You might be coming to a yoga practice with the intention to lose weight. First of all, as any form of exercise, but especially yoga, with its cultural, religious and spiritual origins, is not purely about the amount of calories burned. Exercise should be enjoyed - it is a freedom and a privilege to be able to move and workout. Weight loss is simply a bi-product of this enjoyment of exercise.
I cannot tell you the exact amount of calories burned from practicing power yoga, because that depends on many things, such as how long are you practicing for, how intense the class is and your individual metabolic system. In general, the amount of calories burned will depend on the intensity of your power yoga practice and for how long this intensity is sustained for. If the yoga flow that you are practicing is very physical and works with a lot of continuous movement for 90 minutes you will find that you are burning more calories than if your yoga flow is slow and steady in pace.
Of course, then you might like to think about the amount of yoga classes or yoga practices you are practicing across the week to increase the number of calories burned. If you are looking to lose weight from practicing yoga, power yoga is the style to practice as one of the more physical practices.
As the nature of yoga is based around the breath in synchronicity with the movement, the focus is usually more on sustaining this intention and connection as you pass through various yoga poses. These poses have health benefits which are in turn more beneficial than the amount of calories burned. For example practicing a shoulder stand can help blood circulation and in turn digestion and concentration. The yoga poses also contribute to toning and shaping the aesthetics of the body as well as giving you better anatomical support.
Trust that your body will be burning calories as you practice yoga. It is natural that the more you move, the more you put your body through a workout session the more it will respond and start to adapt to moving more. In time this will change your metabolic rate which then leads to your body burning more calories at an increased rate. It is not always important the immediacy of the amount of calories that have been burnt, but instead the consistency of practice over time and how that makes you feel. Once you start to experience the benefits of a regular practice, you will start to feel better within your body, experiencing lower stress levels which will contribute to burning more calories in the long run.