There are many forms of energy. Light, sound, movement, heat, chemical energy and electrical energy are some of the most common.

Solar power is likely to become the world's largest source of electricity by 2050 as oil and other fossil fuel sources are phased out.

Solar energy is becoming more important because solar panels are becoming more efficient, cheaper to make and easier to install in fields or in roofs.

What is set to happen is that many roofs will be used to collect energy, as these roofs are a large part of many cities, and otherwise do nothing more than keep the rain out.

In fact, finding places to put up solar panels will become a major challenge, as it is not good to cover farmland with panels if it can be avoided.

Solar energy is limited by sunlight, so it cannot be generated at night and little is generated when it is cloudy or the Sun is low in the sky. That is why some kind of battery will need to be developed to complement the solar panels, storing any daytime surplus to be used at night or when it is cloudy.