The rule, and how much of a rule it is
For a fixed array collecting the most energy over a full year, tilt the panels at roughly the site latitude and face them towards the equator. At 40 degrees north, that is 40 degrees of tilt facing south. Tilt them about 15 degrees flatter to favour summer and about 15 degrees steeper to favour winter, because the sun sits higher in the sky in summer and lower in winter and you are trying to present the glass square to it.
Refinements exist. Several published fits give an annual optimum slightly below latitude at mid latitudes, because the atmosphere is clearer in summer than in winter at most sites and the extra summer sun is worth tilting towards. The difference is a few degrees, and a few degrees is worth a fraction of a percent, so the refinement is real and does not change what anyone builds.
The penalty curve is flat where it matters
This is the part worth internalising. Output near the optimum falls off with the cosine of the angular error, and cosine is flat near zero. Five degrees off costs a fraction of a percent. Ten degrees off costs about one percent. Fifteen degrees off costs roughly two and a half. You have to get around 30 degrees away before the loss reaches high single figures.
| Degrees from optimal tilt | Approximate annual output | What that means in practice |
|---|---|---|
| 0-5 | 99.7-100% | Indistinguishable from perfect |
| 10 | about 99% | Less than the year-to-year weather variation |
| 15 | about 97.5% | Roughly one extra panel in forty |
| 25 | about 94% | Noticeable, still cheaper to fix with panels than with racking |
| 40 | about 87% | A flat roof at high latitude, or a very steep one at low latitude |
| 60 | about 73% | A wall-mounted array, or a roof pointing badly wrong |
The consequence is that almost nobody with a pitched roof should be adding racking to correct tilt. A 5/12 roof is 22.6 degrees; at 40 degrees latitude that is 17 degrees off optimum and costs around three percent. Correcting it means tilt-up frames, wind uplift calculations, extra penetrations, and an array that looks like scaffolding. Adding three percent more panels flat to the roof achieves the same energy for less money and less risk.
Azimuth behaves differently from tilt
Facing the wrong way is more expensive than being at the wrong angle, and the two interact. A dead-flat array has no azimuth at all, so orientation cannot penalise it. As tilt steepens, the array becomes more directional, and the same 45 degrees of azimuth error costs progressively more.
Azimuth error also does something tilt error does not: it moves the production, not just shrinks it. A west-facing array at mid latitude might give 85 percent of the annual energy of a south-facing one, but it gives that energy in the late afternoon, when demand peaks and when time-of-use tariffs are usually at their most expensive. On the right tariff a west array can be worth more money than a south array while producing less electricity. That is an economic argument rather than a solar one, and it depends entirely on what your utility charges by hour, which is not something a calculator should assume for you.
Shading outranks all of this
Angle losses are linear and gentle. Shading losses are neither. A single module in partial shade drags a whole series string toward its own current, so a chimney shadow crossing one panel can cost far more than that panel produces. Module-level electronics limit the damage but do not eliminate it, because the shaded module still contributes nothing while it is shaded.
A useful ordering of effort, from most to least valuable: remove or avoid shading; get the array on the sunniest available plane; keep the modules clean and ventilated; and only then argue about degrees. People routinely do this list backwards, spending a weekend calculating the perfect tilt for a roof with a tree over it.
Seasonal adjustment deserves the same scepticism. Changing tilt twice a year between the summer and winter optima gains a few percent annually on a fixed array, which for a residential system is a handful of kilowatt-hours per panel per year in exchange for climbing on a roof twice a year and building a hinged mount that has to survive wind. It makes sense on a small ground-mounted off-grid array where the winter months are the binding constraint, and rarely anywhere else.
Where the tilt figure goes next
The output of this page is an orientation and a percentage, and both feed the sizing work. Take the percentage into the solar panel array sizing calculator as part of the loss stack, or better, query your peak sun hours at your actual tilt and azimuth so the penalty is already inside the irradiance figure rather than applied twice. For an off-grid system where December is the binding month, the winter-optimised tilt is the one that matters and the off-grid load audit calculator is where the worst-month arithmetic starts. Background on how a residential system fits together is in the home solar basics guide.
A PV array is energized whenever light falls on it. There is no switch on the panel that turns it off, and a module lying face up on the grass is a live source. DC arc faults do not self-extinguish the way an AC arc does, because DC current never passes through zero. A battery bank can deliver thousands of amps into a short circuit without any warning noise, and a dropped wrench across two terminals is a serious burn and fire hazard rather than a spark. Lithium cells that have been damaged, punctured, or charged below freezing can fail violently, and the failure is not always immediate. Nothing on this page is a wiring, commissioning or battery-assembly procedure, and none of it should be used as one.
Questions people ask
What is the best angle for solar panels?
Roughly your latitude, facing the equator, if you want the most energy over a year. At 35 degrees north that is 35 degrees of tilt facing south. Favour summer by flattening about 15 degrees and favour winter by steepening about 15. The honest follow-up is that the question matters less than it sounds: anything within about 15 degrees of that answer gives you 97 percent or more of the energy, so the practical best angle for most people is whatever their roof already is.
Is it worth building a tilt frame on a flat roof?
Usually yes, but not for the reason people expect. A dead-flat module at mid latitude loses roughly 10 to 15 percent of the annual energy compared with an optimal tilt, which alone might not justify the racking. What justifies it is everything else: a flat module never sheds water, dirt or snow, so it soils faster and stays soiled, and standing water dries into a permanent tide mark along the lower edge. Ten to fifteen degrees is enough to get self-cleaning behaviour from rain and is far cheaper than a steep tilt, because steeper frames need wider row spacing to stop each row shading the next, and on a flat roof that spacing can cost you more array than the tilt gains.
Should I adjust the tilt seasonally?
On a fixed residential rooftop array, no, because you cannot without rebuilding the mount and the gain is a few percent. On a small ground-mounted off-grid array where the December energy is what determines whether the batteries hold up, it can be worth doing, and the useful version is two positions rather than four: a steep winter angle and a flatter summer one, changed twice a year. Trackers gain considerably more than seasonal adjustment does, typically 15 to 30 percent depending on type and site, at the cost of moving parts, a foundation, and maintenance that fixed arrays do not have.
My roof faces southeast. How much am I losing?
Southeast is about 45 degrees off south, which on its own costs roughly 3 to 4 percent of annual energy at a typical residential tilt, and 5 to 7 percent once the tilt of an ordinary roof is counted alongside it. That is one panel in twenty, and it is comfortably inside the range where adding a module is cheaper than any structural remedy. East or west, at 90 degrees off, is a different conversation at roughly 15 to 20 percent, and at that point splitting the array across two roof planes, or accepting a smaller array on the better plane, both become worth pricing.
Why does the calculator not use my exact location climate?
Because it cannot. The tilt and azimuth factors here come from conventional relationships that describe how a plane collects sunlight, and they are reasonable for planning at mid latitudes. What they do not know is your cloud climate, your horizon, how reflective your ground is, whether your mornings are systematically foggier than your afternoons, and how much snow sits on the array in February. Any of those can move the real answer by several percent, and a coastal site with burnt-off morning fog will genuinely prefer a west-leaning array over a south-facing one. For a design decision, put your actual tilt and azimuth into a solar resource dataset and read the answer for your coordinates.