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Geeking out on the ‘leccy bill

A good post from Lars Wirzenius on measuring the electricity consumption of his computer hardware. Here’s a previous post of mine on the subject.

With the rising cost of energy, a keenness to reduce consumption for green purposes, and an overweening nerdity in general, I did some more investigation around my house recently.

I have a pretty typical Irish electricity meter; it contains a visible disc with a red dot, which spins at a speed proportional to power usage. (There’s a good pic of something similar at the Wikipedia page).

The fuse-board works out as follows (discarding the boring ones like the house alarm etc.):

  • Fuse 7 – gas-fired central heating (on), fridge (on), kitchen power sockets

  • Fuse 8 – TV in standby, idle PVR, Wii in standby, digital cable set-top box, washing machine

  • Fuse 9 – telephone, DSL router, Linksys WRT54G AP/router

  • Fuse 10 – bedroom sockets, home office with laptop, printer, speakers, laptop-server etc.

The approach was simply to turn off the house fuses at the fuse board, one by one, and measure how long it took the disc to make a full revolution; then invert that (1/n) to convert from units of time over a static power value, to some notional unit of power consumption over a static time interval (I haven’t figured out how to convert to kW/h or anything like that, they’re just makey-uppy units).

Fuses Time/power Power/time
Baseline (all fuses on) 22.71 seconds 0.0440
Fuse 7 off 43.03 0.0232
Fuses 7 and 8 off 57.92 0.0172
Fuse 7, 8 and 9 off 84.88 0.0117
Fuse 7, 8, 9, and 10 off ~20 minutes (I’d guess) 0.0008?

(I stopped measuring on the last one and just estimated; it was crawling around.)

Breaking out the individual fuses, that works out as:

Fuse Power/time
Fuse 7 (central heating, fridge, kitchen bits) 0.0208
Fuse 8 (TV, Wii, set-top box, washing machine) 0.0060
Fuse 9 (phones, routers) 0.0055
Fuse 10 (home office, bedrooms) 0.0109

Good results already: (a) it was pretty clear that fuse 7 was doing all the quotidian legwork, eating the majority of the power, and (b) the TV equipment and internet/wifi infrastructure was pretty good at low-power operation (yay). However (c) the computer bits aren’t so great, but still only half the power consumption of the kitchen bits.

Breaking down the kitchen consumption further:

Appliances Time/power Power/time
Gas central heating on (rechecking the baseline) 20.46 0.0488
Gas central heating off 34.15 0.0292
Washing machine on (40 degree wash) 13.65 0.0732
Dishwasher on 2.53 0.3952
Dishwasher and dehumidifier on 2.53 0.3952

Subtracting the baseline:

Appliance Power/time
Gas central heating 0.0196
Washing machine 0.0244
Dishwasher 0.3464
Dishwasher and dehumidifier 0.3464

So the central heating, despite being supposedly gas-fired, eats lots of power! I guess this is the electric pump, used to drive the heated water around the house to the radiators. Ah well, I’m not skimping on that ;)

More practically: the dishwasher result is incredible. That’s 30 times the power usage of the house’s computer hardware. This is a ~7-year-old standard dishwasher; obviously green power consumption wasn’t an issue back then! We’re running it less frequently now, obviously; the odd hand-wash of bulky and nearly-clean items helps. With any luck when we move in a few months, we can replace it with a greener model.

The washing machine is about what I would expect, so I’m OK with that.

Also interesting to note that our dehumidifier is unnoticeable in the volume of the dishwasher; I could have tried to work it out properly in isolation, but couldn’t be bothered by that stage ;)