Our Sun pumps about 386 billion million gigawatts into space, mostly in the form of electromagnetic radiation. By comparison, a large nuclear reactor generates about 1 gigawatt, and global energy consumption is a few thousand gigawatts. This energy output is typical for a star in the same class as our Sun.
Prior to the discovery of nuclear power, scientists envisioned the Sun as a ball of combusting matter. Because the Sun is so large, it could have theoretically consisted of conventional matter in a state of combustion -- but this burning could only last for a few dozen thousand years before the matter would have been completely depleted. Today we know that the Sun is fueled by nuclear energy.
The Sun's total energy output is slowly declining as it fuses together the nuclei of light elements and is left with non-fusible, heavier matter. Eventually this power output will decline so fast that the explosive energy of fusion is outpaced by the attractive pull of gravity, and the star will collapse. The heat generated from the collapse will cause the diameter of our Sun to expand to the size of Mars' orbit.
Only a minuscule portion of the Sun's energy falls on our Earth, yet this energy is responsible for running almost every living thing on the planet. It is widely supposed that as our civilization progresses, it will begin to discard dirty fossil fuels in favor of the cleaner and ultimately more plentiful solar energy.
The energy output of the Sun is not entirely constant. Solar flares and sunspot activity cause small variations in the amount of light sent outwards. It has been speculated that a period of lowered sunspot activity in the 17th century, the Maunder minimum, may have been responsible for a period of declined heat in Europe called the Little Ice Age. It has also been theorized that the Sun's energy output has increased by about 40% since the formation of the solar system.