CIN, a 900 Watt Microwave is not a particularly powerful one so you are lucky there. The best way to proceed with a microwave for your purpose is to firstly use small bursts at a medium setting with time in between. So say set it to medium about 500 - 600 watts for a minute to 90 seconds and then allow it to rest for similar time. Check it and repeat until it is defrosted. If you can do it, once it defrosts a bit, stirring at the rest points helps in spreading the heated food with the non-heated food. Once you are happy that your meal in a container is pretty well defrosted you can then up the power and give it a blast for a minute, let it rest and check it. Again, resting and stirring is helpful to distribute the energy. Microwave ovens are bad at heating or cooking food evenly because it is concentrated energy applied all around the oven and it is a box shape, so it is up to you to assist in the even distribution of the heat. Rotating, turning, stirring and resting (and even foil covering) are important parts of using a microwave oven effectively and properly.
Unless yours is a new "Invertor" microwave, the lower power settings just turn the magnetron on and off with times of microwave and times of no microwave anyway. If you have the Inverter model then the power level is reduced but the magnetron works continuously at that lower power. Rest times in between shots is still an important thing to master.