So I went through and recorded the power use of every device I could, either by reading the specs or measuring it myself. The TV and associated devices were turned off at the wall when not used. For the 5 hours a day average that they were on, they only drew 70 Watt, giving a daily average of 0.35 kWh. With the air-conditioner never used and only drawing a 3 Watt standby that only accounts for another 0.072 kWh. The 1000 Watt microwave was only used for about 15 minutes a day average, giving another 0.25 kWh. From the specs, the fridge was using only 1.5 kWh a day and her use of the fridge was what I considered average, not leaving the door open too long or opening it too often. The washing machine averaged a load a day, which was 0.6 kWh. Don't forget the dishwasher, it used 1.5 kWh for its daily load. These were the only devices that were plugged in at the time and adding it up, they only account for about 4.3 kWh a day.
So there is still another 10.7 kWh a day to account for. Ahh, the oven and stove I thought, had they been using that a lot? No, their use would average out to about half an hour a day, at about 4 kW of power, giving another 2 kWh, still 8.7 kWh from the target.
I had another look at the power bill and noticed that there was only one tariff, which meant that the electric hot water system had to be included as well. I checked the thermostat and it was already set to the lowest legal temperature, so I tried to find out how much power it was using. After digging around in the Rheem website I found a table which listed the maintenance energy of water heaters. This is the energy that the system uses to keep the water at a temperature when no water is being used, ie the waste heat. An approximate value for this is 2 kWh a day. Now lets say they were to use 50 Litres of hot water a day, which is heated from 20 degrees Celsius to 65 degrees Celsius. That would require 9.4 MJ or 2.61 kWh. So, getting closer, with the total 4.61 kWh from the hot water system, there is only about 4.1 kWh to account for. However, that was it, there were no other power sinks.
This accounted for about three quarters of the power, but what about the rest? Maybe my measurements and estimates were out a little, but I would have to add 35 percent to my measurements to get what was on the bill. Not impossible, but my readings did seem a long way off. So I started taking regular meter readings to get some more data. That's when things got interesting. One day when no-one was there, with no-one taking a shower, no cooking, no washing, and no tv, the meter recoreded an energy use of 19 kWh. WHAT! There had to be something wrong. I wondered if the hot water system was leaking in to the overflow pipe, or if a pipe had a leak somewhere. To find out if this was the case, I decided to turn off the hot water for a day and take another measurement.
Being an apartment building, the switch box had about 20 fuses for 4 or 5 apartments. After looking around I found and turned off what I thought to be the hot water system for my sisters apartment. Before doing this I checked the voltage at the hot water system with a voltage detector (I love this thing, It's the closest I'll ever get to being a Time Lord and having a sonic screwdriver). I then checked again afterwards to see if I had turned off the right fuse, which I had.
|Crescent Voltage Detector|
The next step was to check if for some reason the fridge was on the same circuit. Leaving the fridge without power for 24 hours would be a really bad thing. However, while doing this the neighbours appeared at the switch box. Half their power points were dead. Bingo. Turns out that my sister had been supplying power to the neighbours for a few of their appliances, but mainly the heater they were using at night during winter.
After a call to an electrician everything was sorted. He was at first sceptical, but after testing everything himself he came to the same conclusion. The solution was easy, move the offending circuit over to the neighbours meter and run a new line to the hot water system.
There's no way to tell who did this or when it happened. It could have been a dodgy previous tenant or a mistake during installation. Paying for more than you should is annoying, but more importantly this was a dangerous situation. Let's say that a tradey comes to replace the hot water system and turns the breaker off, the neighbours think that a breaker has tripped, so they go and turn it back on, the tradey is now working with live wires but they don't know it. Or an electrician could come to work in the neighbours apartment, shut off the power and not realise that the power points are still live because they are connected to a different apartment. A seemingly innocuous problem could end with someone being seriously injured or worse.
After it was fixed, the meter readings came back to 10 kWh a day, which is close to what I had calculated from my measurements and estimations. So, if something doesn't feel right, trust your gut feeling and either find out why your wrong or find out what is wrong with the situation. It could be more important than you think.