What is the best solar panel tilt?

How it works

1. Day of the year d: 1 January is 1, 31 December is 365 (366 in a leap year).
2. Sun's declination δ = −23.45° × cos(360/365 × (d + 10)), with the cosine of an angle in degrees.
3. Tilt for this date = |latitude − δ|, the angle from horizontal that faces the panel straight at the sun at solar noon.
4. Face them on this date: south when latitude − δ is above 0, north when it is below 0, flat when it is exactly 0.
5. Noon sun height = 90° − |latitude − δ|.
6. Polar night. When the noon sun height is below 0, the sun does not rise above the horizon at noon on that date. The page then gives no tilt for the date, shows "none (no sun at noon)" as the direction, and still shows the declination and the (negative) noon sun height.
7. Year-round tilt = |latitude|, facing south for a positive latitude, north for a negative one, and flat at the equator.
8. In words. "For the year" and "On this date" say the same thing as a sentence: "lay them flat" for a tilt of exactly 0, otherwise "tilt them T° facing south" (or north), with T rounded like the numbers below; on a polar night date, "the sun stays below the horizon at noon".

The year-round tilt is the tilt PVEducation gives for the most energy over a year from a fixed panel: a tilt equal to the latitude. It is also the average over a year of the tilt for each date. Over a year the declination in step 2 averages 0, so for latitudes of 23.45° or more the noon tilt averages |latitude|. Nearer the equator the page still gives |latitude|, the tilt square to the sun at the equinoxes.

Rounding. Angles show with at most 1 decimal, rounded half up, trailing zeros left off. The sentences use the same rounded tilt as the numbers. On 21 December (20 December in a leap year), δ is exactly −23.45°, so the tilt and the noon sun height are exact decimals, often a half such as 26.55°. The page rounds them to 12 significant digits first, which removes the computer's binary error, so 26.55° shows as 26.6°.

Assumptions

• The panels are fixed and face true (not magnetic) south or north.
• Latitude is from −90° to 90°, north positive.
• Clouds, snow, shade and the time of day you use power can move the best angle a few degrees.

Worked examples by hand

Latitude 40° N on 21 June 2026 (day 172). δ = −23.45 × cos(360/365 × 182) = −23.45 × cos(179.51°) = 23.449°. Tilt = 40 − 23.449 = 16.55°, facing south; noon sun height 90 − 16.55 = 73.45°. Year-round tilt 40° facing south.

Latitude 40° N on 21 December 2026 (day 355). δ = −23.45 × cos(360/365 × 365) = −23.45 × cos(360°) = −23.45°. Tilt = 40 + 23.45 = 63.45°; noon sun height 90 − 63.45 = 26.55°. Rounded half up, the page shows 63.5° and 26.6°.

Latitude 33.9° S on 20 March 2026 (day 79). δ = −23.45 × cos(360/365 × 89) = −0.908°. Latitude − δ = −33.9 + 0.908 = −32.99, so the tilt is 32.99° facing north. Year-round tilt 33.9° facing north.

Latitude 10° N on 21 June 2026. δ = 23.449°, so latitude − δ = −13.45: tilt 13.45° facing north, although the year-round tilt (10°) faces south.

Latitude 70° N on 21 December 2026. δ = −23.45°, so latitude − δ = 93.45 and the noon sun height is 90 − 93.45 = −3.45°: the sun stays below the horizon at noon, so there is no tilt for the date. The year-round tilt is 70° facing south.