Renewable energy is energy that does not use up resources such as coal, oil and gas.

It is hard to tell which is the most important form of renewable energy, but if you said falling water, you would not be correct. Sunlight is by far the greatest source of energy. Wind is another source of natural energy, but not all that easy to harness. The energy in ocean waves and currents is also enormous. So out of all of these, falling water probably comes last. However, it is the easiest to harness and the most flexible to use at the moment.

To have a useful source of energy it has to be available at the time you want it and in the quantities you want it. That has been the advantage of oil and coal. You want more energy, to put on a shovel more coal, or open an oil jet a bit more. When you don't want the energy, you stop shovelling or close the tap. Simple. Because you also have stocks of oil and coal you can plan ahead. Those are all the advantages of energy using fossil fuels, and harnessing energy from them is simple - which is why they have been used.

Harnessing energy from sun, wind, waves and falling water is much more difficult, but of these, falling water is the easiest. But suitable sites do not exist all over the world, whereas it is possible to move coal and oil or natural gas about.

The problem with renewables (except nuclear, which has such a long life it might as well be a renewable) is that they are not very concentrated forms of energy and they vary with the time of year and many other things. They cannot be turned on and off. Most produce electricity, which cannot be stored except in batteries.

That is why what you see at the moment is a mix of energy. You get the most out of sunshine, wind and water when you can, and keep fossil fuels in reserve, so they can be used to bolster the supply when you need it.