where is the power button

On the site calculator (and just about any calculator) the carrot sign that you used "^" would suffice for the power button.

 

However, it is also either of the buttons $${{\mathtt{x}}}^{{\mathtt{2}}}$$ (number squared) , $${{\mathtt{x}}}^{{\mathtt{3}}}$$ (number cubed), or $${{\mathtt{x}}}^{{\mathtt{y}}}$$ (number to any power) in the 5th column from the left on the site calculator:)