The amount of water and how much you want to heat it only tells you the energy required, not the power. Power is energy per time, so it matters how fast you want to get there.
You say you want to heat 500 ml of water by 60°C in 30 minutes. Do the math.
The specific heat of water is 4.18 J/g°C. The total energy required is therefore:
(500 g)(60°C)(4.18 J/g°C) = 125 kJ
This energy spread out over 30 minutes (1800 seconds) is:
(125 kJ)/(1800 s) = 70 W
However, that is only the power that has to go into the water. Your 500 ml of water sitting in some container at near boiling temerature is most likely loosing more than that to the environment. Put another way, your heater has to not only provide enough power to raise the temperature of the water, but also to overcome losses of this heat to the enviroment. At only 70 W into the water, the heat loss will be significant.
With a lot of care spent on insulation, maybe 150 W is enough. It's really hard to guess since we have no idea what your mechanical system looks like. A few 100 W would be better.
Also, consider that 30 minutes is a long time to wait for a cup of boiling water. If the system can just do that in full sun, then it won't work in anything less than ideal circumstances. 500 ml of water is also not a lot of stuff to heat. I think you are grossly underspecifying your heater. You may get 500 ml to boil in 30 minutes, but I expect you won't be happy with the performance of the heater overall.
Figure you get about 1 kW per square meter of full sunlight, so a few 100 W doesn't really take a large collector. However, you have to pay attention to the aligment, and focus the rays on a small spot roughly the size of the bottom of the kettle.