To explain Site and Source Energy, it helps to first understand the two forms of energy:
- Primary energy is a raw fuel (e.g. oil, natural gas, coal). Primary energy provides a complete assessment of the energy requirement to operate your property.
- Secondary Energy is a product created from a raw fuel. For example, electricity (secondary) is a product that is often produced by burning coal or natural gas (primary); similarly, district steam (secondary) is a product that may be produced using a natural gas (primary) fired boiler. Secondary energy will almost always appear as more efficient because it doesn’t include the energy it took to create the secondary energy (e.g. burning the coal), or the energy to deliver the electricity to your property. These “losses” are always incurred when a primary energy is converted into a secondary energy and sent to your property.
Site Energy is the combination of primary and secondary energy that you buy directly for use at your building. It is calculated in Portfolio Manager by converting each of your fuels into kBtu, then adding them all together. Site Energy:
- Includes whatever you buy, in the form you buy it
- Buildings that have more secondary energy will look better than buildings with more primary energy. As a result, Site Energy is not a good metric to compare two different buildings.
Source Energy is your total primary energy consumption, which means your Site Energy plus all the delivery and production losses. To get the delivery and production losses, each fuel is multiplied by a “Site-to-Source” conversion factor. Source energy is the sum of the:
- primary energy you buy directly
- secondary energy you buy directly
- losses incurred when the primary energy was converted into the secondary energy
- losses incurred when both primary and secondary energy were delivered.
- Is your total primary energy consumption
- Accounts for the conversion from primary into secondary energy, whether this occurs at the utility or the property
- Is the EPA-Preferred metric to use (over Site Energy) because it is a more equitable assessment of the true energy needs of the building
- Is the metric used in the ENERGY STAR Score
- Is the best metric for an "apples-to-apples" comparison between properties
Refer to this FAQ about how Source Energy is more equitable if you compare buildings with different heating systems and therefore will also be more equitable if you try to assess the impact of switching fuels.