> ## Documentation Index
> Fetch the complete documentation index at: https://docs.plantpredict.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Heat Balance Model

export const POAIrradiance = () => <Tooltip tip="Solar irradiance on the tilted module surface (beam + diffuse + reflected); drives PV power generation.">
    POA irradiance
  </Tooltip>;

export const STC = () => <Tooltip tip="Standard Test Conditions: 1000 W/m² irradiance, 25 °C cell temperature, AM1.5G spectrum; the reference conditions for rating PV modules.">
    STC
  </Tooltip>;

## Summary

The Heat Balance model is an extended form of the Faiman (2008) model that calculates cell temperature from the energy balance between absorbed solar radiation and heat loss to the environment. It uses experimentally determined conductive and convective heat transfer coefficients to characterize thermal dissipation, and accounts for the fractions of irradiance converted to electricity (via module efficiency at <STC />) and reflected off the surface of the module (via the absorption coefficient).

## Inputs

| Name                                      | Symbol              | Units          | Description                                                                                                                                   |
| ----------------------------------------- | ------------------- | -------------- | --------------------------------------------------------------------------------------------------------------------------------------------- |
| **Effective Front POA Irradiance**        | $G_{POA,front,eff}$ | W/m²           | Front-side POA irradiance after optical corrections, before DC system losses (from [irradiance calculation](/models/poa-irradiance/overview)) |
| **Ambient Air Temperature**               | $T_a$               | °C             | Ambient air temperature                                                                                                                       |
| **Wind Speed**                            | $v_w$               | m/s            | Wind velocity                                                                                                                                 |
| **Absorption Coefficient**                | $\alpha_T$          | —              | Fraction of incident irradiance absorbed by the module                                                                                        |
| **Module STC Efficiency**                 | $\eta_{STC}$        | —              | Module efficiency at standard test conditions                                                                                                 |
| **Conductive Heat Transfer Coefficient**  | $u_c$               | W/m²·K         | Conductive heat transfer coefficient                                                                                                          |
| **Convective Heat Transfer Coefficient**  | $u_v$               | W/m²·K per m/s | Convective heat transfer coefficient                                                                                                          |
| **Cell-to-Module Temperature Difference** | $\Delta T_{c-m}$    | °C             | Temperature difference between cell and module surface                                                                                        |
| **Reference Irradiance**                  | $G_{ref}$           | W/m²           | Reference irradiance (typically 1000 W/m²)                                                                                                    |

***

## Outputs

| Name                           | Symbol | Units | Description                        |
| ------------------------------ | ------ | ----- | ---------------------------------- |
| **Cell Temperature**           | $T_c$  | °C    | Operating temperature of PV cells  |
| **Module Surface Temperature** | $T_m$  | °C    | Back-of-module surface temperature |

***

## Detailed Description

The Heat Balance model calculates cell temperature from heat generation and dissipation:

$$
T_c = T_a + \frac{\alpha_T \cdot G_{POA,front,eff} \cdot (1 - \eta_{STC})}{u_c + u_v \cdot v_w}
$$

where:

* $T_a$ is the ambient air temperature in °C (from weather data)
* $\alpha_T$ is the absorption coefficient (from module definition, fraction of irradiance not reflected or transmitted, typically 0.9)
* $G_{POA,front,eff}$ is the front-side <POAIrradiance /> after optical corrections, before DC losses, in W/m² (from [irradiance calculation](/models/poa-irradiance/overview))
* $\eta_{STC}$ is the module efficiency at STC (from module definition)
* $u_c$ is the conductive heat transfer coefficient in W/m²·K (from DC field definition)
* $u_v$ is the convective heat transfer coefficient in W/m²·K per m/s (from DC field definition)
* $v_w$ is the wind speed in m/s (from weather data)

The thermal behavior is characterized by a thermal loss factor $u = u_c + u_v \cdot v_w$, split into a constant conductive component $u_c$ and a wind-proportional convective component $u_v$. These coefficients depend on many factors (e.g., mounting configuration, module technology, local climate) and are in practice experimentally determined. In alignment with PVsyst, $u_v$ default value is zero, effectively eliminating the convective contribution and reducing the model to a constant thermal loss factor. Users should input a non-zero value if they want wind velocity to affect cell temperature.

### Module Surface Temperature

$$
T_m = T_c - \frac{G_{POA,front,eff}}{G_{ref}} \cdot \Delta T_{c-m}
$$

where:

* $G_{ref}$ is the reference irradiance in W/m² (from module definition, typically 1000 W/m²)
* $\Delta T_{c-m}$ is the cell-to-module temperature difference at the reference irradiance, in °C (from DC field definition)

***

## References

* Faiman, D. (2008). *Assessing the outdoor operating temperature of photovoltaic modules.* Progress in Photovoltaics: Research and Applications, 16(4), 307–315. DOI: [10.1002/pip.813](https://doi.org/10.1002/pip.813)