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Power versus energy

Power is measured in watts (W) or kilowatts (kW).

1 Watt (W)= joule per second
1 Kilowatt (KW) = 1000 watts
1 Magawatt (MW) = 1000 kilowatts
1 Horsepower = 745.699872 watts

When you’re discussing waterpower and say “Power” you have to know what “Power” you’re measuring or talking about because there are 3 spots to measure power.

1. The hydraulic power – or power in the water.
2. The mechanical power – the turbine changes the hydraulic power to mechanical power at the output shaft of the turbine. The mechanical power will be less than the hydraulic power because of efficiency losses due to friction etc.
3. The electrical power – the generator changes or converts the mechanical power in the rotating turbine shaft into electrical power. As power is converted from one form to another some power is lost at each stage.

1Hydraulic Power Calculation

To calculate hydraulic power you need to measure Head (meters) and Flow (liters per second).

Hydraulic Power = Head (meters) x Flow (lps) x 9.81
Hydraulic Power = Watts
or
Hydraulic Power = Heat (meters) x Flow (cubic meters per second) x 9.81
Hydraulic Power = kilowatts

The greater the head and flow the greater the power.

Where did this formula come from?
The energy released by a falling body of water = weight x distance

And weight is the downward force the water exerts which = mass x acceleration due to gravity.

And mass = density x volume

Therefore put those all together and you get:
Hydraulic Power = Joules per second = Watts = Density x Volume x acceleration due to gravity x distance.

And density of water = 1000 kg per cubic meter.

And volume of water = Q = cubic meters (m3) (use cubic meters, not liters per second because density of water is being measured in terms of cubic meters)

And acceleration due to gravity = 9.81 meters per second squared (ms2)

If you assemble all this together in the formula again

Hydraulic Power = Watts = 1000 (kgm3) x Q (m3) x 9.81 (ms2) x H (m)

Watts = 9810 QH
kilowatts = 9.81QH, where Q is cubic meters per second and H is meters of head.

Note: Head referred to above is the vertical distance between upper and lower water levels (and there are some rules for proper measurement of this distance). However, the turbine may not be installed at the current upper water level height. If it’s installed lower then the useful head is less. This can be referred to as the “Net Head” or “Useful Head”.

Therefore we have to recalculate the hydraulic power available to the turbine using useful head.

Net Hydraulic Power Available to Turbine= Useful Head (meters) x Flow (lps) x 9.81 = Watts

2Mechanical Power

Mechanical Power = Net Hydraulic Power (watts) x Turbine Efficiency (%)
Mechanical Power = Watts

3Electrical Power

Electrical Power = Mechanical Power (watts) x Generator Efficiency (%)
Electrical Power = Watts

Energy is the unit of power used over a period of time.

Energy = Power x period for which power is used i.e. kWh= power in kW x hours for which the power is used or produced.

i.e. 1 kWhs = 1 kw of power used or produced for 1 hour.