The short answer
When you run Build Layout, PVX.AI tests every candidate position and places a rack only if it passes all of these checks: it sits fully inside the PV area, it does not overlap another rack, an exclusion area or a road with its setback, and the terrain under it is within the slope limits. An empty patch means one of those checks failed there. On sloped sites the slope limits are the usual reason; on flat sites it is usually setbacks, roads or corridors.
1. The ground is steeper than the layout’s slope limits
In Build Layout, with Use 3D Terrain (Slope Checking) on, three limits apply: N (%) for ground rising to the north, S (%) for ground rising to the south, and EW (%) for east-west slope in either direction. The dialog remembers the values you used last. PVX.AI measures the slope across each rack’s own footprint, from the terrain at its four corners, and skips a rack when the slope reaches or exceeds the limit. For trackers, the north-south slope is measured along the tracker’s length. Terrain-following trackers skip the north-south check, but the east-west limit still applies.
What to do: set the limits from the rack or tracker supplier’s specification. To see where the limits bite, turn on Show Directional Slope Map (EW/NS) on the Terrain Settings tab, and use Layout > Tools > Rack Slope Analysis to colour the racks that were placed. Note that Rack Slope Analysis works in degrees, while the layout limits are in percent. See Multi-Directional Terrain Analysis and Rack Slope Analysis.
2. You graded, but the graded ground still fails the layout limits
The thresholds in the Grading dialog (NS Threshold Slope (%) and EW Threshold Slope (%)) are separate from the layout’s N (%), S (%) and EW (%). Nothing copies one into the other. If you grade to the same value as the layout limit, ground that ends up exactly at the limit is still rejected, because a rack is skipped when the slope reaches the limit.
What to do: set the grading thresholds about 1% below the layout limits, grade, then run Build Layout again. See Terrain Grading. If you only need ground moved under some racks, see Why are my cut and fill volumes so high?
3. The terrain does not cover the whole PV area
If any corner of a rack falls on a spot with no terrain, such as a hole in the surface or ground outside the imported area, that rack is skipped. A strip without racks along one edge of the PV area often means the terrain stops short of the boundary.
What to do: import terrain that extends past the PV area boundary on every side. See Which coordinate system and grid resolution should I use?
4. Boundary roads, setbacks and corridors take up the space
- Boundary Road in Build Layout is on by default. Its Width and Setback shrink the PV area inward before any rack is placed.
- Corridor, when enabled, inserts roads or gaps between rows according to the pattern set with Edit corridor…, each with its own setback.
- Every manual road keeps racks back by its own rack setback, set when the road is drawn or converted (3 m unless you change it). Change it with Road > Road Setback, then build the layout again.
- A rack that overlaps an exclusion area by any amount is skipped. Exclusions have no extra buffer, so check that the exclusion outline is not larger than intended.
What to do: when corridors are on, read the Rack-to-rack gap line in Build Layout: it adds the road width and both setbacks, so you can see how much space each corridor takes. See Fixed Distance PV Layout, Defining Roads, Corridor Pattern Editor and Defining Exclusion Areas.
5. Only long trackers were selected, so short gaps stay empty
You can select several racks of the same type for one layout. At each position PVX.AI tries the largest rack first and falls back to smaller ones, so a shorter tracker can fill a spot where the longest one does not fit or fails the slope check. PVX.AI has no separate minimum tracker length setting: a short tracker, for example a one-string tracker, is placed only if a short rack spec is part of the selection.
What to do: create the shorter tracker lengths your supplier offers in Database > Products > Racks, select them together with the long tracker, and build again. Check that the short spec’s dimensions and pile layout are complete. Mixing fixed racks with trackers, or East-West with standard fixed racks, in one selection is not allowed. See Creating Trackers (Detailed Design).
6. The whole PV area came back empty
If no rack fits at all, the AutoCAD command line shows a line such as 0 racks placed, every candidate was rejected (boundary/exclusions/roads/slope). PVX.AI does not report which check rejected each position, so work through the sections above: first turn off Use 3D Terrain (Slope Checking) and build once. If racks now appear, slope is the cause. If they still do not, look at the boundary, setbacks, roads and exclusions.
How do I reach a target capacity, for example exactly 50 MW?
Use Layout > Tools > Capacity Iteration. Choose a Spacing Mode (Row Spacing (m), GCR (ratio) or Shading Angle (°)), set the range and tilt angles to try, tick Capacity Target and enter the DC target in kWp. PVX.AI predicts every combination with the same setbacks, exclusions, roads, corridors and slope limits as Build Layout, and sorts them by the Delta (kWp) from your target. Pick the closest row and click Generate Selected Layout. It finds the closest layout in the range you gave, not an exact match, so remove or add a few racks by hand if you need the exact figure.
The AC capacity and DC/AC ratio follow from the electrical configuration: with string inverters, the strings per inverter set the ratio. See Layout Using Capacity Iteration and Defining DC Boxes, Inverters and Transformers.
More on layout options is on the solar layout page on pvx.ai.