In a nutshell, what you do is clip it round the incoming feed to your electricity meter (or, in fact, any current-carrying cable), and it has a go at measuring the power consumption through the cable. What the meter connected to it then does is work out the current in your cable based on what's induced in the clamp loop, and then make a bunch of assumptions to calculate the power. Specifically, unless you have a smarter than average device at the other end, it assumes it's seeing 240V (in the UK), and then
power P = voltage V * current I * a fudge factor.
This falls short of ideal in a number of ways:
- your mains voltage can fluctuate a bit
- the actual current induced in the clamp can be affected by precisely how you place the clamp
- it takes no account of the kind of devices connected to your electricity
This latter can be quite noticeable. I won't go into details, but if a device has inductance or capacitance as well as resistance, then the equation isn't just P = V * I, and it depends on the nature of the particular device. Things with heating coils and big motors tend to be particular culprits, and most of us have one or more of those in the house!
There's also another problem which rears its head when you install solar PV cells - when your inverter kicks out more power than your house uses, the rest gets fed back to the grid. Because of the way it works, the clamp sensor can't measure which way the current flows, so it can't tell if you're using or generating power.
So why use a clamp sensor? Well:
- it works on any current-carrying cable
- if you have an old-style electricity meter, it's your only choice
- if you have solar PV, unless you have a smart meter that your PC can talk to, it's your only way of measuring how much electricity you send back to the grid (in conjunction with another sensor)
The latter is why we're hanging on to ours. More on our monitoring system in later posts.