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Your post doesn't mention the method you tried to control the MOC3020, but you will find that using a diac to trigger the conductive part of the cycle will give you that delay-then-jump behavior.

 Thanks for that, have you a schematic with this ic in? and being controlled from the arduino?

All I have tried with the MOC 3020 was with a psu on the led side to see what control it needed and found that when placed in the 15amp dimmer circuit I built which adjusted ok with the preset in circuit the lowest start up was about 4 volts on a circuit 0 - 12volt.  

I haven't used the phase control IC with an Arduino yet, but I have plans to integrate it into my dump load controller. There should be no reason that the DC control logic the TCA785 needs can't be generated by a digital output using analogWrite(). The opto can be driven directly by the output(s) of the control chip.

I, too, am thinking of using a triac to control my 3kW immersion heater. Please, if you would, answer this basic question:

The triac chops the AC supply into ON and OFF states while the solar pv inverter supplies continuous AC. My utility meter is a modern solid state device that does not run backwards. If, say, the solar output is 2kW and my triac system is set to deliver 1 kW rms to the immersion heater, will the utility meter integrate over a mains cycle such that my consumption is -1kW (or, rather, zero because it will not run backwards) or will it ignore the 2kW put back into the system during the part of the cycle when the immersion heater is OFF and bill me for a positive amount (approximately 1kW for the fraction of the cycle that the immersion heater is ON)??

summary:

solar 2kW
triac controlled heater 1kW (but peaking at instantaneous 3kW)
1-2 = -1 (is this what is billed, or rather zero because it won't run backwards)?
or will a positive amount be billed?

(A utility meter was one thing I was not allowed to take apart as a child so I don't know how it works.)