Connect PV into main breaker in CU

solarfred · May 18, 2012

Is it possible to take the PV circuit from the inverter via it's own RCBO in the CU into the incoming breaker thus connecting to the main live tail before the house circuits.
Will the PV then only flow past the breaker if it is required to feed house load and any balance will be exported directly back down the main incomer.

Earthstore · May 18, 2012

Yes, the same as you can connect into the tails using a Henley block, which in essence is the same thing.

Just one point, if the incoming is 16mm, you can not poke some TWE into breaker as you may not have full mechanical connection.

I hope I have worded this correctly.

solarfred · May 18, 2012

That is what I thought. So to measure net usage via energy monitor it is possible to run tail from main breaker to all other circuits and CT clamp that cable and then clamp the solar incoming and thus you have the net demand without the solar export messing up the numbers.
Having checked my monitor against a simple EON single channel monitor I know the CT's measure AC current pretty accurately but when the EON monitor is attached to the main incomer by the meter and my solar monitor is connected as above and if the solar is equal to the house demand the current in the main incomer should be near zero?? But it is 100-150w. I cannot fathom where the 100w difference comes from.
Also both monitors measure about 40w more than the actual inverter output.

Earthstore · May 18, 2012

These monitors can not measure which direction the AC is travelling IE if your PV is producing 2KW and you are using 1.5KW then your incoming monitor will still show 500w, but that would be export not import.

Also some inverters use a little power to convert the DC to AC and there can be other small loses, so even though you may be producing 2KW at the inverter, this might explain the difference.

This type of monitor should only be used as a guide, they are not accurate, they can pick up power from other cables, and the neutral which affect the readings, also some of this type you can adjust or reset, but don't ask me how, sorry.

The generation meter is the only (well should be) accurate monitoring device.

solarfred · May 19, 2012

Precisely why I set up the measurements when the incoming PV (on a dull day) was matching almost exactly my house demand so that I knew the numbers should cancel out. It does not matter in this situation that the CT on the incoming main does not measure direction as the current in the cable should be close to zero ie. I am importing/exporting nothing. If I can measure the true demand (as I have checked the monitors accuracy) and I can see what the inverter is producing and there is 150w "in the main supply" then either I am exporting too much and the numbers are totally wrong or I am importing when the PV should be sufficient to run the house demand. Hope that makes sense.
I have noted that at low levels the monitors do get very inaccurate but as amps rise they get closer to true numbers.
I managed to get all three monitors in shot to show the results. Hope they come out clear enough.
Left is inverter, middle is on incoming live from meter box, right is my monitor measuring solar surplus.

As a further experiment here are the readings when I switch on the 3kw immersion.
http://i1145.photobucket.com/albums/o517/freddyuk1/IMG_0082.jpg
Now the main incomer monitor has a reading of 3.02kw and my monitor net import is reading 3.00kw which suggests it is close enough. The solar is still 100w difference between the inverter reading and the monitor reading. (the CT for solar is PV side of the breaker and there is no generation meter). So I can programme this monitor to control circuits around the house to only come on after the solar balance is positive. It will work the immersion therefore and I should be safe by leaving some small leeway to ensure I do not import anything to switch appliances.
This also means I do not have to rewire a Henley block and new garage unit to get proper net solar readings although I accept the point the connection needs to be secure.

moggy1968 · May 19, 2012

are you sure you aren't picking up induction type currents from the inverter?

solarfred · May 19, 2012

Is there a way I can check that?

Earthstore · May 19, 2012

I think it is a small current that bounce's to and from the inverter and your monitor picks it up both ways, not too sure how you can check for it though.