# Adding A Window Seasonal Net Energy Balance Calculator to the PHPP

In a previous post here on the Accelerator (**link**) I showed how window net energy balance data can be linked to a DesignPH Sketchup model for easy visualizations. However, I skipped right over the step of calculating the actual net energy balance data, and so I’d like to go back and revisit that now.

*Warning: this is a post all about an excel spreadsheet. If thats not your thing … well … consider this your warning.*

**Background:**

Once windows and shading have been builtin in the **PHPP** model, a series of calculations are automatically executed using that information which contribute to the overall Heating and Cooling Annual Demand results, the Heating and Cooling Peak Loads, and the overall Primary Energy values.

Understanding how the windows in your project are performing is critical for both overall energy balance results, but also for assessing the risk of overheating. Therefor it can be very useful to review the overall seasonal energy balance for each window individually. PHPP will already give you an approximation of this net-energy balance for the heating season, but for assessing overheating risk and for a more nuanced picture of your building’s performance it can be helpful to see both winter and summer results in more detail. Since PHPP is Excel based it is quite easy to add additional calculators to it using any standard Excel techniques.

*Note: For the example shown here, I’ll be working in the ‘SI’ version of the PHPP, but the same process would apply for the ‘IP’ version as well.*

**Window Net Energy Balance**

The calculation we’ll execute here will closely mimic what the PHPP is doing in order to keep our results in line with what we’re seeing in the PHPP elsewhere. This calculation will be familiar to anyone who has gone through the Certified Passive House Designer / Consultant course: We’ll first calculate the total seasonal losses for each window individually, then calculate the total seasonal gains for each window. We’ll find the difference between the losses and the gains, and that will be our net energy balance.

And yes: if you’d like to see even more detailed information, you could certainly do this on a monthly basis if you prefer. For now though we’ll look at an overall seasonal value and we can always increase the level of detail later if we want.

**Window Seasonal Heat Loss:**

We’ll follow the PHPP’s pattern with regard to variable naming here. The PHPP shows us the total heat loss of any surface (like a window, for example) as:

Q‑T-srfc = A‑srfc * U‑srfc * f‑t * G‑t

where:

Q‑T-srfc = heat loss of the surface (kWh/winter)

A‑srfc = Area of the Surface (in m2)

U‑srfc = U‑Value of the Surface (W/m2‑K)

f‑t = Temperature Correction Factor (unitless)

G‑t = Seasonal Degree-Hours (k‑K-hrs/winter, like ‘Heating Degree Days’ except its expressed in ‘hours’)

so for each window in the PHPP, we want to harvest those critical four pieces of data (Area, U‑Value, exposure, and climate) and execute our simple heat loss calculation once for winter, then again for summer (the seasons will have a different G‑t value). I’m going to do this all in the ‘Windows’ worksheet, but of course you could also just create a brand new worksheet and do everything there which would maybe be a cleaner way to do it. For now, I’m going to go off to the right hand side of the ‘Windows’ worksheet, and add some new column headings for Heat Loss calculations: