I apologise for any duplication in other threads but I can not find the exact answer to this fundamental question.
If an ON/OFF controller such as a diode or a triac is used to control a load such as an immersion heater, does the electricity meter integrate (over a cycle) the brief ON period with the continuous input to the grid from the solar pv inverter, or not?
There are different kinds of meter and some old ones actually run backwards. I am referring to a modern digital meter such as a Mercury 5236. I have checked at least that it does not run forwards when power is being exported but it certainly does not run backwards. The question is does it integrate over a cycle the exported steady-state solar pv energy with the sudden burst of ON state from the switching controller, or does it ignore the exported energy and thus charge you for the difference between the heater watts and the generated watts integrated over the ON period?
To be numeric, say you have a 3kW immersion heater. Say your solar pv is generating a constant 1.5kW and say that the triac is switching the immersion heater so that it conveys 1kW of heat to the water. Does the utility meter bill you for zero, or does it bill you for some positive value?
Re: fundamental but important question re switching controllers and utility meters
This was discussed at some length in the Solar Energy Management thread. The contention is, and my own experiment confirmed that, as long as you don't move more than one watt-hour total through the meter in either direction, you can "roll back" the count by reversing the direction of the current. Once one watt-hour is exceeded, the count is passed to a register and the process starts over.
For example, you can draw 100 watts of load through the meter for long enough to accumulate .9 watt-hours of energy consumption. If you then reverse the flow (producing energy instead of consuming) at the rate of 100 watts, you can pump back 1.8 watthours without exceeding the one watt hour limit (+.9 - 1.8 = -.9). You can see-saw the energy back and forth across the meter at any rate you desire as often as you wish without incrementing the total display.
The result of this is that partial cycles or skipped cycles don't have enough energy to total one watt-hour, and so do not effect the total registered on the meter's display..
This was brought out in the Solar Energy Management topic here:
http://openenergymonitor.org/emon/node/176?page=2
Of course, ~everything~ depends on how your particular utility meter works internally.
Re: fundamental but important question re switching controllers and utility meters
As Mr. Sharkey says, everything depends on the particular meter, and especially how the utility company set it up. You might find a manual on-line. If you do find it, read it and particularly look for options that can be programmed, e.g. how it responds to reverse power etc.
Re: fundamental but important question re switching controllers and utility meters
Thanks and apologies for cross posting. My utility meter is a Mercury 5236, about which I can find little, and First Utility maintain their traditional silence when asked questions their customer services team have no scripted answers for.
I shall assume one watt-hour bidirectional integration over a mains cycle when used with a triac and look for any red flashing of the meter to indicate power is being registered.