PV size and roof directions

You enter PV as installed capacity in kWp, one entry per roof direction. EnergyFlow turns that into kWh by simulating your site’s weather, hour by hour. Capacity is an input. Annual energy is a result.

What kWp means

kWp, kilowatt-peak, is the power the manufacturer declares the array can produce under standard test conditions: 1,000 W of sunlight per square meter on the panels’ surface, at a panel temperature of 25 °C. It is the figure on a quote (“a 6 kWp array”), and the European Commission’s PVGIS manual defines it the same way. Roof area is not an input. Panel efficiency is already inside the kWp figure, and applying it again would count it twice.

What you enter

The equipment step shows your house with the roof from your house description (flat or pitched, and its rotation). Each of the four plan directions (north, east, south, west) can carry panels, with two numbers each:

• Installed capacity in kWp.
• Panel tilt, the angle from horizontal, from 0° (flat) to 90° (vertical).

On a pitched roof, flush-mounted panels take the roof pitch as their tilt. On a flat roof, you enter the mounting angle of the frame. A direction with no capacity has no panels.

The wizard also shows each direction’s real compass bearing, which is the plan direction plus your house’s rotation. A house turned 30° has its “east” face at 120°. The model calls this the azimuth: the compass direction the panels face, in degrees clockwise from north, so 90° is east, 180° south and 270° west. You never type it yourself. Each face accepts up to 50 kWp, and the total can reach 100 kWp. Three buttons set a starting layout: south, east and west, or no PV. You can edit the numbers afterwards.

Each saved run keeps its own roof layout, so you can change one face and compare the two runs.

How production is calculated

For every hour, the model takes the sunlight from the same weather file as the thermal simulation: the direct beam, the diffuse light scattered by the sky, and the total. Then, for each roof direction:

1. It computes the sun’s position for your latitude, longitude and altitude.
2. It works out how much light lands on the tilted panels. Direct sunlight is scaled by the angle between the sun and the panel: head-on counts in full, a glancing ray for less. Diffuse light uses the Perez-Driesse model, a standard way to spread sky light over a tilted surface, and the ground reflects 20 % of the light that hits it.
3. It estimates how hot the panels run, from the sunlight on them, the air temperature and the wind speed in the weather file. It uses the Faiman model, the one the European Commission’s PVGIS tool uses, with the PVGIS coefficients for panels mounted on a roof with a ventilated gap behind them.
4. It scales that to your capacity: DC power (direct current, what the panels make) equals installed Wp times the sunlight on the panels (in watts per square meter) divided by 1,000, times a temperature factor, times one minus the loss factor. The temperature factor takes off 0.29 % for every degree the panels run above 25 °C, the temperature coefficient of two current TOPCon panel datasheets, and adds a little on a cold, sunny day.
5. It multiplies by the inverter efficiency to get AC power, the alternating current your house uses.

The loss factor, or system losses, covers everything between the panel’s rating and the inverter: dirt on the panels, wiring, and a general allowance for shading. It is not a shading calculation. The default is 18 % per roof direction, and the wizard does not let you change it. The inverter efficiency is the share of the DC that comes out as AC; the inverter, the box that converts one into the other, loses some as heat. The default is 0.96, meaning 96 %, and the wizard does not let you change it either. Together the loss factor and the inverter efficiency remove a little over a fifth of the output at full test sunlight. The panel temperature comes on top. On the Luxembourg typical year, a south-facing roof array at 30° loses about 2 % of its annual output to it (1,105 against 1,083 kWh per kWp), most of it from June to August, while winter output goes slightly up.

The result is one AC production figure per hour for the year, with all faces added before it reaches the battery, the house and the grid.

Why east and west change the daily shape

Panels catch the most light when they face the sun squarely. In the morning the sun is in the east, at noon it is in the south, and in the evening it is in the west. A south-facing roof therefore peaks around noon and gives little at either end of the day. An east-facing roof peaks in the morning and a west-facing one in the afternoon. The model works out each plane separately, so a roof split between east and west gives two smaller curves that peak at different times instead of one tall one.

Added together for the same total capacity, the east and west pair usually gives a flatter day than a south array: less at noon, more in the early morning and the evening. That helps if your demand has morning and evening peaks, because more of the PV falls when the house is using power, and a battery has less midday surplus to absorb. Annual energy per kWp also changes with direction and tilt. EnergyFlow calculates it for your site, so run two layouts with the same total capacity to see the effect.

Limits of the model

• Every array is treated as roof-mounted with a ventilated gap, with a temperature coefficient (how much output falls as a panel heats up) of -0.29 % per degree. The wizard does not let you change either. Free-standing racks run cooler, and PERC panels, at around -0.35 % per degree, lose a little more in summer.
• There is no low-light behaviour, and the loss factor applies equally in every hour.
• There is no inverter clipping, the capping of output when the inverter is smaller than the array’s peak. EnergyFlow does not ask for an inverter size, so a large array on a small inverter is not capped.
• Shadows from trees, neighbouring buildings and chimneys are not calculated. Only the general loss allowance represents them.
• Weather arrives in hourly values, so short peaks from passing clouds are averaged out.
• Panel marks in the roof picture are symbolic. The picture does not check that your kWp fits on the roof.
• Snow cover and panel ageing over the years are not modelled.