Orientation, Tilt and the Path of the Sun

A roof’s orientation and tilt angle have a large influence on how much sunlight a solar array can capture. The U.S. Department of Energy’s solar design basics explain that, for the highest annual energy output in the northern hemisphere, modules should face due south and be inclined at an angle equal to the local latitude. Mounting structures hold the array at a fixed angle, and that angle is determined by the latitude, the direction the structure faces, and the electrical load the system needs to serve.

Short version: South-facing roofs with a slope between 15 and 40 degrees are a strong starting point, but other orientations can still work.

The U.S. Energy Information Administration (EIA) describes how solar resources change by location and time. The intensity of solar radiation at any spot is greatest when the sun is at its highest point in the sky, around solar noon, on clear days. Latitude, climate and weather patterns are major factors that affect insolation, which is the amount of solar radiation received on a given surface area during a specific period. Locations at lower latitudes and in dry climates generally receive more insolation. Seasonal differences in solar resources grow larger as you move farther from the equator.

The DOE’s homeowner guide notes that solar panels perform well on south-facing roofs with a slope between 15 and 40 degrees, though other roofs may be suitable too. If your roof faces east or west, it can still produce meaningful energy, but the annual total will be lower than a south-facing array. A solar professional can model your specific roof and give you a custom estimate. For a deeper look at how rooftop systems work day to day, see How Rooftop Solar Works.

Shade: Buildings, Trees and Seasonal Changes

Shade is one of the most common reasons a roof may not be ideal for solar. The EIA notes that buildings, trees and mountains can shade a location during different times of the day in different months of the year. A spot that looks sunny in summer could be shaded for hours in winter when the sun sits lower in the sky. Even partial shade on one panel can reduce the output of a whole string of panels, depending on the inverter type.

The DOE homeowner guide warns that if trees near your home create excessive shade on your roof, rooftop panels may not be the most ideal option. A solar professional can use tools to measure shade patterns across the year and help you decide whether trimming, panel layout changes or microinverters can make the roof workable. If you want to understand how inverters handle shade, the article Home Solar Inverter Types explains the differences between string inverters, microinverters and power optimizers.

Roof Age and Replacement Timing

Solar panels are built to last for decades, so the condition and remaining life of your roof matter. The DOE homeowner guide says you should consider the age of your roof and how long until it will need replacement. If your roof is near the end of its service life, it often makes sense to replace it before installing solar. Removing and reinstalling panels to re-roof later adds cost and complexity.

A roofing contractor or a solar professional can assess the roof’s condition. The goal is to match the expected life of the roof to the expected life of the solar array so you do not face an unplanned removal a few years into the system’s operation. For more on planning the whole project timeline, visit Installation Planning.

Structure, Mounting and Professional Checks

A rooftop solar array adds weight and wind load to a building. The mounting structures must be stable and durable. The DOE’s design basics state that PV arrays must be mounted on a structure that can support the array and withstand wind, rain, hail and corrosion over decades. Rack mounting is the most common method because it is robust, versatile and straightforward to install.

The Hong Kong Electrical and Mechanical Services Department (EMSD) describes mounting structures as part of the balance-of-system, or BOS, which also includes wiring, switchgear, fuses and protection devices. The EMSD notes that the BOS can account for up to half of the capital cost of a small photovoltaic system and most of the maintenance cost. While the EMSD guide is written for Hong Kong, the principle is the same everywhere: the racking and attachments must be engineered for the specific roof type and local conditions.

The DOE homeowner guide emphasizes that a solar professional determines whether a roof is suitable. Ask the installer how they will check the roof structure, how the racking attaches, and how roof penetrations will be sealed. If you are at the early stage of deciding whether solar makes sense for your home, the step-by-step article How to Plan a Home Solar System walks through the whole decision process.

Estimating Production With PVWatts

Once you know the orientation, tilt and shade conditions, you can get a rough production estimate using the PVWatts Calculator. The DOE homeowner guide explains that the National Renewable Energy Laboratory (now the National Laboratory of the Rockies) developed PVWatts to estimate the energy production and cost of energy of grid-connected PV systems for any address in the world. Homeowners, small building owners and installers can use it to develop early performance estimates.

The PVWatts Calculator itself includes important cautions. Its predictions include many inherent assumptions and uncertainties and do not reflect variations between PV technologies or site-specific characteristics except as represented by the inputs. The expected range is based on 30 years of actual weather data at the given location and is intended to show the variation you might see. PVWatts does not differentiate between better-performing and lesser-performing modules. The National Laboratory of the Rockies and private companies offer more sophisticated tools, such as the System Advisor Model, for more precise modeling.

PVWatts is a helpful starting point, but the DOE homeowner guide recommends working with a solar installer for a custom estimate of how much power your system is likely to generate. For its analyses, the National Laboratory of the Rockies uses an average system size of 7.15 kilowatts direct-current with a 3 to 11 kilowatt range.

Factor What to Look For Who Checks It
Orientation and tilt South-facing, slope between 15 and 40 degrees; east or west can work with lower output Solar professional using site survey and modeling tools
Shade Buildings, trees and terrain that cast shade in different months; partial shade impact on panel strings Solar professional with shade analysis tools or on-site measurements
Roof age Remaining life versus solar array life; need for re-roofing before installation Roofing contractor and solar professional
Structure and mounting Ability to support array weight, wind and weather loads; racking attachment and waterproofing Qualified solar installer, possibly structural engineer
Permits and inspections Local government permit before installation; inspection after installation; utility connection approval Local government and electric utility

If the Roof Is Not Suitable

Not every roof is a good match for solar panels. The DOE homeowner guide states that if a solar professional determines that your roof is not suitable, or you do not own your home, you can still benefit from solar energy through community solar. Community solar lets multiple people share a single solar array that can be installed on-site or off-site. Costs are divided among the participants, who can buy into the shared system at a level that fits their budget.

Community solar programs vary by state and utility territory. The DOE guide points to a tool from SETO awardee EnergySage that aggregates available options and standardizes project information so consumers can locate and compare community solar projects in their area. If you are weighing different ways to access solar, the overview at Home Solar Planning covers rooftop, community and other paths.

Permits, Inspections and Utility Approval

Even when a roof is physically suitable, the system must pass local permitting and inspection requirements before it can operate. The DOE’s permitting and inspection guide explains that local governments generally require solar installers to obtain a permit for rooftop panels before installation. After the installation, a professional from the local government inspects the array to confirm that all safety codes have been followed. The utility then connects the system to the grid, and it can begin generating power.

Details and fees vary across jurisdictions. The DOE notes that administrative errors and backlogs can cause permitting delays, and improving the permitting process is important for reducing soft costs, which account for about two-thirds of the overall cost of residential systems. Programs such as SolarAPP+ and SolSmart work to streamline permitting and lower those administrative burdens. For a walk-through of the full process from contract to turn-on, see The Home Solar Installation Process.

Frequently asked questions

Can I put solar panels on an east- or west-facing roof?

Yes. The DOE homeowner guide says solar panels typically perform well on south-facing roofs with a slope between 15 and 40 degrees, though other roofs may be suitable too. An east- or west-facing roof will produce less energy over a year than a south-facing one, but it can still generate meaningful power. A solar professional can model the expected output for your specific orientation.

Does my roof need to be replaced before I install solar?

It depends on the remaining life of the roof. The DOE homeowner guide advises you to consider the age of your roof and how long until it will need replacement. If the roof is near the end of its life, replacing it before the solar installation avoids the cost and hassle of removing and reinstalling panels later. A roofing contractor can help assess the condition.

What if my roof is too shaded for solar panels?

The DOE homeowner guide notes that excessive shade from nearby trees can make rooftop panels a less ideal option. If trimming trees or rearranging the panel layout cannot solve the shading problem, community solar is one alternative. Community solar allows multiple people to benefit from a shared array installed on- or off-site, with costs divided among participants.

Ready to dig deeper into roof requirements? Visit the Roof Suitability category page for more detailed guides on orientation, shade analysis and structural checks.

Sources