The attached file describes a chopper circuit that has the provision of precisely measuring power. It is based on the fact that a low resistance (~2 Ohm) heating resistor has a <1 uH inductance and therefore appears resistive at a~20 Khz switching frequency. Since the chopper is powered from a DC supply, it is straightforward to measure both voltage and current (and therefore power) to a high degree of accuracy. I have built this circuit, and it functions as designed.
It has advantages over triac or phase control circuits in that both the supply voltage and duty cycle can be precisely controlled. Power output can be controlled either by varying the duty cycle or by adjusting the power supply voltage. I have configured it to do the latter because supply voltage is more easily measured and controlled than duty cycle. As designed, the circuit generates 75% duty cycle, 21 KHz pulses at any voltage up to the limit of the supply which is 40V.