Before embarking on any **hydro power** generation project it is essential to survey the proposed site to calculate the amount of available **hydro power**.

The two vital factors to consider are the **flow** and the **head** of the stream or river. The **flow** is the volume of water which can be captured and re-directed to turn the **turbine generator**, and the **head** is the distance the water will fall on its way to the generator. The larger the flow – i.e. the more water there is, and the higher the head – i.e. the higher the distance the water falls – the more energy is available for conversion to electricity. Double the flow and double the power, double the head and double the power again.

A **low head** site has a head of below 10 metres. In this case you need to have a good volume of water flow if you are to generate much electricity. A **high head** site has a head of above 20 metres. In this case you can get away with not having a large flow of water, because gravity will give what you have an energy boost.

The key equation to remember is the following:

**Power = Head x Flow x Gravity**

where **power** is measured in Watts, **head** in metres, **flow** in litres per second, and **acceleration due to gravity** in metres per second per second.

*The acceleration due to gravity is approximately 9.81 metres per second per second – i.e. each second an object is falling, its speed increases by 9.81 metres per second (until it hits its terminal velocity)*.

Therefore it is very simple to calculate how much **hydro power** you can generate.

Let’s say for example that you have a flow of 20 litres per second with a head of 12 metres. Put those figures in the equation and you will see that:

**12 x 20 x 9.81 = 2,354 Watts**

### Real World Hydro Power Calculation

So in the example above a 12 metre head with a 20 litres per second flow rate equated to just over 2.3kW of available power. Sadly it is not possible to tap all of that power – nothing is 100% efficient. However, **hydro power turbine generators** are very efficient when compared to **wind turbine generators **and **solar panels**.

Efficiencies of around 70% can be expected which is to say that 70% of the *hydraulic energy* of the flowing water can be turned into *mechanical energy* spinning the **turbine generator**. The remaining 30% is lost. Energy is again lost in converting the *mechanical energy* into *electrical energy* (electricity) and so at the end of the day you can expect a complete **system efficiency** of around 50-60%.

In our previous example where 2.3kW of power was available – we can therefore expect to generate around 1.1-1.4kW of electricity.

These same calculations are valid whether you are planning a tiny Pico or Micro Hydro Power system or the next Three Gorges Dam Hydro Project.

Find out much more about **hydro power** by clicking here and viewing our Hydro Power Directory. We also have an **Introduction to Hydro Electric Power**, and information about the most common (small scale) **Run of River Hydro Power** systems. **Waterwheels** are introduced here.