Do you mean that you want to store rain water in a tank then use the gravitational potential to supply energy to a generator? It won't generate much power . . . Even in a country with rainfall of metres per year, a one square metre collector would give only a few tens of  thousand joules of energy per year. A solar panel will give 10 thousand joules of energy every minute.

Are those two questions? 

Hi,

There are many techniques for rain water harvesting and generating electricity. Some of them are as follows,

1. Using Bernoulli principle.

2. Using a collection chamber for collecting rain and then using Bernoulli principle.

Check also piezoelectric energy harvesting. 

To generate electricity by collecting rainwater is possible. It is necessary to collect water large enough height (meters minimum) or to place the generator several meters underground. Used hydro-generator should be multipolar small rotational speeds in order to not induce cavitation of the water, which will reduce the efficiency of the system.

I think these generators are manufactured in the Czech Republic.

To generate electricity through rain water i can suggest you one  efficient way:

1. Collect  water at rooftop  , use a pipeline connection from rooftop to ground , when water will be flowing down can have a turbine in middle of pipeline connected to generator turbine will rotate which makes generator to generate electricity ,its very basic way of generating electricity, for increasing the rate of electricity generation  make pipeline connection more slanting.

2. The waste water which is left after electricity generation form turbine,  collect that left water in small  pond, in that pond you can use the concept of solar pond and can generate electricity again with left water after turbine process.

In the above process water will not be wasted at all, and you can use 100% of water that  you collected to generate electricity.

Hope my idea will help you in efficient way,

Have a nice day ahead.

Electricity can be generated while rainwater is flowing down from roof to underground storage. You can use turbine coupled with DC generator to charge batteries.

E= (p* phi* z/ a)* N/60

The potential head at roof will get converted to kinetic head at the generator.

v= sqrt(2gh)

The generator DC voltage has to be a little higher that that of battery nominal voltage to be able to charge it. So you will have to modify piping size at the turbine to get enough velocity..

A1v1 = A2v2

You can also use suitable gear/ pulley combination to achieve optimum N at the generator. But, in that case applied torque by rainwater at turbine must be sufficient to drive it.

In short, the energy of the rain water (head & quantity) must be studied first for an optimum design of the system & also to check its economic viability.

you can check these two papers for rain water energy harvesting using piezo

1.Piezoelectric model of rainfall energy harvester

2.PIEZOELECTRIC ENERGY HARVESTING IN VARYING SIMULATED RAIN CONDITIONS

This question cannot be answered easily on a portal like this. You will firstly need to put in research from your end as to what you exactly desire from the electricity generation point of view. In general, you may find sufficient knowledge in the field of micro and nano turbines, household renewable energy applications and large scale non-conventional power generators. You will need to combine the understanding of scaling of generation devices with factors such as rainfall collection (geographical dependency) as well as power interfaces to/from grid or energy sink (single home).

Refer about the project "Kalpsar" on web and my related answer on web RG, about rain water harvesting and the consideration of parameters in tidal power.....26Nov'14.....