As I previously discussed, there are really only four types of energy: solar, nuclear, geothermal, and gravitational.
Some of our energy sources are actually the accumulated potential of these types of energy. For example, fossil fuels represent solar energy collected over a long period of time.
Let's say that by using fossil fuels, we can effectively put the power of 1 million years' worth of the sun's energy to work in supplying our day-to-day energy needs (probably a conservative estimate). Every day, maybe we use 10 months worth of collected fossil energy (the actual numbers are less important than my point).
Now, let's suppose fossil fuels run out.
Can the sun itself supply all of the world's energy needs in real-time? Is the amount of energy that we can collect and use per second greater than the total power consumption of the world per second?
If not, there's a lot of opportunity for scientists and corporations to improve devices that collect energy, and optimize devices that use energy.