Solar panel tilt angle calculator
Find the best tilt for your latitude, or enter the pitch and direction you already have and see what it is costing you. The angles are computed from solar geometry, not a rule of thumb.
Inputs
Result
Tilt against direction, for your latitude
Read down a column to see how little the tilt matters, and across a row to see how much the direction does. That asymmetry is the whole point: a wrong angle is a rounding error, a wrong roof face is not.
How this is worked out
Most tilt calculators hand you a rule of thumb, usually your latitude minus 15° for summer and plus 15° for winter. This one computes the energy landing on the plane instead, hour by hour across the twelve mean days of the year, and then searches for the angle that collects the most. The pieces are standard: Cooper's declination equation, the Kasten and Young air mass, a clear-sky beam transmittance, the Erbs correlation to split beam from diffuse, and the Duffie and Beckman transposition onto a tilted surface with an isotropic sky and a ground reflectance of 0.2.
Why we can show you the model is right
A model is only worth anything if it reproduces figures somebody else published. This one lands within 0.6° of Landau's latitude formulas across 20° to 50° N, puts the October–March optimum at 54.5° against his 54.2°, and values two seasonal adjustments a year at 4.1% against a published figure of roughly 4%. None of that is fitted: there is no tuning constant anywhere in the calculation. The Landau's published formula row in the results shows you that comparison on every run.
Tilt is forgiving. Direction is not.
At 40° N the best fixed tilt is about 34°, and being 10° out costs around 1.2% a year. Being 15° out costs under 3%. This is why flush-mounting on whatever pitch the roof already has is almost always right: the racking to correct a 10° error will never pay for 1.2%.
Direction is a different matter. Turning that same optimally tilted array from south to due west costs about 20%. And once a face is well off south, the best thing to do with it is lay the panels nearly flat, because tilting up into a direction the sun is not in loses more than the tilt recovers. The calculator reports that as the best tilt for your face, and it will sometimes tell you flat beats following your pitch.
What the percentage does and does not include
The figure is a comparison between two orientations under the same sky, which is what this kind of model is dependable for. It is not a production estimate: it knows nothing about your panels, your inverter, or the weather where you live. For kilowatt hours, take the derate figure in the results and put it into the solar panel output calculator, which uses NREL sun hours for your state.
Because this is a clear-sky model, it is slightly hard on flat and off-south roofs, which pick up more diffuse light at a real site than the model credits them with. Treat the percentages as a floor.
Questions
What is the best angle for solar panels?
Roughly three quarters of your latitude plus a few degrees, which works out near 34° at 40° N and 26° at 30° N. The calculator computes it properly rather than using that approximation, and it also shows you how little you lose by being 10° off, which for most roofs is the more useful answer.
Is latitude minus 15 degrees correct for summer?
It is close. At 40° N the rule gives 25° and the computed optimum for April to September is about 18°, so the rule is a little steep. For a fixed array it hardly matters, because summer sun is plentiful and the annual optimum is the one worth hitting.
Should I tilt panels on a flat roof?
Usually a little, yes. A flat mount at 40° N collects about 86% of what an optimally tilted one does, so 10° to 20° of tilt recovers most of the gap, helps rain wash the panels, and lets snow slide. Steeper than the optimum gains nothing and catches more wind.
How much do I lose with an east or west facing roof?
About 20% a year against a south-facing roof at the same latitude, by this model, and probably a little less in reality. That is usually still worth doing. A north-facing roof is a different matter: it loses something like 40% and should be your last choice.
Is it worth adjusting the tilt seasonally?
It is worth about 4% a year if you change it twice, which is real but small. On a ground mount or a pole where you can reach the racking in a minute, take it. On a roof, the ladder trips are not worth 4%, and adjustable roof racking costs more than the energy it returns.
Does my roof pitch matter more than the direction it faces?
No, and it is not close. Pitch within 10° of ideal costs you about 1%. Facing 90° away from south costs about 20%. If you have a choice of roof faces, choose by direction and accept whatever pitch comes with it.