Comparing Site Energy vs Source Energy and Why You Should Care?

When we compare things like homes or cars to determine which are more efficient, better for the environment, greener, etc. it is important to make sure we are comparing apples to apples and using the right calculations.

There are two forms of energy use that are often discussed when talking about buildings energy consumption:

• Site Energy – the amount of energy brought into a site (e.g. your home)
• Source Energy – the total amount of energy used to produce and transport energy to a site 

The EPA explains Site vs Source Energy in their article Understanding Source and Site Energy.  Here’s my translation.

When talking overall energy use, we usually convert the energy forms we are discussing into heat in terms of kBTU (1,000 British Thermal Units or the amount of energy to heat 1,000 pounds of water 1°C).  This way we can add different forms of energy like gallons of gasoline and kilo-watt-hours (kWh) of electricity together for the total energy consumption.

Both site energy and source energy are useful depending on the situation. For forms of primary energy, such as natural gas, the site energy is for all practical purposes equal to the source energy.  However, for electricity, which is a type of secondary energy or energy carrier, there can be a huge difference between the amount of energy used at the site versus at the source to create it, making it important to know which to use for a given situation.  Below is how we calculate both the site and source energy values for electricity.

Site Energy
Using site energy consumption for electricity is far easier to calculate.  To calculate the site energy, you multiply the number of kWh by the thermal energy available from a single kWh (3.412 kBTU/kWh) to get the electricity portion of the site energy use. Said another way, you can get 3.412 kBTU of heat from 1 kWh of electricity.

For example, if a home consumes 1,000 kWh of electricity in a month, then here is the calculation for the electricity portion of site energy use in kBTU:

Source Energy Calculation

• Source Energy Factor = Heat Input / Net Generation
  from EPA’s eGrid for your region for 2005

Pay attention to the units (MMBTU/MWh = kBTU/kWh)
Note: all of these are significantly higher than the 3.412 kBTU you can get out of 1 kWh

 Here is an example of how I calculated it for New England (NPCC).

You can see that this is significantly higher than the 3.412 kBTU of heat you can get out of 1 kWh.  For more information on using the data from EPA’s eGrid see our earlier post on What Impact Does 1 Watt Have?

Using the same 1,000 kWh example from above, the electricity component of source energy use for this New England home is:

Total Energy Use

Electricity is usually just one of several forms of energy used at a location.  To get the total site or total source energy, we convert each form at energy to heat (kBTU) and add them together.  As site and source energy fossil fuels are essentially the same, we can use the following conversion factor in table below.

Let us return to that example of the New England Home that consumes 1,000 kWh of electricity in a month.  Let us also consider that it uses 100 gallons of heating oil in that same month and 500 lbs of dry white oak firewood, which is just under a quarter of a cord.

First the heating oil conversion to thermal energy:

Now the firewood conversion to thermal energy:

Now we can add all the energy sources together to calculate the total site and source energy for this home in that month.

You can see that the source energy is significantly higher than the site energy.  In the example we used of the New England home it was 18% higher.  In many homes, electricity is used for heating as well, so the difference would be even greater than this home which heats with oil and firewood.

Now you know how to calculate both the Site Energy and the Source Energy for a given location or building, which will be very useful as we calculate your Home MPG in our next post.  Another place the distinction between source and site energy is important is when talking about Zero-Net Energy buildings (or Net-Zero) as well as the discussion of electric cars.  As more of our nation’s electricity moves to cleaner renewable sources like solar and wind, the site energy versus source energy distinction will become less important.

Happy Greening!
Jon

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