Is the calculation of weight to be suspended inorder to produce electricity from an alternator correct or not?
Yes .. but only so long as the 50cm wheel is maintaining 1800 RPM.
Given that the circumference of the wheel is 1.57m, that means you have to unspool 2.8 kilometers of massless rope with the weight on the end of it every minute to achieve that. The weight will be travelling downwards at 105 miles an hour.