Sunlight is the starting point
Two homes on the same street can get very different results from the same solar panels. The difference usually comes down to how much direct sunlight reaches the roof over a full year. Roof direction, roof pitch, trees, chimneys and neighbouring buildings all play a part. Before comparing equipment brands or financing, it is worth understanding whether your roof gets enough sun to make solar worthwhile.
Roof direction
In the Northern Hemisphere, roof surfaces facing roughly south receive the most sunlight over a year. East- and west-facing roofs still produce useful power, just less in total, and with a different daily pattern: east-facing panels produce more in the morning and west-facing panels more in the late afternoon. That timing can matter if your utility charges more for electricity in the evening, because west-facing panels produce more of their output closer to those hours. North-facing roofs in most of the United States are usually a poor fit for panels unless the pitch is very shallow.
Roof pitch
The tilt of the roof changes how directly the sun strikes the panels across the seasons. Most residential roofs fall within a range where panels perform reasonably well, and installers generally mount panels flush to the roof rather than tilting them further. Very steep or very flat roofs change the picture, so ask installers how your pitch affects their production estimate.
Shade: the biggest variable
Shade has an outsized effect on solar panels. Panels are made of many cells wired together, and shading part of a panel can reduce the output of more than just the shaded area. Common sources of shade include:
- Trees, including trees on a neighbour's property, which may grow taller over the life of the system.
- Chimneys, vents and dormers that cast moving shadows across nearby panels during the day.
- Neighbouring buildings, especially in dense areas or where a taller building could be built next door.
- Hills or terrain that block low winter sun.
Shade changes through the day and through the year. A roof that looks sunny at noon in summer may be shaded for much of a winter afternoon, when the sun sits lower in the sky.
How installers measure shade
Reputable installers assess shade rather than guessing. Many use satellite imagery and modelling software to estimate how much sunlight each part of the roof receives across the year, and some confirm it with an on-site visit and measurement tools. Ask every installer you talk to:
- How did you estimate shading on my roof, and can I see the results?
- Which roof sections did you leave out, and why?
- Does your production estimate already include shading losses?
- What would change if a particular tree were trimmed?
If two quotes show very different production for similar system sizes, differences in shading assumptions are a likely reason, so ask each company to explain its numbers.
Equipment that helps with partial shade
Some inverter setups handle partial shade better than a single string inverter. Microinverters, mounted on each panel, and power optimisers, paired with a central inverter, allow each panel to work more independently, so one shaded panel drags down the rest less. They also add cost and more electronics on the roof. Whether they are worth it depends on how much shade your roof actually has. A roof with no meaningful shade may not need them; a roof with scattered shade often benefits.
Trees: trim, remove or keep?
Trees provide cooling, privacy and value of their own, so removing them is a real trade-off and not always the right answer. Options include trimming specific branches, placing panels only on the unshaded part of the roof, or choosing a smaller system. If the trees belong to a neighbour, local rules on trimming vary, and some areas have solar access laws while others do not. Check with your local government before assuming anything about a neighbour's trees.
When rooftop solar may not be the right fit
If most of your roof is heavily shaded, faces north, or is too broken up by vents and dormers to fit a useful number of panels, rooftop solar may not pay off. Alternatives worth exploring include a ground-mounted system elsewhere on the property, a community solar subscription where available, or focusing first on energy efficiency improvements that reduce your electricity use.
A simple self-check
Before requesting quotes, walk around your home at a few times of day, ideally in different seasons, and note where shadows fall on the roof. Take photos. Note which roof faces get sun in the middle of the day. This will not replace a professional assessment, but it will help you spot unrealistic estimates. You can then use our savings guide to think through what a lower or higher production estimate would mean for you.