A rooftop solar power system, also called a rooftop PV system, is a type of solar energy system that uses solar panels placed on the roof of a home or building to generate electricity. These systems include parts like solar panels, mounting structures, wires, inverters, battery storage, controllers, monitoring tools, energy management systems, net metering equipment, switches, grounding tools, protective devices, combiner boxes, weatherproof enclosures, and other electrical parts.

Rooftop solar systems are smaller than large solar power plants that are built on the ground and can produce millions of watts of electricity. They are considered a type of distributed generation because they are spread out rather than centralized. A study showed that rooftop solar systems are better for the environment than large-scale solar plants over their lifetime. Most rooftop systems are connected to the electrical grid. Residential systems usually have a power range of 5–20 kilowatts (kW), while those on commercial buildings often range from 100 kW to 1 megawatt (MW). Very large rooftops can support industrial systems with capacities from 1–10 MW.

As of 2022, about 25 million homes worldwide use rooftop solar power. Australia has the highest amount of rooftop solar power per person compared to other countries.

Installation

The urban environment offers many unused rooftop areas and naturally reduces land use and environmental issues. Calculating the amount of sunlight rooftops receive is a complex process because sunlight levels depend on several factors:

• Time of year
• Latitude
• Weather conditions
• Roof slope
• Direction the roof faces
• Shading from nearby buildings and plants

There are several ways to estimate the potential for solar power systems on rooftops, such as using lidar and orthophotos. Advanced models can even calculate how much sunlight is blocked by buildings or trees over large areas for city-wide solar projects.

The following section lists the most common parts of a rooftop solar system. While designs may differ based on roof type (such as metal or shingle roofs), roof angle, and shading, most systems include these parts:

• Solar panels create electricity without producing carbon when sunlight hits them. Usually made of silicon, solar panels are made of smaller solar cells, with six cells per panel. Many panels connected together form a solar array. Solar panels are protected by strong glass and held in place by an aluminum frame. The front of a panel is very strong, while the back is more likely to be damaged.
• Mounting clamps typically include aluminum parts and stainless steel screws that hold solar panels together on the roof and attach them to rails. Clamps are designed in different ways to fit various roof and rail setups.
• Racking or rails are made of metal and are often placed side by side on the roof to support the panels. It is important for rails to be level so panels can be mounted evenly.
• Mounts connect the rails and the entire system to the roof. These mounts are often L-shaped brackets that are fastened through flashing and into the roof’s support beams. Mounts vary in design to match different roof types and materials.
• Flashings are strong metal plates that create a watertight seal between the mounts and the roof. Often, caulk is used to attach the flashing to the roof, and it looks similar to a metal roof shingle.
• DC/AC wiring connects wires between panels to a micro inverter or string inverter. Cables should not touch the roof or hang from the array to prevent damage from weather.
• Micro inverters are attached to the bottom of each panel and change the electricity from direct current (DC) to alternating current (AC) so it can be sent to the power grid. Micro inverters help each panel work efficiently even when shaded and can provide details about each panel’s performance.

Finances

Solar incentives in the United States vary by state and can help reduce the cost of installing solar power systems, making them more affordable. Each state offers different programs, such as tax credits, rebates, and net metering, which allow solar owners to receive money for extra electricity sent back to the power grid.

In the mid-2000s, solar companies used financing plans like leases and power purchase agreements. These plans let customers pay for solar panels over several years, with some help from net metering credits. By May 2017, the average cost to install a rooftop solar system was about $20,000. This was less expensive than it had been in earlier years.

A report by Utility Dive noted that solar systems are often seen as a luxury because they add to other household costs. Most homes that install solar panels have higher incomes, with an average household income of around $100,000. However, a study found that some low-income households also use solar systems. Researchers estimate that over 100,000 solar installations are at low-income properties in four major solar markets.

A 2018 report by the Consumer Energy Alliance showed that a mix of federal, state, and local incentives, along with lower installation costs, has increased the use of rooftop solar nationwide. In 2016, residential solar power grew by 20% compared to the previous year. The average cost of residential solar systems dropped by 21% between 2015 and 2017, reaching $2.84 per watt in 2017. In eight states studied, government incentives for solar installations covered the full cost of installation.

In 2019, the average cost of a 6 kW residential solar system in the U.S., after tax credits, was $2.99 per watt, with prices ranging from $2.58 to $3.38 per watt.

Industrial-sized ground-mounted solar systems are often cheaper to operate than smaller roof-mounted systems because they benefit from economies of scale. These large systems can produce electricity at about half the cost of smaller systems, or about 2 cents per kilowatt-hour compared to 4 cents per kilowatt-hour.

In grid-connected rooftop solar systems, extra electricity can be sold to the local power company. This helps the system owner recover some of their investment. A public utility commission usually sets the price the company pays for this electricity, which can be at the retail rate (higher) or the wholesale rate (lower). This affects how quickly solar systems pay for themselves and how many people choose to install them.

Feed-in Tariffs (FITs), which guarantee payments for solar electricity, have helped the solar industry grow worldwide. These programs have created thousands of jobs and reduced power losses by allowing electricity to be generated closer to where it is used. However, FITs can sometimes lead to problems if they are removed suddenly, causing a drop in solar industry growth.

Solar shingles

Solar shingles, also called photovoltaic shingles, are solar panels that look and work like regular roofing materials, such as asphalt shingles or slate. They produce electricity while covering a roof. Solar shingles are a type of solar energy system called building-integrated photovoltaics (BIPV).

Hybrid systems

A solar power system on a rooftop (connected to the electrical grid or not) can be used with other energy sources such as diesel generators, wind turbines, and batteries. These combined solar and energy systems can provide a steady supply of electricity.

Advantages

Installers can send solar electricity into the public power grid and receive a fair higher payment for each kilowatt-hour of electricity they produce. This payment helps cover the extra costs of using solar power and shows the benefits of solar energy. In countries where electricity from the grid is unstable or very expensive, rooftop solar systems are often a better option.

For homeowners, a solar PV system can help them use less electricity from fossil fuels by using sunlight to create power for their homes. This can lower the amount of carbon dioxide they produce and help them save money on their electricity bills.

Disadvantages

A power system with 10% energy from solar panels needs 2.5% more control to keep the system stable than a system without solar panels. This challenge can be solved by using special devices called synchronverters in the part of the system that changes direct current to alternating current. In 1996, it was found that producing less than 10% of energy from solar panels was expensive. However, using more solar energy lowers these costs. Still, system stability and cost limits mean that solar energy should not provide more than about 10% of total power.

When replacing a roof covered with asphalt shingles, solar panels must be removed before the roof work begins and reattached after the roof is finished. This process may cause a temporary loss of electricity at the home. Solar panel installers would need to visit the home twice—once to remove the panels and again later to reattach them. The cost of this work is usually higher than the cost of replacing the roof itself.

Technical challenges

Adding many rooftop PV systems to the power grid has many technical problems.

Thin film solar on metal roofs

Thin film solar panels are becoming a cost-effective choice for use on metal roofs, similar in price to traditional monocrystalline and polycrystalline solar cells. These panels are flexible and can be placed along the seams of metal roofs. They use adhesive to stick to the roof, so no holes are needed during installation. The connection wires are placed under the ridge cap at the top of the roof. Thin film solar panels have an efficiency range of 10–18% and cost about $2.00–$3.00 per watt of installed capacity. In comparison, monocrystalline solar cells are 17–22% efficient and cost $3.00–$3.50 per watt. Thin film panels are lightweight, weighing 7–10 ounces per square foot. They last 10–20 years and provide a quicker return on investment than traditional solar panels. Metal roofs typically last 40–70 years before needing replacement, while asphalt shingle roofs usually last 12–20 years.