Degree Days — a measure of how much, and for how long, the outdoor temperature deviates from a base (balance point) temperature. Each hour that the dry-bulb temperature is below the heating base contributes (base − Tdrybulb) ÷ 24 heating degree-days (HDD); each hour above the cooling base contributes (Tdrybulb − base) ÷ 24 cooling degree-days (CDD). Annual totals are used to estimate heating and cooling energy consumption.
Balance Point Temperature — the outdoor temperature at which a building neither needs heating nor cooling. It depends on internal heat gains (occupants, lighting, equipment), solar gains, and how well-insulated the envelope is. A lower balance point means the building retains heat well and needs heating only in colder conditions.
More efficient buildings may have lower heating bases (internal gains and solar gains keep the building warm longer without mechanical heating) depending on climate zone.
kW
%°° (180 = South)
Terminology
Capacity Factor = annual AC energy ÷ (DC nameplate × 8760 h) —
fraction of time the system runs at full rated output; 10–20 % is typical for fixed-tilt PV.
Performance Ratio = annual AC energy ÷ (annual POA irradiation × DC nameplate) —
how efficiently the system converts available sunlight into delivered energy
(accounts for temperature derating, inverter losses, and system losses); 0.75–0.85 is typical.
Calculation Method
The solar PV calculations in this tool follow the
PVWatts® Version 8
algorithm developed by the National Renewable Energy Laboratory (NREL). There will be differences between the results from this tool and PVWatts® due to differences in the underlying weather data and minor differences in the implementation, but the general trends and magnitudes should be similar.
The JavaScript implementation is ported from NREL’s open-source
SSC (SAM Simulation Core)
library.
Key sub-models:
Sky-diffuse irradiance —
Perez, R., Ineichen, P., Seals, R., Michalsky, J., & Stewart, R. (1990).
Modeling daylight availability and irradiance components from direct and global irradiance.
Solar Energy, 44(5), 271–289.
doi:10.1016/0038-092X(90)90055-H
Incidence-angle modifier (IAM) —
De Soto, W., Klein, S. A., & Beckman, W. A. (2006).
Improvement and validation of a model for photovoltaic array performance.
Solar Energy, 80(1), 78–88.
doi:10.1016/j.solener.2005.06.010
Cell temperature —
Steady-state NOCT model per
IEC 61215 / Fuentes, M. K. (1987).
A Simplified Thermal Model for Flat-Plate Photovoltaic Arrays.
SAND85-0330. Sandia National Laboratories.
[OSTI]
Inverter —
King, D. L., Gonzalez, S., Galbraith, G. M., & Boyson, W. E. (2007).
Performance Model for Grid-Connected Photovoltaic Inverters.
SAND2007-5036. Sandia National Laboratories.
[OSTI]
Case
Shading Reduce direct solar radiation
Wind Block Reduce ambient wind speed
Added Wind (Fans) Supplement airflow
Active Case for UTCI Variable
Baseline Case
No Shading
No Wind Block
No Added Wind
Ideal Case
Ideal Shading
Ideal Wind Block
High Fans
Outdoor Thermal Comfort — Strategy Implementation
The Universal Thermal Climate Index (UTCI) integrates dry-bulb temperature, mean radiant temperature (MRT), wind speed, and humidity into a single physiologically-equivalent temperature. Strategies are applied in order: (1) shading, (2) wind block, (3) fans. Variable strategies apply only in hours where they improve comfort.
Shading Strategies
No Shading: Baseline direct normal radiation (DNR) is used unchanged. MRT includes the full solar radiant load via the ASHRAE 55 ERF model.
Partial Shading (fixed): DNR × 0.5 every hour, regardless of comfort impact.
Partial Shading (variable): DNR × 0.5 only in hours where it improves comfort.
Full Shading (fixed): DNR = 0 every hour. Only diffuse radiation contributes to MRT.
Full Shading (variable): DNR = 0 only in hours where it improves comfort.
Ideal Shading: For each hour, the DNR level (none, ×0.5, or 0) that improves comfort is selected.
Wind Block Strategies
No Wind Block: Measured EPW wind speed is used as-is.
Partial Wind Block (fixed): Wind speed × 0.5 every hour.
Partial Wind Block (variable): Wind speed × 0.5 only in hours where it improves comfort.
Full Wind Block (fixed): Wind speed set to the UTCI minimum (0.5 m/s) every hour, simulating a complete windbreak.
Full Wind Block (variable): Wind speed set to 0.5 m/s only in hours where it improves comfort.
Ideal Wind Block: For each hour, the wind speed level (none, ×0.5, or 0.5 m/s minimum) that improves comfort is selected.
Added Wind (Fan) Strategies
Fans add airflow on top of the post-windblock wind speed. Fans are always only applied in hours where they improve comfort, so they help heat-stressed hours without worsening cold-stressed ones.
No Added Wind: No supplemental airflow.
Fans — Low (+0.25 m/s): Occupants may begin to notice the fan is on, but it is not enough to disturb loose paper or napkins. Applied only in hours where it improves comfort.
Fans — High (+0.8 m/s): Produces a noticeable draft and noise; loose paper and napkins will blow around. Above this level is not recommended without occupant control. Applied only in hours where it improves comfort.
Chart & Filters
Each bar shows the distribution of UTCI stress categories for the hours in the current crossfilter selection. Selecting a case as "Active Case for UTCI Variable" overrides the UTCI values in the left-sidebar value filter charts, enabling filtering by strategy-adjusted UTCI.
References
Bröde, P., Fiala, D., Błażejczyk, K., Holmér, I., Jendritzky, G., Kampmann, B., Tinz, B., & Havenith, G. (2012). Deriving the operational procedure for the Universal Thermal Climate Index (UTCI). International Journal of Biometeorology, 56, 481–494. doi:10.1007/s00484-011-0454-1
American Society of Heating, Refrigerating and Air-Conditioning Engineers. (2023). ANSI/ASHRAE Standard 55-2023: Thermal Environmental Conditions for Human Occupancy. ASHRAE. ASHRAE Standard 55
Chartered Institution of Building Services Engineers. (2015). CIBSE Guide A: Environmental Design (8th ed.). CIBSE. CIBSE Guide A
Liu, S., Lipczynska, A., Schiavon, S., & Arens, E. (2018). Detailed experimental investigation of air speed field induced by ceiling fans. Building and Environment, 142, 342–360. ISSN 0360-1323. doi:10.1016/j.buildenv.2018.06.037