The Kardasheve Scale classifies civilizations according to their total energy consumption.
The need for Reliable Supply
Energy is vital to power the technological civilization. Furthermore, while in the past humans needed energy "sporadically", to power various machines, to process a product, today we need an uninterupted supply of electricity just so that our society can function.
- In most developed countries blackouts stop normal activities. Elevators don't work, cities sink into darkness, shops close because they can't operate the checkouts and process credit card payments. Only the automobile transport works and those few islands of self-sufficiency that have their own generators, such as hospitals and some factories. But even these need fuel constantly replenished and an interuption of supply would stop them after a few days.
In the future we can expect that some system will be even more sensitive to power interruptions.
- Maglev trains,
- superstructures such as giant Skyscrapers,
- floating and flying cities,
- and, of course, the global computing and communications infrastructure.
These all may need energy supply that simply doesn't fail.
The energy systems of tomorrow will need to be more reliable and redundant.
- One approach to achieving this would be to continue to gradually improve all aspects of power generation and power transmission.
- Another approach is local generation. As technologies improve, local generation of electrical power becomes more economical. Small nuclear reactors could ensure decades of uninterrupted energy supply. Radioisotope thermal generators could provide power to small digital devices and vehicles, just like they provide power to spacecrafts today.
Current Energy Use
The world produces 15 trillion (1.5*10^13) watts of power a day as of 2007. 86% comes from fossil fuels. The Earth receives (1.0*10^16) watts of sunlight from the sun each day.
The current market share of CFL bulbs in the United States is about 6 percent, up from less than 1 percent before 2001. But that compares dismally with CFL adoption rates in other wealthy countries such as Japan (80 percent), Germany (50 percent) and the United Kingdom (20 percent). Australia has announced a phaseout of incandescent bulbs by 2009, and the Canadian province of Ontario decided to ban them by 2012. 
Transmission of Energy
Some of the important future technologies for energy transmission are
- hydrogen based engines,
- Fuel cells and
- Superconductive power lines.
- Wireless transmission of electrical power has been demonstrated with >90% efficiency (Richard M. Dickinson, James McSpadden, 1998, NASA Center for Space Power, NASA NPO-20321.
- In WiTricity project researchers transmitted 60 Wt of energy 2 meters with 40% efficiency.
Another problem is the depletion of energy resources. We may have already passed Hubbert peak and it's now obvious that certain types of fuel will soon end or will stop being economical.
Renewable energy sources will increasingly be used, including
However, all these sources combined (except solar) do not provide nearly sufficient capacity to fullfill all needs of our technological civilization. They will, of course, be successfully used, especially in more favourable areas, just like hydroelectric power is used today. According to Ray Kurzweil's book The Singularity is Near, only 0.3% of the nearly infinite energy of the Sun is needed to power today's buildings.
Non Renewable sources could include:
Most promising is Nuclear power, because it is safe, relatively cheap and the uranium reserves are very large. Another source is coal, which can be burned relatively cleanly with modern technologies.
Eventually thermonuclear fusion will become viable. The construction of ITER will soon commence. Fusion, in its various forms has vast potential which is much bigger than other currently available sources of energy.
Off-Earth Power Stations
Some schemes call for use of Helium-3 as fuel for fusion, which is extremely rare on Earth, but available on the Moon. It may become economically feasible to mine it there and use for energy generation (or transport to Earth).
In more remote future human civilization will find ways to directly tap into the energy of the Sun and other stars, use black holes for power generation, etc.
- Cost of energy will be higher
- Cities and accommodation designed to be more efficient
- Solar cells with efficiency > 30%
- Multi layer solar cells with efficiency > 50%
- Solar becomes competitive with grid electricity 
- Large area amorphous solar cells with efficiency > 20%
- Conversion/storage of solar energy as biochemical energy
- Common use of solar cells for residential power supply
- Space solar power stations
- Water decomposition by sunlight
- the ultra-reliable BetaBatt installed in satellites and space probes
- Distant Future of Humanity from the energy perspective
- Clean Energy Sources at Futures