Past EED rants


Live leaderboard

Poker leaderboard

Voice of EED

Tuesday 27 March 2007

Measuring your power [Lurks]

With the big issue of climate change on the table, more people than ever are thinking about what they can do on their patch. This is just as well because it turns out that the household makes up a large fraction of the carbon footprint of our activities. This fact is all brought all the more home to me recently in getting a bit of a shock electricity bill whereby they're asking we pay a whopping £87 a month to clear the balance.
This situation arises because like most electicity firms in the UK, they send a man around to read the meter once in a blue moon. From that they just extrapolate (I might be giving them credit, perhaps guess is a better word) your usage and give you bills estimated on readings. You can, however, go read the number on your leccy meter and call them up in which case they revise. We did this and found that we owed them a lot of money.
Now, we have a somewhat unusual situation of both working from home. So that means our stuff is on during the day and much of it is necessary business expense. Stuff that normally folks wouldn't have to pay for because they go into work and use their bosses dime, including the leccy for putting the kettle on all the time.
Anyway, I was raised by my parents to be fairly concious of electricity consumption because in the far north of Australia, electricity was very expensive. I instinctively turn lights off when leaving a room and make sure I don't leave the room with a tele on or whatever. The thing is, these days that's not enough and as a pretty extreme example of a technology consumer, I think I have a responsibility to update my practices.
First of all, I wanted a picture on why our bill is so goddamn high. Even after this analysis I'm still not sure I know but I'm on the way and I think these techniques could be useful to you so here they are.
Most houses have an electric meter, here's one here:

I could bore you with the theory but let's cut to the chase. On the dial there is written 166 and 2/3rds revolutions per kW/H of electricity. What's probably easier to work with is if we can work out, right here and now, how much power the house is drawing. Then we can, hopefully, make some changes and go back and measure it.
This is how it's done: Meters sometimes have something called Kh written on them. This stipulates how many watt-hours of power for one revolution. Mine, as you can see from the picture, didn't. it had how many revolutions per kilo watt-hour of power. So we simply take 1000 and divide by the revolutions (166.6666 basically) and you see we get a figure of 6. This is the Kh figure. You might have it written directly on yours. It's typically from 6 to 7.2.
Now what we need to do is time one revolution of the disc. Do that with a stopwatch and just click on it when the leading edge of the black strip hits the little centre marker. Any delay you present in clicking should be the same, roughly, on both ends so you ought to be fairly accurate. If you want a more accurate picture still, feel free to measure several revolutions and just divide the total time by the number of revolutions to get the average. There's a good reason to do a wider average as I'll indicate later but for quick and dirty, one rev will do.
Then we have a formula which is as follows:

Power = 3600 * (Kh / time)

Since I established my meter's Kh to be 6 and I timed a revolution as 24 seconds, this gives me a figure of 900 Watts. Assuming the disc took 24 seconds to do one revolution, that's how much power the entire house is drawing right here and now.
As I indicated before, a spot reading wont be that accurate because there are a few high current devices which switch on and off, most notably fridges and freezers on a scale of minutes and of course things like immersion electric heaters (in particular!), electric kettles, ovens and cookers in the wider time scale. There's no more accurate figure over time than the actual numbers marked on your meter. If you want a real picture you should check this and see if you can lower it.
However, returning to the point in hand - I wanted to see if I could reduce my current 24/7 draw by any amount at all because, I knew, there is likely to be some dead stuff which serves no purpose.
In particular I mean devices on standby, DC power packs shoved in at the mains but not usefully powering a device and that sort of thing. I measured the power before and then I went through the house and checked for such things. I don't have a lot of standby devices but I found that our video projector and AV surround sound system and the printer were all in standby. So I simply used their main switches on the back. Then I went on to discover no fewer than SIX DC plug packs which were in sockets but not powering equipment. A few of these were mobile phone chargers, just switching off at the wall would do. Some of them were lost relics left in the birdsnest of wiring out the back of a communications cabinet for my work stuff.
I also switched off at the mains the PC speakers and the Squeezebox in our conservatory. I would say I found some stuff, but I didn't find a lot. Still, every little helps right? So I went back and measured the power again and worked out I had saved about 30-40W of power. Nothing huge but 30-40W of stuff doing nothing.
Our house is gas heated but I deeply suspect an electric immersion heater in the hot water system since I have on occasion seen the meter spin like crazy for reasons that weren't necessarily obvious. We have an dishwasher that uses a fair whack of power since it electrically heats the water. Oven is electric, but it's used once every few nights. The kettle is used a lot but we don't boil any more than we need to. All the lights in the house are energy saving or flourescent tubes.
There's not much I can put my finger on that's excessive but I've narrowed it down and have a better idea than I did before which was the object of the excersize. In some ways the gas, being a heating issue, is easier to tell what's going on. I have a good indication of the state of the house's insulation, the temperature on a given day and how much the heating needs to be on. We've got work to do there too, I think, but this other stuff can be accomplished with a calculator and a run around the house. You might want to give it a shot.


  1. So you're both working from home... so home is your office... so pretty much everything you do can be funneled through your accountant as a business expense, right?

  2. Not all of it no, but quite a lot of it. Basically a fraction of the floorspace of the home that is office space ends up being tax deductable, in our case mortgage outstanding. A fraction of the bills as well and then most of the gear ordered to do stuff, computer stuff, Net connection, that sort of thing. All deductable. This stuff ends up being enough that my tax bill is a good whack lower than it was when I wasn't self employed. The missus just does PAYE though, so we're not doing the best we can. Although it looks like her firm is going tits up anyway so she might have to end up commuting to a regular job before too long.