To define alternative energy you need to start with energy. Energy is measured the same no matter where it comes from. Alternative energy just comes from a different source than conventional energy. Most energy except nuclear, tidal, and geothermal energy is some form of solar power. And there is a lot of solar energy.
The sun puts out more power to earth in about one hour than all the people on the planet use in one year. Far more solar energy falls on the earth in one year than all oil, gas, coal and uranium energy combined.
Conventional fuel sources include the fossil fuels: peat, coal, oil and natural gas. These sources store a lot of energy in relatively small and portable places like chunks of coal and barrels of oil. It took millions of years however to pack all that solar power into those nice portable packages. It burns up a lot faster.
Burning conventional energy sources provides most electric power today. The power is very convenient and useful. However, the byproducts of burning are building up and becoming harmful.
When you define alternative energy, realize that most sources are some form of solar power. This includes direct sunlight, wind, hydro, and wave power. Tidal and geothermal energy may be considered alternative, and renewable, but they are generated by the moon and earth's internal core heat respectively.
These energy sources are not as concentrated as the conventional sources and take up more space as a result. However, alternative energy sources simply change energy already present into useful electric power. There is no burning, and no byproducts of the energy conversion that are harmful to health. If there is an effect on the total solar energy balance, it has not yet been noticed or calculated that we are aware of.
Utilizing alternative energy sources provides only a fraction of total world power at this time. The chart below shows the comparisons to help define alternative energy by percentage of use.
However, the balance of large scale energy production is changing. In some countries, alternative energy sources are moving up. Countries like Germany, China, and the UK, for example, are investing heavily in renewable alternative sources.
The United States is also investing in wind and solar power. Brazil has for years been using large amounts of ethanol derived from sugar cane for transportation.
The top level of the chart below shows that worldwide, oil and the fossil fuels coal and natural gas, provide 85% of the energy used for power production. The middle level energy sources give an additional 13% of total worldwide power. That leaves 2% on the bottom row. This is where most of the alternative energy sources reside at the current time. This is changing as more and more alternative sources come online.