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.