Maybe this has been fully wrapped up by now, but I have some test data which may be useful confirmation for some people thinking of using "all" their PV power.
I have a normal grid & PV system, where I have fitted 3 conventional utility meters, for (1) Power used by house, (2) Power imported from Grid, (3) Power generated by PVs. All three give 1000 pulse/kWh and I have built a (PIC based - sorry) system to read the pulses & display everything in the house.
Wanting to build an addition to use all my "spare" PV power to heat water, I was worried whether Grid meter might not see my "chopped" power going to the immersion heater, but may see "full power" of the applied heater. I see there was some discussion here earlier in the year.
With a bread-boarded zero crossing cct driving a Triac in "integral cycle" mode, I put power through to a 1400W room heater in various multiples of full cycles ( 0, 4, 8, 16, 24 out of every 24 cycles) Results are as follows : fully confirms that the Grid meter sees what I want it to see, and only imports the necessary power to make up my "reduced" load.
It was a dull day, but the arithmetic pans out.
cycles/24 0 4 8 16 24
Heater kW(assumed) 0 0 .23 0.47 0.93 1.40
PV generated 0.13 0.13 0.13 0.13 0.13
House using 0.37 0.63 0.87 1.36 1.85
Grid supplying 0.24 0.50 0.74 1.23 1.72
Hope this is useful
Re: utility meter response to triac control
I like your objective of using "all" of your surplus PV power to heat water.
Always good to hear that someone else has independently made the same observations.