Definitions for every metric shown in the solar opportunity tables. All data sourced from Google Project Sunroof and NREL Cambium grid emissions data.
The number of buildings in a region whose rooftops are geometrically and physically suitable for solar panel installation. Sunroof evaluates shade patterns, roof orientation, roof area, and local sun angle to determine suitability.
The share of buildings in a region for which Project Sunroof has collected satellite imagery and completed a rooftop analysis. A high % Covered means the Suitable Buildings count is comprehensive; a lower value means some buildings were not analyzed.
Of the buildings that Sunroof has analyzed (% Covered), the share whose rooftops are classified as viable for solar installation. A building is "qualified" if it has enough unshaded area in a favorable orientation to host at least one panel.
The total annual electricity generation potential across all qualified rooftops in the region, measured in kilowatt-hours per year. This represents the maximum theoretical output if every viable rooftop were fully outfitted with solar panels.
The total number of standard solar panels that could physically fit across all qualified rooftops in the region, given typical panel dimensions and spacing requirements.
The metric tons of carbon dioxide emissions that would be avoided per year if every qualified rooftop in the region were converted to solar. Calculated using regional grid emission intensity factors.
The median number of solar panels that would fit on a qualified rooftop in this region. The median is used rather than the mean to avoid distortion from unusually large commercial rooftops.
The combined nameplate generating capacity, in kilowatts (kW), of all the solar systems that could be installed across every qualified rooftop in the region. Also expressed as MW or GW for large regions.
The median system size, in kilowatts, for a single qualified building in this region. This is a useful proxy for the "typical" solar installation size a homeowner or small business in the region could install.
The number of solar systems that are already installed and operating in the region, based on permit data, utility interconnection records, and other administrative sources aggregated by Project Sunroof.
Solar irradiance is the amount of light (electromagnetic energy) that falls on a given surface area at a given moment. It is measured in kilowatts (kW) — a unit of power, or the rate of energy delivery. A surface exposed to peak sunlight receives roughly 1 kW of irradiance per square meter. Irradiance varies continuously with cloud cover, time of day, season, and geographic latitude.
Solar insolation is the average solar irradiance an area receives over a period of time, expressed in kilowatt-hours per kilowatt (kWh/kW) — or equivalently, in "sun hours." While irradiance tells you how intense the sunlight is right now, insolation tells you the total usable sunlight over a year. A roof with 1,614 kWh/kW/year of insolation will produce 1,614 kWh of electricity per year for every 1 kW of solar panels installed on it. A 10 kW system at the same location would produce roughly 16,140 kWh/year.
A sun hour (also written as a "peak sun hour") is defined as one hour in which the intensity of sunlight averages 1,000 Watts (1 kilowatt) of energy per square meter. 1 kWh/kW of insolation equals exactly 1 sun hour. Sun hours are the most intuitive way to communicate a location's solar productivity: a city with 5 sun hours per day will produce 5 kWh per day from a 1 kW panel array, or 50 kWh per day from a 10 kW system, before system losses.
In the Google Solar API, flux is defined as the annual amount of sunlight falling on a roof surface, measured in kWh/kW/year (equivalent to sun hours per year). The API calculates flux for every point on a roof by combining: (1) hourly solar irradiance data from weather datasets on a 4–10 km grid; (2) the computed position of the sun at every hour of the year for that location; (3) shading from nearby trees, buildings, and other roof sections; and (4) the pitch and compass orientation (azimuth) of each roof facet. The result is a spatially detailed map of how much productive sunlight each square foot of a roof receives.
The Google Solar API defines "sunniness" as the relative level of sunlight received by a particular section of a roof compared to the rest of the same roof, averaged annually. Different parts of the same roof can have dramatically different sunniness due to chimneys, dormers, shading from nearby trees, or adjacent buildings. The API reports sunniness as 11 quantile buckets (deciles) — from the darkest 10% to the sunniest 10% of a roof's surface area. This lets installers identify which roof facets are worth covering with panels and which are shaded enough to drag down system performance.
The Investment Tax Credit (ITC) lets homeowners and businesses deduct 30% of the total cost of a solar energy system directly from their federal income taxes. Enacted as part of the Inflation Reduction Act of 2022, the 30% rate is locked in through 2032, stepping down to 26% in 2033 and 22% in 2034 before expiring for residential installations. On a $33,500 gross system cost, for example, the credit is worth $10,050 — reducing your net cost to $23,450.
Net metering is a utility billing arrangement that credits solar system owners for excess electricity they export to the grid. When your panels produce more power than your home uses — typically on sunny midday hours — the surplus flows back to the grid and your utility credits your bill, usually at or near the retail electricity rate. At night or on cloudy days, you draw from the grid and those credits offset what you owe, effectively using the grid as a free battery.
A projection of the marginal energy cost ($/MWh) for a specific U.S. grid region through 2050, drawn from NREL's Cambium 2024 dataset. Each line represents a distinct scenario: Mid-case (reference), Low/High Renewable Energy Costs (sensitivity to how fast solar and wind costs fall), Low/High Natural Gas Prices (fuel cost sensitivity), and High Demand Growth (electrification pressure). The county detail page shows the trajectory for the GEA (Grid Emissions Avoided) sub-region that contains the county. The heatmap companion chart shows the same Mid-case costs broken down by hour of day and month of year.
In addition to the federal ITC, most U.S. states offer their own financial incentives to encourage solar adoption. Common programs include: state income tax credits (e.g., New York's 25% credit up to $5,000); upfront cash rebates from state energy offices or utilities; sales tax exemptions on solar equipment purchases; and property tax exemptions that prevent the added home value from solar from raising your annual property tax bill. The combination of available programs — and their dollar value — varies widely by state.