Problem The state of Connecticut gets 44 inches (1.1 meters) of precipitation annually.  The land area of Connecticut is 5000 square miles. If a dam were to be built around the state to a height of 200 feet (60 m) and used for hydropower, how much power would result?
Convert the area in square miles to square meters; multiply by the 1.1 meters to get the volume.  Multiply by 1000 kg per cubic meter to get the mass of water falling on Connecticut in one year; multiply by 9.8 to get the weight in newtons.  Multiply by 60 meters (the height of the dam) to get  the energy (in joules) released as the water goes through the turbines.  Divide by the number of seconds in one year (31.6 billion) to get the power in watts.  (Multiply by about the efficiency, 0.85, if you're so inclined.)
About 225 megawattsif you use the 85% efficiency, 265 megawatts if you assume 100% efficiency.)

Connecticut uses over 3000 MW on the average ( 5500 MW during the hottest days in the summer).  Damming the state enough to inundate the major cities could thus produce less than 10% of the electrical power needed in the state and less than 1% of the total power used

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