About Project Sunroof

Why are we doing this?

Rooftop solar can be a great option for lowering energy bills. At Project Sunroof we want to make installing solar panels easy and understandable for anyone.

This work is core to Google's values and our aspiration to help individuals, cities, and other partners collectively reduce 1 gigaton of their carbon equivalent emissions annually by 2030. Project Sunroof and the Solar API are efforts to lead the transition to a more sustainable future by making information accessible, and by driving innovation forward.

Project Sunroof puts Google's expansive data in mapping and computing resources to use, helping calculate the best solar plan for you. When you enter an address, Project Sunroof looks up the building in Google Maps and combines that information with other databases to create your personalized roof analysis. (Don’t worry, Project Sunroof doesn't give the address to anybody else.)

How it works

Project Sunroof computes how much sunlight hits your roof in a year. It takes into account:

• Google's database of imagery and maps
• 3D modeling of your roof
• Shadows cast by nearby structures and trees
• All possible sun positions over the course of a year
• Historical cloud and temperature patterns that might affect solar energy production

Project Sunroof estimates values for the percent of electricity exported to the grid using data from NREL. The estimates are based on the relationship between the total solar electricity production and the total amount of electricity consumed by the household. The larger the solar installation is relative to the household electricity consumption, the higher the estimate of solar electricity exported to the grid.

Project Sunroof also calculates potential savings in utility costs as a result of installing rooftop solar. Actual savings can vary from projected savings for a variety of reasons. Fast-growing trees can shade solar installations, reducing production over time. Utilities can change how much they charge their customers for electricity, changing the savings from solar. Policies that are beneficial to solar installations may change (e.g., net metering). For states without net metering, savings may also vary by the amount of solar electricity consumed in the household compared to the amount exported to the grid.

To calculate solar energy production and projected savings, Project Sunroof uses the following data sources:

• Imagery, 3D modeling, and shade calculations using Google’s machine learning algorithms. Note that translating imagery into 3D models is not always precisely accurate, and the imagery can sometimes be out of date.
• Weather data from the National Renewable Energy Laboratory (NREL) and Meteonorm. Sometimes sharp transitions between nearby stations are reflected in the map.
• Utility electricity rates information from Clean Power Research.
• Solar incentives data from Clean Power Research, relevant federal, state and local authorities, and relevant utility websites.
• Solar Renewable Energy Credit (SREC) data from Bloomberg New Energy Finance, SRECTrade, and relevant state authorities.
• Aggregated and anonymized solar cost data from EnergySage and OpenSolar.

Solar energy production estimates depend on many factors, such as shading, typical weather in your area and equipment used. NREL, your solar provider, Project Sunroof and others may have different estimates, leading to different outputs. Additionally, Project Sunroof mapping data may be from a different period in time than other estimates, and thus may not show recent growth or removal of trees. Many solar providers will give you a final solar production estimate based on shade readings they take while inspecting your roof.

Solar potential estimates

Technical potential includes electricity generated by the rooftop area suitable for solar panels, assuming economics and grid integration are not a constraint. There are many definitions of technical potential, and other definitions may affect results by 25% or more. Based on Project Sunroof’s definition of technical potential, installations meet the following criteria:

• Sunlight: Every included panel receives at least 75% of the maximum annual sun in the county.
• Installation size: Every included roof has a total potential installation size of at least 2 kW.
• Space and obstacles: Only areas of the roof with enough space to install 4 adjacent solar panels are included. Obstacles like chimneys are taken into account.

Project Sunroof’s model makes the following assumptions:

• Each panel is assumed to be 400 W with an efficiency of 20.4%, a DC to AC derate factor of 85%, and industry-standard assumptions about other factors.
• Panels are assumed to be mounted flush with the roof, including on flat surfaces.
• Arrays are between 2 kW and 1000 kW.
• Only arrays on buildings are considered, not other spaces such as parking lots or fields.
• In cases where a building is not in a county, the maximum sun is determined by the max sun received in the city.

Because we are continuously improving the model, estimates are subject to change.

If you’re a company looking to leverage Sunroof data to improve your solar business, or looking for recent updates, please see the Solar API in Google Maps Platform.

Existing installations

• Project Sunroof trained a machine learning algorithm to identify solar installations in overhead imagery.
• As is the case with most machine learning algorithms, the results are not always 100% accurate. Things that are not solar installations sometimes get marked as solar, and sometimes the algorithm misses actual solar installations.
• In this algorithm both solar PV systems (for electricity) and solar hot water systems are identified as existing solar installations.
• Google’s imagery is collected periodically, and due to the rapid growth of solar in some regions, comparisons of the Project Sunroof data to other sources may yield different values.

Choose what you need

Project Sunroof recommends an installation size to generate close to 100% of your electricity use, based on roof size, the amount of sun hitting the roof, and your electricity bill.

We recommend an installation that covers less than 100% of your electrical usage because, in most areas, there is little financial benefit to producing more power than you can consume.

Computing your savings

Project Sunroof uses current solar industry pricing data to run the numbers on leasing, taking a loan, or buying solar panels for your house to help you choose what's best for you.

Project Sunroof also compiles the following incentives to calculate your final cost:

• Federal and state tax credits
• Utility rebates
• Renewable energy credits and net metering

More about financial estimates

Taking next steps

If you’d like to know more or to get started on going solar, contact a solar provider in your area.