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To convert building site energy to source energy, convert each fuel’s metered quantity into a common energy unit, multiply it by the source-site ratio for that fuel and reporting program, then add the results. There is no single universal ratio: for example, the U.S. ENERGY STAR Portfolio Manager reference dated August 2023 uses 2.80 for grid electricity, while U.S. Department of Energy federal reporting guidance for FY 2025 lists 3.14. This guide focuses on U.S. building energy calculations and explains how to choose and apply the right method.
What is the difference between site energy and source energy?
Site energy is the energy delivered to and used at a building, as recorded in utility bills or meters. Source energy accounts for energy used or lost upstream to produce, convert, transmit, and deliver that energy. It is calculated from site consumption; it is not a separate meter reading.
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The distinction matters because a unit of electricity delivered to a building does not represent the same upstream energy input as a unit of fuel burned onsite. The U.S. Environmental Protection Agency’s explanation of source and site energy describes why source energy includes those upstream effects. ENERGY STAR calls the factors used to restate site consumption as source consumption “source-site ratios.” Its Source Energy Technical Reference says source energy is “the most equitable unit of evaluation” for assessing energy efficiency.
In a construction or building-operations context, the calculation can help compare a building’s energy use across fuel types or benchmark it under a specified program. It does not change the underlying bills or show that the building used more energy onsite; it expresses the measured use on a different accounting basis.
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Source energy is particularly useful when a building uses multiple energy sources. A site total alone can combine quantities that have different upstream production and delivery impacts. Applying fuel-specific factors puts those measured inputs onto the reporting program’s source-energy basis. It does not, by itself, explain why a building uses energy, identify equipment problems, or replace a program’s other performance metrics.
How do you convert site energy to source energy?
Use the formula separately for each fuel, then sum the converted values:
Source energy for a fuel = site energy for that fuel × its source-site ratio
If bill quantities are not already in an energy unit, first convert them to a common unit such as Btu. The native-unit conversion and the source-site ratio are two different steps: for example, Btu per kilowatt-hour converts electricity from kWh to site Btu; the electricity source-site ratio then converts site Btu to source Btu.
- Choose the governing method. Identify the country, program, and reporting year or factor-table vintage. Do not mix a factor from one program’s table with another program’s instructions without a clear reason.
- Separate consumption by fuel and source. Keep grid electricity, onsite renewable generation used by the building, natural gas, district energy, and other fuels distinct where the method differentiates them.
- Convert each bill quantity to a common energy unit. Use the applicable utility or program conversion value. Preserve the units in your calculation notes.
- Apply the matching ratio to each fuel. Multiply each site-energy subtotal by the ratio for that energy type and framework.
- Add source-energy subtotals. Only add values after they have been converted to the same energy unit, such as Btu or kBtu.
- Record the method and factors. Note the source of the table, geography, vintage, units, and treatment of onsite generation and exports so another person can reproduce the result.
For a compact calculation schedule, create one row for each fuel or separately reported energy stream. Record the bill quantity, native unit, conversion to site energy, site-energy subtotal, applicable source-site ratio, and resulting source-energy subtotal. Keep enough precision during the calculation to avoid rounding each line too early; round the displayed totals only after summing. The ratio is a multiplier, not a percentage to add to the bill quantity.
Which source-site ratio should you use?
Use the factor table required by the benchmarking or reporting program for the building. The values below demonstrate why it is important to name the framework: U.S. Portfolio Manager factors in the August 2023 technical reference and the DOE FY 2025 federal reporting option differ for grid electricity and chilled water.
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| Energy type | U.S. ENERGY STAR Portfolio Manager ratio (August 2023) | DOE federal reporting guidance, FY 2025 |
|---|---|---|
| Grid-purchased electricity | 2.80 | 3.14 |
| Onsite solar or wind electricity | 1.00 | 1.00 |
| Natural gas | 1.05 | 1.05 |
| Fuel oil and diesel | 1.01 | 1.01 |
| Propane or liquid propane | 1.01 | 1.01 |
| Steam | 1.20 | 1.20 |
| District hot water | 1.20 | 1.20 |
| District chilled water | 0.91 | 1.00 |
| Wood | 1.00 | Not listed in the cited federal table |
| Coal or coke | 1.00 | Not listed in the cited federal table |
| Other | 1.00 | Not listed in the cited federal table |
The Portfolio Manager values are from EPA / ENERGY STAR’s August 2023 Source Energy Technical Reference. Its U.S. table is not a universal set of constants: for example, it lists Canadian grid electricity at 1.83 and Canadian natural gas at 1.06. Use the geography that applies rather than transferring U.S. factors to a building in another country.
The DOE values are from its FY 2025 federal annual energy reporting guidance, which identifies EPA ENERGY STAR calculator ratios as a reporting option. Its grid electricity factor of 3.14 is not interchangeable with the 2.80 grid factor in the Portfolio Manager technical reference. For compliance or formal benchmarking, consult the current instructions for the specific program and reporting period instead of selecting whichever factor seems preferable.
When comparing factor tables, check more than the headline electricity number. Confirm the jurisdiction (for example, U.S. or Canada), purpose (Portfolio Manager benchmarking or federal annual reporting), energy type (grid electricity, onsite renewable electricity, gas, or district energy), vintage of the table, and generation accounting rules. Also check whether the number you have found is a source-site ratio or a conversion from a native meter unit to site Btu. Similar-looking figures can describe different steps.
A ratio of 1.00 does not mean the energy stream should be omitted. It means that, under that particular table, the source-energy value equals the site-energy value for that category. Likewise, the chilled-water values in the table show why a factor must be matched to the reporting framework: 0.91 in the cited Portfolio Manager reference and 1.00 in the cited DOE FY 2025 table produce different source subtotals from the same site input.
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How do you convert bill units before applying a ratio?
Convert each utility’s native unit to site Btu before multiplying by a source-site ratio. DOE’s FY 2025 federal guidance provides these examples of native-unit-to-site-energy conversions:
| Bill or meter unit | Conversion to site Btu | Source |
|---|---|---|
| Electricity | 3,412 Btu/kWh | DOE, FY 2025 |
| Natural gas | 1,026 Btu/cubic foot | DOE, FY 2025 |
| Distillate No. 2 fuel oil | 138,000 Btu/gallon | DOE, FY 2025 |
| Propane | 92,000 Btu/gallon | DOE, FY 2025 |
| Steam | 1,000 Btu/pound | DOE, FY 2025 |
These conversions are taken from the DOE’s FY 2025 reporting guidance. They are not source-site ratios, and they should not be assumed to be the required conversion for every utility bill or program. If the bill already reports energy in Btu or another energy unit, check whether it is site energy and confirm the unit before applying the ratio.
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For example, using the DOE FY 2025 values, 5,000 kWh × 3,412 Btu/kWh gives 17,060,000 site Btu. The units cancel correctly: kWh in the bill quantity is multiplied by Btu per kWh, leaving Btu. Applying a source-site ratio of 2.80 then gives source Btu. Keep this unit check in each line of the calculation, especially when working with spreadsheets or combining utility records.
Do not treat 3,412 Btu/kWh or 1,026 Btu/cubic foot as the source multiplier. Those values only translate a meter or billing quantity into site energy. The ratio is applied afterward. Similarly, if a bill uses therms, gallons, pounds, or another unit not shown in a table, obtain the applicable native-unit conversion from the governing program or reliable bill information; do not substitute an unrelated conversion or guess a value.
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Suppose a building reports 5,000 kWh of grid electricity and 10,000 cubic feet of natural gas. Using DOE’s FY 2025 native-unit conversions and the U.S. ENERGY STAR Portfolio Manager ratios from August 2023, the calculation is:
| Fuel | Convert bill quantity to site Btu | Apply Portfolio Manager ratio | Source Btu |
|---|---|---|---|
| Grid electricity | 5,000 kWh × 3,412 = 17,060,000 Btu | 17,060,000 × 2.80 | 47,768,000 |
| Natural gas | 10,000 cubic feet × 1,026 = 10,260,000 Btu | 10,260,000 × 1.05 | 10,773,000 |
| Total | 58,541,000 Btu |
The arithmetic is illustrative, not a substitute for a particular program’s reporting instructions. If the same electricity quantity were converted using the DOE FY 2025 grid ratio of 3.14, its source electricity would be 53,568,400 Btu instead of 47,768,000 Btu; the gas calculation would remain 10,773,000 Btu under the listed factors. The total would therefore differ because the grid-electricity ratio changed, not because the meter reading changed.
For readability, results can be reported in kBtu by dividing Btu by 1,000. In the example, 58,541,000 Btu equals 58,541 kBtu. Keep the unit label with every subtotal and total; a number without a unit is not a reproducible energy result.
The example deliberately keeps the framework visible: the site-Btu conversions come from DOE’s FY 2025 guidance, while the first source calculation uses the Portfolio Manager ratios dated August 2023. When the FY 2025 federal electricity ratio is substituted, that substitution is called out rather than silently blended into the first total. For an actual submission, follow the reporting instructions as a set and use the matching conversions and ratios required there.
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How should onsite solar, wind, and exported electricity be handled?
Keep grid purchases separate from onsite renewable generation and use when the selected method distinguishes them. The cited U.S. Portfolio Manager reference assigns onsite solar or wind a ratio of 1.00, while assigning grid-purchased electricity 2.80; the DOE FY 2025 table likewise lists separate entries for onsite renewable electricity and grid purchases.
Do not automatically count all onsite generation as building consumption. Record the quantities the way the applicable reporting method instructs, distinguishing generation used onsite from electricity exported to the grid when required. ENERGY STAR Portfolio Manager help lists separate annual energy metrics for grid electricity, onsite renewable generation and use, natural gas, district steam, district hot water, district chilled water, and other fuel types; see the Portfolio Manager Help for its available meter types and metrics. Export accounting can affect the reported totals, so follow the program’s rules rather than treating exported generation as energy consumed by the building.
For a building with solar, keep the utility’s grid-purchase quantity and the onsite system’s generation/use information as separate records rather than netting them together by default. Whether a reported onsite renewable amount represents generation, consumption, or another defined metric depends on the program’s meter setup and accounting rules. If electricity is exported, do not add it to building consumption simply because it was generated onsite. Preserve the underlying quantities so the applicable method can be applied transparently.
What common calculation mistakes should you avoid?
- Applying the electricity ratio to every fuel. Ratios are fuel-specific. Use the gas ratio for gas, the relevant district-energy factor for district energy, and so on.
- Confusing a unit conversion with a source-site ratio. Btu/kWh or Btu/cubic foot converts a native bill unit to site Btu. It does not by itself account for upstream energy use.
- Adding unlike units. Do not add kWh to cubic feet, gallons, pounds, or therms. Convert the quantities into a common energy unit before summing.
- Mixing factor tables. A federal reporting factor and a Portfolio Manager factor may serve different frameworks. Identify the table and purpose before calculating.
- Using U.S. factors for a building elsewhere. ENERGY STAR’s reference includes separate Canadian ratios; other jurisdictions may have their own requirements.
- Counting exports as building consumption without checking. Separate grid purchases, onsite generation used, and exports as the applicable method requires.
- Calling the result a meter reading. Source energy is a calculated value based on metered site quantities and selected factors.
- Omitting the factor date or geography. Without the table, location, and vintage, another reader cannot tell why a result differs or reproduce it.
Another frequent pitfall is comparing two reported totals without checking their boundaries. A total using grid purchases alone is not necessarily comparable to a total that also records onsite renewable generation under a separate metric. Similarly, a result using a Canadian factor table cannot be compared as if it used the U.S. Portfolio Manager table. First align the reporting period, energy categories, units, geography, and factor method; only then interpret the difference.
How can you make the result reproducible?
For a project record, benchmarking submission, or energy report, preserve a short calculation schedule with one row per fuel. Include the billing period, meter quantity and unit, site-energy conversion factor and source, calculated site Btu, source-site ratio and table, source Btu, and any treatment of onsite generation or exports. State the program and geography in the report narrative, and give the factor table’s date or reporting year.
Keep separate values for measured site consumption and calculated source energy. DOE’s Energy Intensity Indicators terminology distinguishes delivered/site, source, and adjusted source energy. If a program asks for an adjusted measure or another metric, do not substitute ordinary source energy without confirming the definition and instructions.
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A useful record also notes whether a value came directly from a utility bill, a meter, or a program calculation; identifies any unit conversions; and explains which data streams were kept separate. Save the factor table name and vintage with the worksheet, not just the final total. This makes it possible to explain a later difference caused by a revised factor, a different framework, or a corrected meter quantity without mistaking it for a change in the building’s actual use.
How we picked the calculation method
This guide presents the U.S. ENERGY STAR Portfolio Manager method as the primary worked example because the question concerns converting building site energy to source energy and the official reference provides fuel-specific ratios. It also shows DOE FY 2025 federal reporting values where they differ, so readers can see why framework, geography, fuel, and factor vintage must be identified. The factors shown are attributed to their official references rather than presented as universal values.
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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteENERGY STAR’s Source Energy Technical Reference is dated August 2023. DOE’s cited annual reporting guidance is for FY 2025 and explicitly provides native-unit conversions alongside its federal reporting factors. These dates identify the editions used for the examples; they do not guarantee that an edition is the right one for every later reporting cycle. For a formal benchmark or compliance calculation, check the current directions issued by the relevant program.
Frequently Asked Questions
Why is source energy higher than site energy for grid electricity?
The ratio accounts for upstream energy used in generating electricity and delivering it to the building, not just the electricity recorded at the meter. The exact multiplier depends on the program and factor table, so a larger source figure does not mean the building’s meter recorded additional consumption.
Can I use the same source-site ratio every year?
Do not assume a factor remains valid indefinitely. Check the applicable program’s current guidance and retain the factor table’s publication or reporting vintage with your calculation, especially for formal benchmarking or compliance reporting.
What is adjusted source energy?
Adjusted source energy is a distinct metric, not simply another name for source energy. DOE’s terminology distinguishes it from delivered/site and source energy; use the definition and calculation required by the reporting program if it asks for an adjusted measure.
Can I calculate source energy from a utility bill that reports therms?
Yes, provided you have the applicable conversion from therms to site energy and the correct natural-gas source-site ratio for your reporting method. The DOE FY 2025 conversion examples cited here give cubic feet, not therms, so use the governing guidance or bill-specific information for the therm conversion rather than assuming an unstated value.
What is the difference between a Btu conversion and a source-site ratio?
A native-unit conversion, such as 3,412 Btu/kWh in DOE’s FY 2025 guidance, converts the billed quantity into site Btu. The source-site ratio is applied after that step to restate site energy as source energy. They are separate factors with different purposes.
The Bottom Line
Bottom line: Convert each fuel’s measured site use to a common energy unit, apply the matching source-site ratio for the correct geography and reporting framework, then sum the source-energy subtotals. Record the factor table and vintage, and keep grid electricity, onsite renewables, and exports distinct where the applicable method requires it.
