Electricity Meters – not as smart as you thought?

There are two basic types:

1. In-line meters - you plug it into a power socket and then plug the appliance that you want to monitor into the meter.  The meter has several buttons and an LCD display which can show any of the following: the mains voltage, the current that the appliance is taking from the mains, the frequency of the alternating voltage (usually around 50Hz), the power being consumed by the appliance (measured in watts) and the energy used since the appliance was plugged into the meter (in kWh).  There is often another feature that is not much talked about, that is a number called the power factor.

2. The second type monitors the power for the whole house/flat.  You have to clip a small magnet coil around one of the leads coming in from the external electricity supply, just close to your meter.  This magnetic loop is connected into a small battery powered wireless transmitter that in turn sends information to the monitor unit (this may be battery or mains powered).  When wireless communication has been established the monitor displays the power being used in the whole house (in watts).  Usually the monitor will also allow you to add up the energy you use over a given period and convert it into a cost (provided you enter what you pay for each kWh).  Various more sophisticated readouts are often available, such as weekly consumption, amount of CO2 generated etc.

So what are the pros and cons of each type?

1. The inline monitor is very simple to use, nothing much can go wrong (no wireless communication to fail, no batteries to go flat,….). But of course this type can only monitor the power from a single appliance at a time and it's tedious moving it from appliance to appliance (maybe 20 or 30 appliances in the average house and ideally you would check them all).  Also some of the most energy consuming appliances, eg cookers, do not have easily accessible wall sockets into which the meter can go, and fridges and washing machines often have sockets that are hidden below work surfaces.  Also, the meter is not able to measure power consumed by lighting circuits so doesn't help with checking how much energy your lighting is using.

2. At first sight the whole house monitor seems much better (provided you don’t mind batteries and wireless links).  You get an instant idea of just how much you are using and this can be a valuable wake-up call to you and your housemates.  And you can still get an idea about what each appliance uses by watching the power consumption display as you turn each item on and off and noting the change.  This certainly works very well with most of the heavy-using appliances like kettles, tumble dryers, irons etc.  A downside of the whole house monitor usually only becomes clear when you start to look at the lower power-rated devices such as laptop computers, printers, ipod docking stations and appliances on stand-by.  One thing to notice is that, even when all the electrical devices are running at a constant rate the power consumption indicated by the monitor is not constant.  The fluctuations depend on the actual meter but are generally around 10W that sets a basic limit below which you are not able to sensibly check consumption of any particular item.  Thus the basic sensitivity of the monitor is much worse than the in-line type.

Note for Geeks: Looking even more closely (as geeks tend to do) you might happen to notice that turning on a particular light, for example, causes an apparent fall in the power consumption.  You may also find that comparing the consumption of a particular appliance using both types of meter can yield two quite different answers.  For example we have a microwave oven which, when plugged in to the inline meter but not cooking, shows a ‘standby’ consumption of just 3W.  But switching this same appliance off at the mains switch showed a drop of 50W when measured with the whole house monitor.  This discrepancy by a factor of just short of 20 was deeply disturbing (to a nerd) until we clicked on the inline monitor button to select the power factor function.  This revealed that for this particular oven on standby the power factor is 0.05 whereas when it is actually operating as a microwave the power consumption shoots up and the power factor becomes almost exactly 1.  When the power factor is close to 1 it means the current and voltage are in step (in-phase in technical language).  When it is much less than 1 it means current and voltage are out of step.  This matters because the amount of actual power consumed is the result of multiplying the current flowing by only the in-step part of the voltage.  The result is actually zero when the current and voltage are exactly out of step, as they are when the power factor is zero.  (Being exactly out of step means that when the current is at either a positive or negative maximum (remember, it’s a.c.) the voltage is exactly zero and vice versa). The in-line meter allows for this out-of-stepness when calculating the power consumption but the whole house meter does not.  It cannot, in fact, because it only measures current with its small magnetic sensor and assumes both the value of the voltage and that it is exactly in step with the current.  That’s why we got such a big difference in the readings from our microwave.  (In fact a power factor value of 0.05 explains exactly why the reading were different by a factor of around 20 and also makes clear that the in-line meter reading is correct).


Electricity Meters