Determine the amount of Btu's per volume of each substance that is used in your particular application. This technical information is published and readily available on the Internet (for example, http://www.engineeringtoolbox.com/heating-values-fuel-gases-d_823.html), and can be found by searching the technical data for each specific energy source. Say you heat your home by natural gas and by wood, and you use 10,000 cubic feet of natural gas and 3 cords of wood. One cubic foot of natural gas contains 1,000 Btu of energy, while 1 cord of wood contains about 20,000,000 Btu. Which would mean that this home would use 10 million Btu's of natural gas, and 60 million Btu's of wood.
Add all the Btu of each energy source together. In the example given, your total energy expenditure is 70 million Btu.
Divide the energy source you wish to know the percentage of by the total. In the example above, if you wanted to know the total percentage of energy used in the form of natural gas, you would divide 10 million by 70 million. Use the calculator to divide the numbers. Your result should be 0.1428.
Multiply this number by 100. The digits on the left side of the decimal point will be the whole percentage points of this energy source used. In the above example, this would tell us that natural gas makes up 14 percent of the total energy used to heat the home.