So 'roof' is clearly out of the question. Now let's look at basement. I install a 4x4x2 meter tank, then drill down 100 meters to create another such tank. 32 tonnes lifted 100 meters gives me 32MJ, perfect.

Now how expensive is it to build a water storage area underground? Is this a 50 thousand dollar project or a 50 million dollar project?

If the ground water is at the right depth, you would only need to drill a well. So this should be practical somewhere, at least.