Powerpal Technical
Calculating the resource energy

With a few simple steps you can determin just how much energy is in your water resource.
The power formula for water is:

Watts = m.g.h. x efficiency

Where m = flow in litres per second
          g  =  9.81 gravitational constant
          h =   net head (after any pipe losses have been removed)
         eficiency = 0.5 (a rough estimate for small hydro plant)

What is the approximate electrical energy yield of 200 l/sec falling 10 metres.
Answer. Gross power (W) = m.g.h x .5
            = 200 x 9.81 x 10 x 0.5
            = 9,800 Watts
              or 9.8 kW  

Unfortunately a universal law of nature is that converting energy from one form to another consumes energy, it is never possible to convert all the Gross energy to say electricity without incurring LOSSES.

Similarly STORING energy in batteries is a chemical process, it consumes energy. Derate the energy stored by 20%, in practise this will vary considerably given temperature type of battery, its age and depth of discharge. Only ever consider deep cycle SOLAR or LEISURE batteries.

If you have mains electricity a grid connect system is best, even a very small hydro system will generate considerable energy over the year.

The difference between ENERGY and POWER

Energy is the quantity of power you use, the term to use is the 'kiloWatt hour' also known as a 'unit' . 1000 Watts consumed for 1 hour is 1kiloWatt hour or 1 unit of electricity. (Watt was a chap so the W is caplitalised) you pay for these units in the quarterly or monthly bill (plus rental, tax and any other nonsense they can sting you for!)

Power is the rate  that energy is produced or consumed, it is a real time description of energy transfer. A current flow of 1amp at 250 Volts is a load of 250 Watts, a current flow of 250 Amps at 1 Volt is also 250 Watts. Watts are a time related expression so ading extra time is muddling the picture


A turbine maintains an export rate of 500 Watts for a complete day, it is wrong to describe this as 500 Watts in an hour or per hour as the media commonly does.  It is correct to describe it as generating 500 Watts for 24Hours or all day, thats still a rate. The correct way is to do the sums and describe it as an energy volume: so 24hrs x 500Watts (or  0.5kW ) = 12 kWh of energy generated daily or 12 units a day.

If  that rate is maintained then the site would generate 12 x 365 = 4380 kWh (or units) each year, a considerable energy contribution for most households. Clearly you do need the water flowing and machine not stopping to acheive this, and this raises the need to ensure the installation is done really well.

DW (2008)