Energy units

Energy consumption and output is measured in British Thermal Units regardless of energy source. BTUs are convenient because each energy is measured differently: LP gases are measured in gallons, natural gas is measured in cubic feet, electricity is measured in kWh, etc. The same goes for price (dollars per gallon, per thousand cubic feet, per kWh). With BTUs there is a common measuring standard for energy consumption and cost.

Using BTUs to calculate average consumption and expenditure

The US Energy Information Energy Agency’s 2009 Residential Energy Consumption Survey provides the following figures on national household annual averages:

Source: US EIA 2009 RECS

Using BTUs to work out energy costs

Consumption in BTU illustrates how popular each energy is; expenditure indicates how much an average household spends but not which is cheaper. But BTU can also help with the cost of one energy relative to another: US EIA data also enables a comparison between energy source prices per million BTU (2011 dollars, for 2012 residential consumption):

Source: US EIA

Propane appliances and BTU ratings

For propane, BTUs measure each propane appliance’s consumption and deliverable power. Whereas a 100,000 BTU/hr furnace will be half as powerful as a 200,000 BTU/hr furnace, the latter will consume twice as much propane. This is easily converted into gallons: one only needs to divide the number of BTUs by 91,574 (number of BTUs per gallon), so a 200,000 BTU/hr furnace will use around 2.2 gallons of propane each hour it is running (this seems like a lot and is only true if the appliance is running at full power all the time). If you know the price your propane company uses you can also calculate cost: at December 2011 national average propane prices ($2.856/gal) this comes to $6.28.

Propane appliances all have BTU ratings. This enables you to decide whether you need a powerful appliance (for example, a powerful heater for a large room) or one that uses fewer gallons of propane per hour (for example, air conditioning in a small space). BTU ratings are also used by propane companies to decide what kind of tank, pipes and regulators a propane installation requires to function properly.