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:

1,000 kWh x 3.412 kBTU/kWh = 3,412 kBTU
Source Energy
In order to calculate the electricity portion of source energy we have to look a little deeper at how electricity is produced, which varies throughout the country and the world.  Today, the vast majority of electricity is produced by burning fossil fuels (including nuclear) which produces heat that is then converted through a turbine into electricity. Due to the laws of thermodynamics, the amount of heat needed to produce 1 kWh of electricity is higher than the amount of heat you can get out of 1 kWh of electricity.  How much higher is dependent on the fossil fuels and technologies being used as well as any transmission losses. If you are interested in going deeper, look at the Laws of Thermodynamics. If you just want to figure out how to calculate the source energy for electricity, then you are in luck because the EPA publishes summary data by power grid region, that includes the total heat input and total electricity output for the region. Therefore, you can look at the table (reproduced below) to get the numbers to use in your calculations.
 

Source Energy Calculation

  • Source Energy Factor = Heat Input / Net Generation
    from EPA’s eGrid for your region for 2005
NERC region acronym NERC region name NERC region annual heat input (MMBtu) NERC region annual net generation (MWh) kBTU to 1 kWh
ASCC Alaska Systems Coordinating Council 53,811,571.9 6,576,653.1 8.182212302
FRCC Florida Reliability Coordinating Council 1,673,508,351 207,884,617.6 8.050178847
HICC Hawaiian Islands Coordinating Council 123,159,469.2 11,522,803.2 10.68832532
MRO Midwest Reliability Organization 1,901,339,243 209,004,904.2 9.097103486
NPCC Northeast Power Coordinating Council 1,677,467,698 282,945,253.9 5.928594577
RFC Reliability First Corporation 7,217,406,640 997,522,914.7 7.235329167
SERC SERC Reliability Corporation 8,073,256,711 1,109,683,765 7.275276945
SPP Southwest Power Pool 2,056,674,383 214,392,155.4 9.593048678
TRE Texas Regional Entity 2,572,493,111 321,072,020.3 8.012199596
WECC Western Electricity Coordinating Council 4,232,044,098 695,836,845.6 6.081948843
    29,581,161,276 4,056,441,933 7.292391156

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).

1,677,467,697.8 MMBTU / 282,945,253.9 MWh
5.929 kBTU/kWh

pay attention to the units (MMBTU/MWh = kBTU/kWh)
 

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:

1,000 kWh x 5.929 kBTU/kWh = 5,929 kBTU

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:

 
50 gal x 138.69 kBTU/gal = 6,935 kBTU

Now the firewood conversion to thermal energy:

0.25 ton x 15,380 kBTU/ton = 3,845 kBTU



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

Energy Form
Site Energy (kBTU)
Source Energy (kBTU)
1,000 kWh Electricity
3,412
5,929
50 gallons Heating Oil
6,935
1/4 ton firewood
3,845
Total
14,192
16,709

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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Comments

  1. This is very great thing you have shared with us. Now I found enough resources by your tips about this issue, Thank you.

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