Saturday, March 8, 2008

Kill-a-Watt, SmartStrip, etc.

[Sunday, March 9th. After I posted this last night, I switched off the printer and again the SmartStrip failed to switch the other stuff off, as a reslt of which I have made substantial revisions to the posting.]

I bought a Kill-a-Watt power monitor, among other things to help me work out why the HP printer did not seem to reliably switch the SmartStrip (see last Saturday’s posting). The Kill-a-Watt fits between the outlet and whatever device is plugged into it, allowing one to measure the power used by that device. Unlike the Cent-a-Meter described last week, which measures total household usage and is very useful in its own right, this is not influenced by extraneous factors going on in the house like the refrigerator cycling on and off.

I found that the printer is quite consistent: it uses 7 watts when plugged in but switched off, and after going as high as 37 watts during its power-up sequence it settles at 13 watts when switched on. This seems not to be a large enough difference to make the SmartStrip work reliably. I adjusted it as carefully as I could, and at first it seemed to work, but as noted above it later failed to switch the SmartStrip off. I also wonder why the printer draws any power when switched off; that 7 watts costs $10 a year. Maybe time for a new printer.

I also tried using the DSL controller, which draws only 8 watts when on, as the controlling device. This turned out to be inadequate to switch the SmartStrip on, even when the sensitivity was turned up to he maximum.

I had similar trouble with the entertainment systems being switched by the tuner/amp when this was done using the remote. The unit draws 24 watts when working and 14 when on standby, and again the SmartStrip does not seem to be sensitive enough to detect the difference. If I switch off using the switch on the unit instead of using the remote it works fine, so I guess I will be doing that in future. (Saving that standby power is worth $20 a year, so I should be doing that anyway.)

The Kill-a-Watt is quite expensive at $45.95 (from but it has a number of other features. Most useful, in addition to measuring the rate of power usage in watts, it can be used to integrate that over time to measure kilowatt hours used over a measured time interval. This is perfect for monitoring things like refrigerators which cycle on and off. It can also be used to switch between active power and apparent power readings, and can also display the power factor. These are rather esoteric measures which take account of phase differences between the voltage and current, and I doubt many users will understand or care about them. Other features include monitoring voltage, current, and frequency, and again I am not sure what use these would be put to.

To test the kilowatt hour capability, I turned my attention to our ancient refrigerator and found that it used 0.16 kWh in 2 hours, so averaging about 80 watts. This does not sound much, but it is on 24/7 so over a year this is about 700 kWh. At my price of 16 cents this is $112 a year. According to the energy star site, a typical modern top-freezer 18 cubic feet energy star rated refrigerator consumes about 400 kWh per year, so replacing our old model could save about $50 a year and pay for itself in 9 years. That may not sound like a great return, but what bank is gong to give you 11 percent? And that assumes energy prices do not go up in those 9 years. Bottom line is that old fridge will soon be history.

Finally, I used the Kill-a-Watt to test some chargers. It seems almost a cliché to say one should unplug chargers when not in use, but I found this not to be worthwhile. I tried two Dell computer chargers. Both registered 0 watts when the not plugged into the laptop. When plugged into a fully charged laptop, they registered 1 and 2 watts respectively. I also tried two mobile phone chargers and found that both registered 0 watts when not charging a phone. I also tried an electric toothbrush charger and a cordless phone charger and found that these registered 0 or 1 watt even when they were charging. I therefore think one should not lose sleep over chargers.

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