Design decisions
Tilt, azimuth and row spacing
Three numbers decide how much energy a given number of modules produces and how many of them fit on the land. None of them is chosen by the racking supplier's default template.
- Design latitude
- 35.4 degrees north
- Annual optimum tilt
- About 20 to 25 degrees
- Our default GCR
- 0.58 fixed tilt
- Spacing set by
- Winter solstice shadow
Orientation
Annual yield index by tilt and azimuth
100 is the best annual energy this latitude can produce from a fixed plane. Everything else is expressed against it, so you can see what a given orientation actually costs.
| Azimuth | 5 deg | 10 deg | 15 deg | 20 deg | 25 deg | 30 deg | 35 deg |
|---|---|---|---|---|---|---|---|
| 135 deg (SE) | 95 | 94 | 92 | 90 | 87 | 84 | 80 |
| 157 deg (SSE) | 97 | 97 | 97 | 96 | 94 | 92 | 89 |
| 180 deg (S) | 97 | 99 | 100 | 100 | 100 | 98 | 96 |
| 202 deg (SSW) | 97 | 97 | 97 | 96 | 95 | 93 | 90 |
| 225 deg (SW) | 95 | 94 | 93 | 90 | 88 | 85 | 81 |
| 270 deg (W) | 88 | 84 | 80 | 75 | 70 | 65 | 60 |
The curve is flat at the top
Between 10 and 30 degrees facing south, the annual difference is a few index points. That flatness is what lets us drop tilt on rooftops to cut wind load, ballast and row pitch for almost no energy cost.
West of south is a tariff decision
At 202 degrees and 25 degrees tilt the index is 95 rather than 100. On an irrigation account that pumps late into the afternoon, or a tariff with a 4pm peak, those five points buy something worth more than they cost.
Due west is a real loss
At 270 degrees the index collapses into the sixties and seventies as tilt rises. Facing an array west only makes sense on a flat plane, and even then only with a strong afternoon load.
Row spacing
How far apart the rows sit
Tighter rows fit more kilowatts per acre and lose more winter energy to the row in front. Net delivered kilowatt hours per acre is the number that matters, and it is usually not maximised at either extreme.
| Row pitch | GCR | kW DC per acre | Shading loss | Net kW per acre | When we use it |
|---|---|---|---|---|---|
| 2.6 m | 0.85 | 415 | 6.4% | 388 | Too tight for this latitude. Winter mornings are lost. |
| 3.2 m | 0.69 | 336 | 3.1% | 326 | Aggressive. Works where land is expensive and winter load is low. |
| 3.8 m | 0.58 | 283 | 1.6% | 279 | Our default for fixed tilt at 25 degrees in Kern County. |
| 4.6 m | 0.48 | 234 | 0.7% | 232 | Generous. Chosen when land is plentiful and winter output matters. |
| 5.5 m | 0.40 | 195 | 0.3% | 194 | Tracker territory. Fixed tilt rarely needs this much space. |
Method
Setting pitch from the winter solstice
Around 21 December the sun reaches its lowest declination of the year, so a tilted row casts its longest shadow. That day sets the minimum row pitch, because if the rows clear each other then, they clear each other all year.
We pick a design window, usually three hours either side of solar noon, and calculate the solar altitude at the edge of that window on the solstice. The shadow a row casts is a function of its height above the row in front and that altitude, so the geometry gives a minimum pitch directly.
Then comes the judgement. That minimum protects the design window on the worst day of the year, and going wider protects more of the day at the cost of fewer rows. We model annual energy across at least three pitches and put net kilowatt hours per acre side by side, which is the comparison that actually answers the question.
The free half of the fix
Inter-row shading arrives as a horizontal band creeping up the bottom of the row. Wire each string along the row and the whole string dims together, which a string inverter tolerates reasonably well. Wire across rows and a single shaded module drags an otherwise healthy string down. Same hardware, same cost, materially different output.
Answers
Array geometry: questions
Not covered here? Our engineers answer directly, not through a call centre.
Ask a question(661) 555-0183For maximum annual kilowatt hours at this latitude, close to it. For maximum value on a time-of-use tariff, often not. Rotating west of south gives up a small amount of annual energy and moves output into the late afternoon, which is when the expensive peak period starts.
The annual energy optimum at our latitude sits in the low to mid twenties of degrees, but the curve is flat near the top. Dropping from 25 to 15 degrees costs very little annual energy and buys tighter row spacing, lower wind loads and less ballast. On rooftops that trade is almost always worth taking.
We start from the winter solstice, when the sun is lowest and shadows are longest, and set the pitch so rows are unshaded through a chosen design window either side of solar noon. Then we model annual energy at that pitch and at wider and tighter values and compare net kWh per acre.
Module area divided by the ground area it occupies. A GCR of 0.58 means the modules cover 58 percent of the land. Higher GCR fits more kilowatts on the parcel and loses more energy to inter-row shading in winter.
Yes, considerably wider. A tracker rotates through the day, so it casts a long shadow at both ends of the day in a way a fixed row does not. Applying a fixed-tilt GCR to a tracker field is a common and expensive mistake.
It applies to tilted rooftop rows exactly as it does on the ground. It is one reason so many commercial roof arrays are set at 5 to 10 degrees: the low tilt reduces the shadow, allows tighter rows and fits more kilowatts on a fixed roof area.
We will model three layouts, not one
Tilt, azimuth and pitch get compared on net kilowatt hours per acre before anything is ordered. That comparison is part of every ground-mount assessment.
CSLB #1071482 (sample). Prevailing wage on public works. Engineers answer, not a call centre.