider using a power strip and placing all of your electronics that don't need to be used full time or don't need to charge all the time on it.
posted by robofunk at 8:16 AM on August 15, 2008

To take the calculation further, the 40.8 Watt-hours of energy you're saving amounts to about:

40.8 x 30 = 1224Wh = 1.224 kWh per month

In terms of the average US household's monthly consumption of 920kWh, that's about 1/750th.

I hope I don't come across as lecturing when I suggest that the envionmental cost of manufacturing the timer will probably take quite a long time to recoup.
posted by le morte de bea arthur at 8:19 AM on August 15, 2008

Response by poster: I hope I don't come across as lecturing when I suggest that the envionmental cost of manufacturing the timer will probably take quite a long time to recoup.

Yes, this is something I'm aware of.

Energy cost of manufacturing the timer, plus costs of getting it to me
vs.

Interesting equation and I'm as clueless about that one as I am the rest of it. I would note that these are cheap devices, but also pretty damn simple. Just plastic and a little metal.

I know you're not worried about saving pennies, so I'll assume you're trying to be environmentally friendly.

You don't have to assume anything. You just have to read the entire question. But what you wrote sounds awfully like a lecture...

Anyway, thanks all for the answers. I'm sure there will be some more pointing out exactly the same thing. If anybody wants to continue the debate about the efficacy of timers, feel free (at last, my liberal arts degree is useful!).
posted by deeper red at 8:33 AM on August 15, 2008

I bought one of these watt meters and found that the consumption numbers when totaled were greater than my electric bill. This was without recording the single phase 220V consumptions (hot tub etc.) Treat your numbers as relative references.
posted by Raybun at 8:49 AM on August 15, 2008

Response by poster: I bought one of these watt meters and found that the consumption numbers when totaled were greater than my electric bill.

I did wonder about the accuracy (what about the consumption of the meter itself?), but I didn't think it was that bad. What do you think causes it?

What the meter is good for:

(a) Showing relative values, like you say. My notebook computer sucks up 35 watts on average. So it's not a huge power draw. My desktop PC (an iMac) sucks about 100 watts all-in (including peripherals)---much more. Maybe I ought to work on my notebook more, and not leave my iMac turned on for hours while I'm out of the room.

(b) Showing if a device sucks power when it's 'off', and if so, how much. My TV and satellite receiver suck 15 watts on standby---all just to light a few red LEDs, and wait for an infra-red signal from the remote. That's the same as one of my energy efficient bulbs that light my living room at night. My battery charger apparently sucks 1.5 watts when not actually charging any batteries, as does the charger for my electric drill. Most of my transformer blocks (phone chargers etc) suck zero power when nothing is attached, but this kind of thing is illuminating in itself.

Here's another reason why this has been a useful experience: I make my notebook computer hibernate, rather than go into standby mode. But it's still sucking power, despite being "off", so the best policy is to disconnect the power lead each night too. I know the notebook's transformer block takes zero power when nothing is attached, because the watt meter says so. I also plan to remove the battery from the notebook when it's plugged-in, because that needs constant topping-up, and charging seems to take about 15 watts.
posted by deeper red at 9:02 AM on August 15, 2008

Response by poster: You can also do cool things like reducing the brightness of a notebook's screen, and seeing what happens.

Full brightness: 22.8 watts
Half brightness (still plenty most of the time): 20.2 watts

Accessing a CD adds about 8 watts of power. Listening to audio at a moderate volume: +3 watts.
posted by deeper red at 9:07 AM on August 15, 2008

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