Current Cost EnviR shortcomings regarding Power Factor

We recently installed a Current Cost meter

The system allows you to clamp a sensor around your mains cable which broadcasts readings of current to the display.  It also allows connection of up to 9 appliances.  The display aggregates the data into two hourly, daily, and monthly bins.  It also transmits the data on an RS-232 to any listening PC. The main sensor clamps onto the live output to the main fuse box in the house, which measures all current being consumed by the house.

Our challenge was to try to use this data to monitor where our energy consumption was going, as well as collect information on our solar PV installation.

In order to monitor additional systems in the house I bought a pack of three of the sensors on the left.  One clamps around the live feed from the Solar PV system, measuring Generation. I clamped another around the live leaving the fuse box going to the underfloor heating system. The final sensor was intended to be clamped around the mains cable to the cooker.

Current clamps work with a single core only

Unfortunately, my strategy of clamping around a cable in the loft to measure the current does not work.  You must clamp it around only the live cable, or the current on the neutral cancels out the live and you get no reading.

Because I had an extra transmitter, I made a special extension cable where the live passes through the sensor, so I can plug in any set of appliances into this cable.

What’s using all that “power”

When my system eventually was up and running after the initial set backs, and some opening of my fuse box to get the clamps on the circuits, I noticed that my under-floor heating was reporting 35 watts when it was not actually heating.  This struck me as too much.  I wouldn’t leave a light bulb on all the time.  While the thermostats were warm, they weren’t that warm.

So I powered down the entire house, switched on the under-floor circuit only, and found that they were not using any measurable amount of power.

Power Factor strikes!  This happens in several circumstances.  Large magnetic loads (refrigerators, and pretty much anything with motors) only use a portion of the power, returning some of it back out of phase.  The link above tells you more about it.

Now, it’s really hard to monitor “power”

Unfortunately, this makes life difficult as the sensors are describing power that is not actually being billed to you (though it does cause inefficiencies for the power companies, and the amperage actually does flow through the circuits in the house, increasing load).

What else suffers from this? My PC for one appears to show a power factor of 0.7, which means that I have to multiply the reading on my meter by this factor to get real power.

Oh, and the Inverter for the Solar PV as well.  It shows 80 watts at night, supposedly consuming this amount, though in fact it only consumes less than 1W.

There is a solution for individual appliances.   These measure the real power on the device.  Unfortunately they don’t also let you know the power factor, so as a result, you cannot subtract the “phantom” consumption from the total. They do though let you measure real cost for the individual appliance.

Note that in the UK, consumers are not billed for Power Factor.

Dimmer switches have no effect?

I also tried measuring what happened when I used the dimmer switches on eight 50W halogen lamps.  Apparently nothing, they use almost exactly the same power. 

Of course, that can’t be right, as the dimmer switch isn’t acting like a 500W heater when the lights are dimmed.

While researching the new LED lights I talked about yesterday I was warned by my electrical supplier that they’d had some problems dimming them.

Philips have a document that describes the way these things actually work.  Again, you can see what’s going on with the power factor, as it’s only consuming a portion of the load.

Too many Sensors?

So, now I’ve populated my system with a main meter, 3 additional clamps an an appliance monitor.  The system is supposed to be able to handle 10 inputs with up to 3 readings on each.

Unfortunately, it drops packets as they are colliding, and not being received by the main unit.  There is no protocol to avoid collision.  So we now have lumpy sampling to deal with as well.

Buyer Beware

I hope this has been a useful overview of the shortcomings of the Current Cost system, which while flexible isn’t really good enough for highly accurate measurements, due to the power factor issues, and lost packets.

We’re doing our best to produce some software that alleviates some of these issues, so stay tuned.