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Tuesday, January 6, 2009

Calculating energy usage is harder than it seems

In my last post, I said a Christmas project was to monitor my usage - but I confess I've got a problem of how to measure my energy usage which is so far unsolved.

As I said, I have a currentcost meter (m1) which reads the current going into and out of the house. Of course, it cannot determine which way current flows, so it reads 50W regardless of whether the 50W is imported to, or exported from, my house. Therefore, I put a second meter (m2) round the wire from the solar panel, which told me how much the panel was generating and was thinking that if the value on m2 > m1 I must be exporting, otherwise I would be importing. However this thinking is flawed:

If I am generating 100W, (m2=100) and I am using 50W, then I will export 50W (so m1=50).
If I am generating 100W, (m2=100) but I am using 150W, then I will import 50W (so m1=50).

So from my two readings, I cannot conclusively say whether I am importing or exporting electricity, unless I am missing something obvious? The bright green import/export meter which Southern Electric fitted manages to work out what is being imported and what is being exported - but there doesn't seem to be a nice little serial port anywhere obvious! The other option is to buy a wireless unit as an addition to the inverter but this costs £120 and doesn't appear to give me open access to the data.

BTW - in 20 days we've generated 16.2 kWh and exported 9 of them. A grey day typically generates 200Wh the whole day - a sunny winter's day like today seems to generate about 2kWh.

4 comments:

shawdm said...

Could you not put the second sensor around the wire at the point after they have been combined into one circuit. I assume at that point it can only ever be flowing in to your house. If that number is smaller than that being generated by the panels, then you'd know you were exporting.

There's probably a reason you didn't do that. :-)

Kevin X Brown said...

Hi @shawdm Thanks for the idea! Took me a while to think about that - it makes sense that the "solar slip road" traffic joins the "mains motorway", and measuring it just afterwards would give the total traffic going on in.

Unfortunately though, the current will find it's way to Earth in the quickest way possible - so if an appliance is on in my house, it will go through it - if not, it will find the nearest neighbour's appliance to go through. So the "mains motorway traffic" just flips direction, and the meter, wherever it is along the motorway, will be none the wiser.

Graham White said...

Sounds to me like you need to have a word with an electrical clever git. Maybe time to talk to Current Cost themselves or Andy SC to see if there's a non-invasive way of measuring the direction of current?

andrew@ugh.net.au said...

An overly complex solution might be to (presumably under the control of some piece of computing equipment) periodically flip on something of known load. If the reading from the meter showing your flow to/from the grid goes up then you are importing and if it goes down you are exporting.