Building Energy Performance Calculator
Estimate annual energy use, Energy Use Intensity (EUI), operating costs, and savings from efficiency upgrades for residential and commercial buildings.
Building Energy Inputs
Used to set typical operating hours and internal loads.
Includes occupancy, lighting and equipment use.
Results
Annual energy use
- Total site energy
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- Electricity use
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- Gas use
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- Energy Use Intensity (EUI)
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Annual cost & emissions
- Annual energy cost
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- Cost per ft²
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- Estimated CO₂ emissions
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CO₂ factors: 0.4 kg/kWh (electricity), 5.3 kg/therm (natural gas). Adjust locally as needed.
Quick upgrade scenario (20% efficiency improvement)
See the impact of a package of measures (better envelope, HVAC, and lighting) that reduces total energy use by 20%.
- New annual energy
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- New annual cost
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- Annual savings
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How this building energy performance calculator works
This tool provides a quick, engineering-based estimate of annual building energy use, Energy Use Intensity (EUI), operating costs, and potential savings from efficiency upgrades. It is designed for early design decisions, benchmarking, and retrofit screening – not to replace detailed energy modeling software such as EnergyPlus, eQUEST, or code-compliance tools.
Key concepts and formulas
At a high level, the calculator estimates annual energy use as:
Annual energy use ≈ Floor area × Base EUI × Adjustment factors
where:
- Base EUI depends on building type and climate.
- Adjustment factors account for operating hours, envelope quality, HVAC efficiency, and internal loads.
1. Base EUI by building type and climate
Typical Energy Use Intensity (EUI) values are derived from public benchmarking datasets (e.g., U.S. DOE, CBECS, EU building stock studies). For example, for temperate climates:
- Office: ~70–90 kBtu/ft²·year
- Retail: ~60–80 kBtu/ft²·year
- School: ~50–70 kBtu/ft²·year
- Hospital: 150+ kBtu/ft²·year
- Warehouse: 20–40 kBtu/ft²·year
- Residential: 30–50 kBtu/ft²·year
The calculator starts from a reference EUI for the selected building type and then scales it up or down based on climate (cooler climates increase heating demand; hotter climates increase cooling demand).
2. Operating hours adjustment
Internal loads (lighting, plug loads, occupants) scale roughly with operating hours. We apply a linear factor:
\( \text{Hours factor} = \dfrac{\text{Hours per week}}{\text{Reference hours per week}} \)
\( \text{Adjusted EUI} = \text{Base EUI} \times \text{Hours factor} \)
3. Envelope, HVAC and internal load efficiency
The three sliders represent multiplicative factors on the heating, cooling and internal load components of the EUI:
- Envelope quality affects heating and cooling loads (insulation, air tightness, glazing).
- HVAC efficiency affects how much energy is required to meet those loads (boilers, chillers, heat pumps).
- Lighting & equipment efficiency affects internal gains and electricity use.
A value of 1.0 represents a typical existing building. 0.5 represents a high-performance design (e.g., deep retrofit or near-passive building), while 1.5 represents a poorly performing building.
4. Converting to kWh, therms and EUI
Internally, the calculator works in kWh and then converts to therms and kBtu for reporting:
\( 1 \text{ kWh} = 3.412 \text{ kBtu} \)
\( 1 \text{ therm} \approx 29.3 \text{ kWh} \)
\( \text{EUI} = \dfrac{\text{Annual site energy (kBtu)}}{\text{Floor area (ft²)}} \)
For metric reporting, EUI in kWh/m²·year is computed directly from annual kWh and floor area in m².
5. Cost and CO₂ emissions
Annual energy cost is computed as:
\( \text{Cost} = E_{\text{elec}} \times P_{\text{elec}} + E_{\text{gas}} \times P_{\text{gas}} \)
where \( E_{\text{elec}} \) is electricity use (kWh), \( E_{\text{gas}} \) is gas use (therms), and \( P \) are the unit prices.
CO₂ emissions are estimated using default emission factors:
- Electricity: 0.4 kg CO₂ per kWh (typical grid mix; adjust for your region).
- Natural gas: 5.3 kg CO₂ per therm.
Interpreting your results
The calculator classifies your EUI qualitatively (very efficient, efficient, typical, inefficient, very inefficient) based on typical ranges for the selected building type. Use this to:
- Benchmark an existing building against typical stock.
- Test the impact of envelope and HVAC upgrades on energy and cost.
- Support early-stage design decisions before detailed modeling.
Limitations and when to use detailed modeling
This is a simplified model. It does not explicitly simulate hourly weather, thermal mass, ventilation strategies, or advanced controls. For:
- Code compliance (e.g., IECC, ASHRAE 90.1, national energy codes).
- Performance-based design and certification (LEED, BREEAM, Passive House).
- Utility incentive programs or legally binding energy performance contracts.
you should use full building energy modeling tools or work with a qualified energy modeler.
Practical ways to improve building energy performance
Common, cost-effective measures include:
- Upgrading to high-efficiency boilers, chillers, or heat pumps.
- Improving insulation, air sealing, and window performance.
- Converting to LED lighting and adding occupancy/daylight controls.
- Optimizing schedules and setpoints via building automation systems.
- Right-sizing equipment during major renovations or replacements.
Use the sliders to approximate the impact of these measures and the “upgrade scenario” box to see how a 20% energy reduction translates into annual cost and CO₂ savings.
FAQ
What is building energy performance?
Building energy performance describes how efficiently a building converts energy into useful services like heating, cooling, lighting, hot water and plug loads. It is usually measured as annual energy use per unit of floor area (EUI) and total energy cost.
What is a good EUI value?
“Good” depends on building type, climate and code vintage. As a rough guide in temperate climates:
- New, efficient offices: 40–60 kBtu/ft²·year.
- Typical existing offices: 70–90 kBtu/ft²·year.
- High-performance or deep-retrofit offices: < 40 kBtu/ft²·year.
Always compare against local benchmarks and regulations where available (e.g., national building energy codes, city benchmarking ordinances).
Can I model all-electric buildings?
Yes. To approximate an all-electric building, set the gas price but interpret the gas use as zero by assuming heating is provided by electric heat pumps. In a future version, a dedicated “fuel mix” control can explicitly allocate loads to electricity vs. gas.
How can I use this tool with energy codes and policies?
Many jurisdictions (e.g., NYC Local Law 97, EU EPBD, national energy codes) set performance targets in terms of EUI or CO₂ intensity. This calculator helps you quickly see whether a concept design or retrofit strategy is in the right ballpark before investing in full compliance modeling.