You've probably read the rule: tilt your panels at your latitude. It's close in the southern US, and it runs up to 14° too steep as you go north. We calibrated the actual optimum against PVGIS satellite irradiance data for 52 cities, and the corrected numbers are below: 27° at latitude 25, 37° at latitude 40, 42° at latitude 50, always facing true south in the northern hemisphere.
Fixed mount: °. If you adjust twice a year: ° in winter, ° in summer.
Don't know your latitude? Search your city, or use your zip code or UK postcode: every city page has the exact angles plus a monthly table.
Why the correction? Winter sun sits low and a growing share of light arrives as diffuse sky light rather than direct beam, and both effects flatten the true optimum as you go north. The latitude rule ignores them.
| Latitude | Old rule says | Calibrated optimum | Rule's error | Example city |
|---|---|---|---|---|
| 20° | 20° | 23° | +3° | Honolulu, HI |
| 25° | 25° | 27.1° | +2.1° | Miami, FL |
| 30° | 30° | 30.8° | +0.8° | Houston, TX |
| 35° | 35° | 34.1° | -0.9° | Albuquerque, NM |
| 40° | 40° | 37.2° | -2.8° | Denver, CO |
| 45° | 45° | 39.8° | -5.2° | Minneapolis, MN |
| 50° | 50° | 42.1° | -7.9° | London, UK |
| 55° | 55° | 44.1° | -10.9° | Glasgow, UK |
| 60° | 60° | 45.7° | -14.3° | Anchorage, AK |
South of latitude 35 the old rule is fine. By London's latitude it's 8° too steep; by Anchorage's, 14°. If someone tells you to tilt Glasgow panels at 56°, they're quoting the rule, not the sun.
Fixed rooftop panels sit at whatever pitch the roof has. Here's what common pitches cost against the optimum for a mid-US roof at latitude 40 (optimum 37.1°):
| Roof pitch | In degrees | Loss vs optimum at latitude 40 |
|---|---|---|
| 3/12 | 14° | about 8.1% |
| 4/12 | 18.4° | about 5.3% |
| 5/12 | 22.6° | about 3.2% |
| 6/12 | 26.6° | about 1.7% |
| 8/12 | 33.7° | about 0.2% |
| 10/12 | 39.8° | about 0.1% |
| 12/12 | 45° | about 0.9% |
A 6/12 roof gives up about 1.7%. Tilt racks on a pitched roof rarely earn back their cost and wind risk for that; spend the attention on facing the panels south instead.
| Tilt error | Direct-beam loss |
|---|---|
| 5° | about 0.4% |
| 10° | about 1.5% |
| 15° | about 3.4% |
| 20° | about 6% |
| 30° | about 13.4% |
These are beam-geometry figures (1 minus the cosine of the error); diffuse light makes real annual losses smaller still. The takeaway: 5° is noise, 15° is a percent or three, and only being wildly off, flat in Scotland, vertical in Texas, costs real money.
A fixed mount at the calibrated optimum collects the most for zero effort. If you're willing to adjust twice a year, go steeper in winter (latitude × 0.9 + 19°, about 54° in Denver) and shallower in summer (latitude × 0.9 − 21°, nearly flat in the southern US). Monthly adjustment squeezes out the last few percent; every city page has the twelve-row table.
True south (true north below the equator), which differs from compass south by your local magnetic declination: up to 15° in parts of the US. Check a declination map or enable "true north" in your phone's compass settings before you drill.
The curve behind this page and every city page is a quadratic fitted against PVGIS v5.2 optimal-slope data for 52 cities spanning latitudes 1 to 69. Against that reference it lands within 3.4° RMS, versus 7.3° for the piecewise latitude rules it replaced. What's left is mostly climate, which latitude can't see: Paris and Harbin sit within 3° of latitude of each other and want 38° and 47° respectively, because one is cloudy and one is not. All data here is CC0; cite it freely.
The rules change when your roof moves. See when tilting RV panels beats lying flat, how many watts a campervan needs, and how to aim portable panels at camp.