They come in different shapes and sizes but the one at my house is a Landis+Gyr EM1000 model, installed on 13 May 2009 just before 2pm. (I know this because I have the data that was downloaded later and that is the end time of the first half hour with a normal numeric consumption value - before that point they are all just hyphens, which is what appears in the data I get for these meters when they don't have power, including during outages when some kind of internal backup battery seems to keep the internal clock ticking over.)
Here it is earlier today, on the 7th of April.
Note how the meter is showing the time (in HH MM format on it's LCD at the top left) but is an hour "fast" compared to the camera time at bottom left - which I know is accurate because I set it myself! This difference is because my particular meter is one of the many from earlier batches that was incorrectly configured in terms of the daylight saving dates and so won't switch over to standard time this year until at least next Sunday! This is one of a number of date and time issues that need to be considered when the data from these meters is processed. Their internal clocks are also not automatically updated or synchronised with any external time source. This can be important as the internal clock also determines the date/time stamp associated with each half hour data record stored by the meter and that has to be reconciled with the load control times from TLC later on. The EM1000 is meant to be able to store 284 days of data and after that the oldest data is discarded or overwritten as required.
The EM1000 model is the most basic model or TOU meter, only suitable for smaller single phase installations as far as I know. TLC currently also use other later (and perhaps more capable) models. I believe these include the EM51000 and EM5300. Click here for more technical details! All of these have a unique meter ID - that's the 2902959 number in the picture above. Then there is another smaller group of older TOU meters that I don't have manufacturer or model numbers for but as far as I know they all have eight digit meters numbers that start with the digit '9'.
The EM1000 has a LCD display that cycles automatically between the date (DDMMYYYY format, or 07042011 for today), time (HH MM), total day ("A" - 6am to midnight, as far as I know) consumption in kWh and total night ("B" - midnight to 6am) consumption. Pressing the big red "scroll" button lets you flick through those four display modes more quickly.
To the left of the scroll button is a small LED (red when lit) that basically flashes at a rate that indicates the "demand" your installation is generating at that time. As the labelling says, one flash means you've used 1 watt hour (Wh, 1/1000th of a kWh or "unit"). This means if your home is using a constant 1 kW of power then that LED will flash steadily at a rate of 1000 times per hour, once every 3.6 seconds, or 10 times in 36 seconds. Therefore I can test how much kW load (aka demand) my house is generating by dividing 36 by how long 10 flashes take (in seconds). If ten flashes take about 10 seconds - but remember to count from zero so you include 10 gaps also - then the average load or demand over that period was about 3.6 kW. Other meters might have different details and some models might even show you the load or demand on the LCD but you can usually use your meter to find out the current load by one method or another.
It was actually quite a bit easier to measure the demand on my old style total consumption meter with a rotary dial (as I used to have before my first "demand meter" was installed, but that's another story all on it's own).
Below the flashing LED is a strange looking roughly circular plate with some smaller round structures within that in an upside down V arrangement. All very mysterious and the whole thing reminds me vaguely of those crop circles the aliens leave behind, but instead that's actually where the optical reader device "connects" when the half hour data is downloaded or the meter needs to be reconfigured in some way.
One thing this this meter knows nothing about though is the load control signals arriving at my house. That job goes to the "ripple control receiver" instead, a completely different device in my meter box. Here's what my one looks like:
At the lower left you can see three panels with some strange plus signs and numbers written vertically besides positions 1:, 2: and 3:. These correspond to the three spaces for actual "relays" immediately to the right behind the transparent plastic with screw and orange anti-tamper seal. My one only has one relay installed, in the leftmost position, corresponding to the leftmost position:code label on the left, 1:+101+6. This apparently means that relay is on channel 6 and as the label indicates is controlling the power supply to my hot water cylinder.
The green switch on the relay should tell us whether TLC is load controlling on the relevant channel or not. The up position is "On", meaning power is available to my hot water cylinder, and the down position is "Off", meaning TLC are load controlling and my hot water is gradually becoming cold water again!
There's a saying, however, that what can go wrong, will go wrong!
So what could go wrong with all of this? Well, for starters, perhaps the meter's internal clock will do something weird. Perhaps it won't handle daylight saving transitions properly! Perhaps the backup power system for the meter clock will fail if it has no external power for a long enough period and then the clock will be out. Perhaps some power surge or other external event will cause it to lose data or whatever. Perhaps the ripple control receiver will fail or not detect some control signal properly meaning it is on when it should be off or off when it should be on. Perhaps the channel information recorded by TLC doesn't match what is actually in your meter box. Murphy is everywhere and I like to go looking for him!
So, I carefully record various details every so often, meaning I can do some checks against the data downloaded from the meter when that eventually happens and also against the load control timing information that TLC supply me. Some of us are just born this way and there's really no point in trying to change it!
I record the true date and time using my very accurately set wristwatch. I check the meter date display is correct and then I cycle rapidly through the different modes on the meter until the time display flips over to the next minute so I can record how far out the meter clock is when compared to my watch, to a precision of a second or two. (Since TLC checked and adjusted it in June last year it has held good time, being a more or less constant 14 seconds fast.) I also record the two displays of total units used, A (for day) and B (for night). Finally I note whether the green switch on the ripple control receiver is in the up or down position. And if it's down, I try to avoid using high demand appliances because this could be one of those periods of load controlling that turns out to be long enough to contribute to the "kW load" or "demand" calculations later on!
Other people buy "centameters" and "switchits" and other such newfangled hi-tech gadgets, but I like to go out in the rain on cold, dark and windy nights, trying to see how fast the red light is flashing and which way the little green switch is pointing... :-)