How can I best answer a question such as 'what factors most affect the supply of and demand for energy on a national scale?'

In order to best answer this question, we must first break it down. We are being asked to outline which factors affect both the supply and demand for energy before evaluating which of these we believe to be most significant. Furthermore, we have been given a scale which for this question is national. 
Let's break the question down into three parts: supply, demand and evaluation. 
For the first paragraph, you might explore how factors such as climate, resource endowment, technology and capital contribute to the supply of energy on a national level. It is essential that for each of these factors that we link the theory back to the practical and provide an appropriate case study to exemplify the importance of the factor in the real world. In order to take your answer to the next level, it is important to consider how factors may relate to one another as well as the limitations of each factor. In the second paragraph, you might adopt a very similar structure but instead this time discuss how factors such as level of development, energy policy, and world energy prices affect demand for energy on a national scale.
In the third paragraph, we must offer our own opinion on what we believe to be the most significant factors in the supply of and demand for energy. It is vital to consider the national scale at this point as we must show that we recognise that the factors may differ between countries based on a variety of reasons such as the climate, resource endowment or economic power. Here it is useful to use your case study examples to support your argument as well as considering how the importance of the factors you have chosen is linked to the contemporary global context. For example, it might be useful to focus on sustainability and energy policy given that climate change is at the forefront of the global attitude towards energy at the moment. 
Overall, the most important thing to remember is to clearly read the question and plan out your answer considering the four main parts of the question (supply, demand, scale, evaluation) before you start writing.