How Much Less Does an East- or West-Facing Roof Produce Than a South-Facing One?
Summary
In a New York model at a 20° tilt, turning the array from due south to due east left about 86% of the annual output, due west about 84%, and southeast or southwest 95% to 96%. Winter is where the gap opens up: in December the west-facing array made only about six-tenths of the south-facing one. DOE says modules in the Northern Hemisphere give the highest annual output when pointed due south and tilted at the local latitude.
One kilowatt, six directions
All runs use NLR's PVWatts V8 for New York (40.7° N), a 1 kW roof-mounted array, and 14% system losses.
| Azimuth, 20° tilt | Annual output | Share of due south |
|---|---|---|
| South, 180° | 1,291 kWh | 100% |
| Southeast, 135° | 1,242 kWh | 96% |
| Southwest, 225° | 1,229 kWh | 95% |
| East, 90° | 1,106 kWh | 86% |
| West, 270° | 1,087 kWh | 84% |
| North, 0° | 869 kWh | 67% |
The monthly figures tell a seasonal story. In June the west-facing array produced 134 kWh against 138 kWh facing south, 97% of it. In December it was 40 kWh against 63 kWh. The north-facing array's December output was 16 kWh, about a quarter of the south-facing figure. In this run east edged out west.
Tilt matters less than direction
Holding the array due south and changing only the tilt:
| Tilt, facing south | Annual output | December |
|---|---|---|
| 5° | 1,180 kWh | 47 kWh |
| 20° | 1,291 kWh | 63 kWh |
| 30° | 1,323 kWh | 72 kWh |
| 40° | 1,320 kWh | 78 kWh |
Near-latitude 40° and 30° end up with almost the same annual total, with 40° shifting some output from summer to winter. A nearly flat 5° array still reached 91% of the 20° case.
Sources
- U.S. Department of Energy — Solar Photovoltaic System Design Basics
- NLR — PVWatts V8 API (New York 40.71, -74.01; 1 kW; roof mount; losses 14%; runs 2026-09-23)
- National Laboratory of the Rockies — PVWatts Calculator
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