By Robert Wilson

If a coal power plant is closed and replaced by a wind farm and that wind farm produces the same amount of electricity as the coal plant, what happens to energy consumption? Depending on the coal power plant, and how you define energy consumption it could do anything from stay the same to falling by a factor of three. Welcome to the perplexing world of measuring energy consumption in an increasingly renewables world.

In popular discussion the phase "energy consumption" is used with little regard for its meaning. Yet, in many respects it is a problematic term. Take the European Union. It has a target of getting 20% of its energy consumption from renewable energy by 2020. Let me put that more accurately. It has a target of getting 20% of its final energy consumption from renewable energy. The word final is key, yet you will almost never see it appear in discussions about renewable energy targets.

There are two ways of measuring total energy consumption: primary and final energy consumption. Primary energy consumption is a measure of the energy content of all the oil, coal, natural gas etc that is taken out of the ground. Essentially we ask how much energy is released when we burn the stuff. Typically this is reported in tonnes of oil equivalent, that is how many tonnes of oil would release the same amount of energy if burned.

Final energy consumption is slightly different. It is the energy delivered to the final consumer. For example it only measures the electricity that is produced by a power plant, it does not care about the heat produced from burning coal or natural gas that was not converted to electricity. So, for a 50% efficient power plant the primary energy consumption is two times higher than the final energy consumption.

This sounds simple enough. But here is another problem. How do we measure primary energy consumption for renewable sources such as wind, solar, and hydro-electricity? In the case of coal we can ask how much energy is released when we burn the stuff. Obviously we do not burn anything for wind, solar and hydro. So, what do we do? Here we have two choices. We can use the energy content of the electricity generated as the primary energy. This is called the "physical energy content" method, and is used by groups such as the International Energy Agency. The second choice is to ask how much fossil fuel energy would have been required to produce the same amount of electricity. This is called the partial substitution method, and is used by BP in their often cited Statistical Review of World Energy. In the case of BP they add up all of the wind and solar electricity generated and convert it to primary energy assuming that it would have been burned in a 38% efficient fossil fuel power plants, that is BP say primary energy consumption from wind and solar is more than two times higher than the IEA does.

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