# U-values and the thermal balance model

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- How do I convert the coefficients, a and b, used in the Sandia thermal model to the U-values used in the PVsyst thermal balance model?

Regarding the thermal balance equation: U · (Tcell - Tamb) = Alpha · Ginc · (1 - Effic):

- Are U, Tamb, Alpha and Ginc the inputs of the above equation and Tcell and Effic are determined so as to satisfy the above equation?

- In a previous post in this forum it was stated that U has a high uncertainty. How is this uncertainty implemented in the above equation? i.e. what is the relationship between the U used in the above equation and the U-values defined by the user.

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• 1 month later...
Does anyone have any suggestions for this?
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- How do I convert the coefficients, a and b, used in the Sandia thermal model to the U-values used in the PVsyst thermal balance model?

One way could be to plot the Sandia model over the temperature range of interest and do a regression fit with the PVsyst model.

Regarding the thermal balance equation: U · (Tcell - Tamb) = Alpha · Ginc · (1 - Effic):

- Are U, Tamb, Alpha and Ginc the inputs of the above equation and Tcell and Effic are determined so as to satisfy the above equation?

For the purpose of the simualtion, Tcell is the unknown and all the others are given or estimated.

- In a previous post in this forum it was stated that U has a high uncertainty. How is this uncertainty implemented in the above equation? i.e. what is the relationship between the U used in the above equation and the U-values defined by the user.

The U in the above equation is one of the given or estimated parameters. You could try a few different values to see how the simulation is affected by this uncertainty.

I hope it helped!