Solar Potential by Zip Code

Solar irradiance measures the amount of solar energy hitting a surface per unit area. This calculator expresses it in kWh/m²/day — kilowatt-hours per square meter per day — which is the same as peak sun hours. One peak sun hour equals one hour of sunlight at an intensity of 1,000 watts per square meter (full midday sun).

A location with 5 kWh/m²/day receives the equivalent of five full-intensity sun hours daily when averaged across the year. Higher numbers mean more raw solar energy available to convert into electricity.

The figures shown here come from long-term averages that account for cloudy days, seasonal changes, and local climate — not just clear-sky maximums. This makes them a realistic baseline for estimating how much solar energy your area gets throughout the year.

Several factors determine how much solar irradiance a zip code receives:

• Latitude — Locations closer to the equator receive sunlight at a more direct angle, concentrating more energy per square meter. This is why Phoenix outperforms Fairbanks by a wide margin year-round.
• Cloud cover and weather patterns — Coastal areas and the Pacific Northwest have persistent marine clouds that reduce annual averages even at favorable latitudes. The Southwest's dry, clear skies make it the highest-producing region in the US.
• Altitude — Higher elevations have less atmosphere to scatter sunlight, which slightly increases irradiance. This benefits parts of Colorado and New Mexico.
• Seasonal swing — Northern locations see dramatic month-to-month variation. A Minnesota winter averages around 2 kWh/m²/day while summer can reach 6+. Southern states stay more consistent year-round.

The US average is roughly 4–5 kWh/m²/day, with the Southwest reaching 6–7 and the far North dipping below 3 in winter months.

Your area's irradiance number is the starting point for sizing a solar system. A rough output estimate uses this formula:

Daily output (kWh) = Panel wattage × Peak sun hours × System efficiency

For example, a 400-watt panel in a location with 5 peak sun hours and 80% system efficiency produces about 1.6 kWh per day (0.4 kW × 5 h × 0.80). A typical US home uses 30 kWh/day, so you'd need roughly 19 panels at that location.

The same panel in Phoenix (6.5 peak sun hours) produces about 2.1 kWh/day — 30% more output than at the US average. In Fairbanks (2.5 peak sun hours), output drops to 0.8 kWh/day. This directly affects how many panels you need and how quickly a system pays for itself.

Keep in mind that irradiance is just one piece of the picture. Your roof's tilt and orientation, local shading from trees or neighboring buildings, and equipment efficiency all affect real-world production. Use this tool to understand your area's raw solar potential, then consult a local installer for a site-specific estimate.

Most of the contiguous US has enough solar resource to make rooftop solar financially viable — the annual irradiance rarely drops low enough to rule it out entirely. That said, some locations are meaningfully better than others:

• Above 5 kWh/m²/day — Excellent. The Southwest (Arizona, Nevada, New Mexico, Southern California) falls here. Systems are smaller, cheaper, and pay back faster.
• 4–5 kWh/m²/day — Good. Most of the South, Texas, the Mountain West, and the Midwest. Solar pencils out well at standard electricity rates.
• 3–4 kWh/m²/day — Moderate. The Pacific Northwest, Great Lakes region, and parts of New England. Solar still works but requires a larger system for the same output.
• Below 3 kWh/m²/day — Low, mostly seen in Alaska. Payback periods are longer, though high electricity prices can still make solar worthwhile.

High local electricity rates can make solar economically attractive even in moderate-sun areas. A location with 3.5 peak sun hours but electricity at $0.25/kWh may offer a better return than a sunnier location where power is cheap.