Friday, 28 October 2011

Electricity Monitors and PV don't mix

If you have PV panels, then you are probably either keen to live your life more sustainably or keen to manage your finances sensibly. Either way you will be interested to keep an eye on your electricity consumption as well. So you may be thinking of getting one of these clip-on electricity monitors that allows you to keep an eye on your electricity usage real time and also keep track of how much you are using over the days and weeks (without taking a torch to your meter cupboard every day). Well, if you have PV panels or a wind turbine that feeds into the grid you can forget the clip-on monitor.  The trouble is that it measures the current going through the cable - whichever direction it is going in.

The diagram shows how your panels will be connected, through the generation meter into your consumer unit with all the circuit swtiches. The grid supply is on the other side of your meter. When you are generating more power than you are using then current flows onto the grid. When you are using more power than you are generating the current flows into your consumer unit. Your supply meter only measures current flowing into your unit (unless you have a very old meter in which case you may be lucky and it runs backwards when you are feeding onto the grid). The clip-on monitor however, doesn't care which way the current is flowing it still counts positive. The real time watts figure it tells you is either how much you are using (U-G) or how much you are feeding onto the grid (G-U) and you can probably guess which at least some of the time.  At night G will be 0 so it's U. When its sunny it is probably G-U. However, the total it gives you for the day is the amount you have put onto the grid plus the amount you have taken off the grid which means nothing whatsoever.

If you think about this when you have the panels installed you can ask for an extra junction box so that you can measure your usage directly - see this post on a navitron discussion group. Unfortunately, we didn't think about it at the time.


  1. I think the energy meter firms are starting to catch up with all the PV installs, eg see the Wattson Solar Plus:

  2. Thanks for posting this! I'm trying to find information on Electricity Saving Devices and this has definitely helped me in this process.

  3. And another reason - apparently some inverters appear to draw power when the panel is not generating, for example at night, but in fact this is almost entirely reactive power, not real power (Wiki answers has a reasonable explanation of this A simple current clamp monitor can't tell the difference and will report power drawn when in fact there is none. A friend with 12 panels, each with a converter, reports that his Owl meter over-reads by up to 100W because of this.