The picture shows a simple system of a floating weight like a pontoon or even an old ship.

As it rises and falls on the water it pulls on a line that turns the shaft of a generator. First instincts tell us that if we get a really big weight, say an old warship or ocean liner, then we should be able to generate a lot of power this way. The answer may be surprising.

Firstly we must explain how we are going to measure the power. Power is measured in watts, which is the rate of converting energy from one form to another. (See the article on thermodynamics). The power in watts is regularly quoted for kettles, about 2 thousand watts (2 kW), or a powerful light bulb, say 100 watts. Energy is measured in Joules and converting energy at the rate of one Joule every second gives us a power of 1 watt. (See the article on Energy and Work) One Joule of energy is the amount of energy needed to lift a weight of one Newton through one metre. A Newton is a unit of force that is equal to about one tenth of the weight of a Kilogram (to be more precise a mass of one Kilogram weighs 9.81 Newtons). See the article on Energy and Power for more details.

So if you raise a kilogram through a height of 1 metre in one second it would take about ten watts of power to do it, and you would convert ten Joules of energy. If you reverse the process then a weight of one Kilogram falling through a distance of one metre in one second could generate about ten watts of power. This process would generate 10 Joules of energy. So for each metre distance of tidal fall each Kilogram of weight in the pontoon represents about 10 Joules (1 metre x 10 Newtons) of potential energy.

If we assume that our pontoon weighs 100,000 Kg (100 tonnes), and calculating using the more precise ratio of Newtons to Kilograms of 9.81 then the potential energy would be about 1 million Joules (100,000 x 9.81 x 1 = 981,000). If it takes 1 hour for the water level to fall 1 metre this is 3,600 seconds and the average power output would be 981,000 Joules/ (1 x 3600 seconds) = 273 watts. This is approximately one tenth of the power of an electric kettle. If the mass of the boat were raised by a factor of 10 to 1000 tonnes then the falling boat would generate enough power to drive the electric kettle.

Obviously this is an oversimplified explanation and many other factors need to be taken into account such as the efficiency of the generator, but it can be seen that the average power output of such systems is very low for all the trouble and expense necessary to set it up.

The wave power devices are more useful because now the power in our pontoon could be released for each passing wave; say every 20 seconds for a 1 metre high wave. Now the pontoon generator can release its 1.0 Mega Joules of energy in 10 seconds and the power output is now 1,000,000/10 or 100,000 or 100 Kilowatts. However the inertia of very large masses means that they will only move a fraction of the wave height. Think of being on large boat in a rough sea. It may be uncomfortable but the boat is actually moving quite slowly compared to the waves and it is not moving very far vertically.

For many reasons wave power devices don't reach high levels of efficiency or power but it can be seen that it is the rate of change of the energy in the system that is the key factor.

In summary, the use of a large mass rising and falling with the tide does not generate significant amounts of power and much higher power outputs can be generated using similar equipment to extract power from sea waves or tidal flows.