Two houses in the same French town, Lyon. Same solar panels, same price. One roof faces south, the other faces east, and the south one makes about a fifth more electricity every single year.
The difference is just the roof orientation: which way it points and how steep it is.
A panel that points the wrong way gives up a chunk of its energy production before it even starts, and no fancy tech can fix that later. So before you spend money on more solar panels, it's worth getting this part right.
Orientation And Tilt: The Two Things That Matter
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A solar panel is basically a flat board catching sunlight. Two things decide how much of that sunlight it can receive, and so how much electricity it can produce.
First is the tilt. That's how steep the panel sits, measured in degrees from flat ground. Lying flat is 0 degrees, and standing straight up is 90.
Second is the azimuth. Scientists call it that, but it really just means the direction the panel faces: north, south, east or west.
Get one wrong and the performance drops. Get both wrong and you might as well have bought less.
Roof Pitch: Finding The Ideal Panel Angle
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Builders call the slope of a roof its pitch, and you'll often hear "tilt your panels at your latitude". It's a rough starting point, not the answer, because the ideal angle depends on where you live and what you want the solar system to do.
If you want the most energy over a whole year (normal for a grid-tied house), the optimal tilt usually comes out a bit flatter than that, because summer days are long and bright. The EU's free solar tool PVGIS puts the best yearly tilt at about 37 degrees for Lyon (45.8° N), 39 for Utrecht in the Netherlands (52° N) and 41 for Minneapolis (45° N). A few degrees either way barely matters: in Lyon, 45 degrees instead of 37 loses less than 1% over the year.
If you need power in the dark months (normal for an off-grid home, which has to get through December), you design the system for those, and that means steeper. In Lyon, 60 degrees makes about 6% less over the year than 37, but about 13% more in December. Steep solar panels also shed snow more easily.
Some people with adjustable mounts change the tilt twice a year to maximize output: steeper for the low sun, flatter for the high one.
The best way to choose is to put your own address into PVGIS (free, works for Europe, Africa, Asia and the Americas) or NREL's PVWatts (US) and compare the monthly energy production for a few tilts before the installation. Pair it with how many sun hours your area gets.
Which Direction Should Solar Panels Face?
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Face them south if you live above the equator, and the other way if you live below it. That's the whole rule.
In Europe, the US, Canada and most of Asia, the sun spends most of the day in the southern sky. In Australia, South Africa and Argentina it does the opposite, so the sunny side of the sky is flipped.
A panel aimed at the part of the sky where the sun spends most of its time catches the most light, and that is the optimal orientation. Simple as that.
Do Solar Panels Have To Go On A South-Facing Roof?
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No. An east or west roof still works, it just makes less: about 15 to 25% less over a year than the best south-facing roof, depending on how steep it is.
In Lyon, PVGIS gives about 17% less for an east roof at 10 degrees, 18% less at 20 degrees and 22% less at 37 degrees. December is a different story, though. An east roof at 37 degrees makes roughly half as much then as the best roof. For a grid-tied installation, that often just means one or two extra panels, but off-grid you should check the dark months before you count on it.
There is even a clever trick called an east-west split. Half the solar panels face east and catch the morning sun, while the other half face west and catch the afternoon. You never get the big midday peak, but production is spread more evenly throughout the day. If you use most of it in the morning and evening, that can actually suit you better.
Shading is the other thing that can spoil a good roof. A tree or chimney on the sunny side can hurt performance more than a bad direction, so read how shade affects solar panels before you decide.
Flat And Low-Slope Roofs: Propping The Panels Up
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Lying flat, they catch less light and also collect dust, leaves and puddles. In Lyon, a flat panel makes about 16% less over a year than one tilted at 37 degrees.
The fix is called a ballasted tilt frame. It's a metal frame that sits on the roof and props them up, while heavy concrete blocks hold it down so the wind cannot toss it off. No holes drilled in your roof is a nice bonus.
The steeper the panel, the harder the wind pushes on it and the more ballast the design requires. That's why flat-roof frames are often set at only about 10 degrees, which still gets most of the gain (about 9% less than 37 degrees in Lyon). Your roof also has to be strong enough to carry all that weight, so check its load requirements first.
A Real Example: The Impact Of An East Roof
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Say you live in Lyon. Your roof faces south and slopes at about 35 degrees, which is almost the sweet spot, so the system catches close to the maximum sunlight your area can give.
Now take the same house and the same 4000-watt solar system, but the roof faces east. PVGIS says you lose about a fifth of your yearly energy, so it makes about as much as a 3200-watt system facing the sun.
To make up the gap, you would need roughly 1000 watts more: two or three extra solar panels. Not a disaster, but worth knowing before you sign anything.
If the roof faced north, the loss would be much bigger: at 37 degrees in Lyon it makes only about half.
Check The Roof Before Your Solar System Is Installed
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Most solar quotes assume your roof is good. Nobody mentions the fifth you lose because your house was built to face the street, not the sun.
If you check the tilt and direction first, you can plan around it. The cheapest way to get more solar energy is not buying more. It's pointing what you have the right way, then working out your yearly kWh with the real numbers.
Sources
- European Commission JRC, PVGIS photovoltaic geographical information system
- NREL, PVWatts calculator
- Spirit Energy, Angling for a flat roof solar system?
Hard Words
| Term | What it means |
|---|---|
| Tilt | How steep the panel sits, in degrees from flat ground. 0 is lying flat, 90 is standing straight up. |
| Azimuth | Which direction the panel faces: south, east, west or north. |
| Latitude | How far north or south of the equator you are. A rough starting point for tilt; the best yearly tilt is often a bit flatter. |
| Hemisphere | One half of the Earth. North of the equator panels face south; south of it they face north. |
| East-west split | Half the panels face east and half face west, so the electricity is spread across the day instead of peaking at noon. |
| Ballasted tilt frame | A metal frame held down by concrete blocks. Props panels at an angle on a flat roof without drilling holes. |
| Fixed-angle mount | A mount that holds the panel at one tilt forever. Cheapest and most common. |
| Adjustable mount | A mount that lets you change the tilt by season. A bit more work, a bit more output in the dark months. |
| Sun's path | The arc the sun traces across the sky. Higher in summer, lower in winter, so the best tilt changes with the season. |
Common Questions
Which way should my panels face?
South in the northern hemisphere and north in the southern one. Point them where the sun spends most of its time.
What angle should they sit at?
For the most energy over a year, usually a bit flatter than your latitude: about 37 degrees in Lyon (45.8° N), says PVGIS. For winter power off-grid, go steeper, like 55 to 60 degrees there. Check your own address in the free PVGIS tool.
What if my roof faces east or west?
You make about 15 to 25 percent less over a year than on the best roof, and about half in December. Grid-tied, one or two extra panels usually fix it; off-grid, check the winter months first.
Is north-facing solar ever ok in the northern hemisphere?
Usually no. In Lyon a roof facing that way at 37 degrees makes only about half as much. It can make sense as a cheap top-up when every other surface is shaded.