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

Rooftop systems are smaller than large solar power plants that are built on the ground and can produce millions of watts of electricity. Studies show that rooftop solar systems are better for the environment than large solar plants. Most rooftop solar systems are connected to the power grid. Solar systems on homes usually have a power capacity of about 5 to 20 kilowatts (kW), while those on businesses often range from 100 kilowatts to 1 megawatt (MW). Very large rooftops can support industrial-scale systems with capacities between 1 and 10 megawatts.

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 city environment has many unused rooftop areas and can naturally avoid issues related to land use and the environment. Calculating the amount of sunlight that reaches rooftops is a complex process because sunlight levels depend on several factors:

• Time of year
• Latitude
• Weather conditions
• Roof slope
• Roof direction
• Shade from nearby buildings and plants

There are multiple ways to calculate the potential for solar power systems on rooftops, such as using lidar and orthophotos. Advanced models can even calculate how much shade affects solar power systems across large areas for city planning.

The following section describes the most common parts of a rooftop solar system. While designs may differ based on roof type (e.g., metal vs. shingle), roof angle, and shading, most systems include the following components:

• Solar panels generate electricity without producing carbon when sunlight hits them. Usually made of silicon, solar panels are made of smaller solar cells, with about six cells per panel. Many panels connected together form a solar array. Solar panels are protected by tempered glass and held in place by an aluminum frame. The front of a solar panel is strong, but the back is more fragile.
• Mounting clamps are made of aluminum brackets and stainless steel bolts that hold solar panels together on the roof and attach them to rails. Clamps come in different designs to fit various roof and rail setups.
• Racking or rails are made of metal and are often placed side by side on the roof for panels to rest on. It is important that the rails are even so the panels can be mounted properly.
• Mounts connect the rails and the entire array to the roof. These mounts are often L-shaped brackets that are bolted through flashing and into the roof’s rafters. Mounts vary in design because of the many types of roofs and materials.
• Flashings are strong metal plates that create a water-resistant seal between the mounts and the roof. Often, caulk is used to attach the flashing to the roof, and it looks like a metal roof shingle.
• DC/AC wiring connects wires between panels and to a micro inverter or string inverter. Cables should not touch the roof surface or hang from the array to prevent damage from weather.
• Micro inverters are attached to the bottom of the panel and change the direct current (DC) power from the panels into alternating current (AC) power that can be sent to the electrical grid. Micro inverters help each panel work efficiently even when shaded and can provide data 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 system owners to sell extra electricity back to the grid.

In the mid-2000s, solar companies provided customers with financing options like leases and power purchase agreements. These plans let customers pay for solar panels over several years, and some payments were covered by credits from net metering programs. 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 from 2018 by the Consumer Energy Alliance found that federal, state, and local incentives, along with lower installation costs, helped increase the use of rooftop solar across the country. In 2016, residential solar power capacity grew by 20% compared to the previous year. The average cost of residential solar installations dropped 21% between 2015 and 2017, reaching $2.84 per watt. In eight states studied, government incentives for solar systems covered more than the cost of installation.

In 2019, the average cost for 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 rooftop systems because they benefit from economies of scale. These large systems can produce electricity at about half the cost of small rooftop systems.

In grid-connected rooftop solar systems, extra electricity can be sold to the local electric company for use elsewhere on the grid. This helps solar system owners recover their investment. A public utility commission usually decides the price the company pays for this electricity, which can be at the retail rate or a lower wholesale rate. These rates affect 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 grow the solar industry globally and created thousands of jobs. However, FITs can sometimes lead to problems if they are removed, causing a sudden drop in demand. These programs also support local energy production, reducing the need to transport electricity over long distances and lowering transmission losses.

Solar shingles

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

Hybrid systems

A rooftop solar power system (either on-grid or off-grid) can be used with other power parts like diesel generators, wind turbines, and batteries. These solar hybrid systems may be able to provide a continuous power supply.

Advantages

Installers can send solar electricity to the public power grid and receive a higher payment for each kilowatt-hour (kWh) of electricity they produce. This payment helps cover the extra costs of making solar electricity. In countries where grid electricity is unstable or more expensive, rooftop solar systems are more practical.

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

Disadvantages

A power system with 10% solar power would need 2.5% more load-frequency control (LFC) than a regular system. This problem can be solved by using synchronverters in the DC/AC circuit of the solar system. In 1996, the break-even cost for solar power was high when solar power made up less than 10% of the system. However, as solar power increases, the break-even cost decreases. But economic and LFC factors limit solar power to about 10% of the total system.

When replacing an asphalt shingle roof, solar panels must be removed and stored before the roof is fixed. They are reinstalled after the roof work is done. This process may cause power outages at the home. Solar installers would need to visit twice—once to remove the panels and again later to reinstall them. Their labor is usually more expensive than that of roofers who install asphalt shingles.

Technical challenges

There are technical problems when adding many rooftop PV systems to the power grid.

Thin film solar on metal roofs

Thin film solar panels are becoming a cost-effective choice for installation 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 wires connecting the panels run 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 but provide a faster 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.