Peak Sun Hours Calculator — Find Your Location’s Daily Solar Energy by City and State

A peak sun hours calculator tells you how many hours of usable, full-strength sunlight your location receives each day — the single most important number for sizing a solar system correctly. Select your US city or region, adjust for local weather, shading, and mounting type, and the calculator returns your effective daily PSH, peak summer output, winter low, seasonal drop percentage, and a month-by-month chart showing exactly how your solar resource shifts across the year.

How to Use the Peak Sun Hours Calculator

Step 1 — Name your project and select your location.

Enter a project or property name for the PDF report, then choose your location from the region dropdown. The calculator includes real historical PSH data — not generic approximations — for eleven specific cities and regions. US options include Phoenix, Arizona (6.5 PSH annual average, the strongest US solar market), Los Angeles, California (5.5 PSH), Chicago, Illinois (4.2 PSH), and New York, New York (4.0 PSH).

International locations include Madrid, Berlin, London, Sydney, Cape Town, and Mumbai, each with city-specific monthly curves that reflect actual irradiance data including monsoon dips and coastal fog patterns.

Step 2 — Use Custom Input for locations not in the list.

Select Custom Manual Input at the bottom of the dropdown to enter your own annual daily average in kWh/m²/day. Find this figure for your exact address using NREL’s free PVWatts tool at pvwatts.nrel.gov — enter your address and the tool returns your location’s annual average PSH based on 30 years of historical TMY3 weather data. Also select your hemisphere when using custom input, as this determines whether the seasonal curve peaks in June-July or December-January.

Step 3 — Set your microclimate and weather modifier.

This adjustment accounts for local conditions that deviate from the regional historical average. Choose Highly Clear or High Altitude (+10%) for locations with consistently clear skies — high-elevation areas in Colorado, Nevada, or New Mexico qualify because thinner atmosphere scatters less sunlight. Choose Average Historical Data (0%) for standard conditions.

Choose Frequent Fog or Marine Layer (-10%) for coastal locations where morning overcast regularly suppresses early-day production — San Francisco, Seattle, and Portland are prime examples. Choose Heavy Rain or Cloudy Climate (-20%) for consistently overcast markets.

Step 4 — Choose your mounting and tracking type.

Fixed Mount applies no multiplier — this is the standard residential rooftop installation where panels are set at a fixed angle and never move. Single-Axis Tracker adds 20% to your effective PSH by rotating panels east to west throughout the day. Dual-Axis Tracker adds 35% by adjusting both horizontal rotation and tilt angle — used in commercial and utility ground-mount installations.

For most US homeowners, Fixed Mount is the correct selection. The tracker options are most relevant for commercial ground-mount system design.

Step 5 — Set your local shading penalty.

Use the slider to subtract a percentage from your effective PSH based on shading from trees, neighboring buildings, chimneys, or other obstructions. At 0% no shading penalty is applied. At 10–20% the tool models partial shading that affects a portion of the array during peak hours. At 30–50% the tool models severe shading scenarios where obstacles block significant sun during midday.

The insights section below the results quantifies exactly how many PSH hours your shading penalty costs you per day and flags when trimming trees would be more cost-effective than adding panels.

Step 6 — Read the results.

The Effective Daily Peak Sun Hours banner shows your site’s adjusted daily PSH — the usable figure to use for all system sizing calculations. The Annual Averages card breaks down your regional baseline, tracking multiplier, and combined weather-and-shade modifier so you can see exactly how each factor contributes to your final number.

The Seasonal Extremes card shows your peak summer PSH, winter minimum PSH, and the percentage drop between them — critical data for off-grid sizing and battery storage design.

Step 7 — Analyze the monthly chart.

Twelve amber bars show your effective PSH for each month of the year. Hover over any bar to see the exact figure. For northern US states, the drop from July (peak) to December (trough) can be 60–80% — meaning a system sized for summer production will dramatically underperform in winter. Off-grid designers must size their battery and panel capacity for the worst winter month, not the annual average.

Step 8 — Export your site report.

Click Export PDF to save a printable PSH analysis report labelled with your project name. Useful for system sizing documentation, installer bid comparisons, or off-grid energy planning worksheets.

The Peak Sun Hour Formula Explained

Peak Sun Hours is not a measure of daylight duration — it is a measure of energy density. The formula is:

1 PSH = 1 hour at 1,000 W/m² irradiance

A 14-hour summer day in Chicago might deliver only 6.2 PSH because the early morning and late evening hours contribute only 100–300 W/m², not the full 1,000 W/m² reference intensity. The calculator uses this accumulated energy concept to predict system output:

Estimated Daily Production (kWh) = System Size (kW) × Effective PSH

Example for a New York home:

• System size: 8 kW
• Regional baseline: 4.0 PSH (New York)
• Weather modifier: 1.0
• Fixed mount: 1.0
• Shading penalty: 10% → modifier 0.90
• Effective PSH = 4.0 × 1.0 × 1.0 × 0.90 = 3.6 PSH
• Estimated daily production = 8 × 3.6 = 28.8 kWh/day
• Estimated annual production = 28.8 × 365 = 10,512 kWh/year

Frequently Asked Questions

Q: What are peak sun hours and how are they different from daylight hours?

A: Peak sun hours measure the total solar energy received in a day, not the duration of daylight. One peak sun hour equals one hour of sunlight at an intensity of exactly 1,000 watts per square meter — the standard reference intensity used to rate solar panels.

In practice, the sun is only at full 1,000 W/m² intensity around solar noon on a clear day. Morning and evening hours contribute a fraction of that intensity.

So a city with 12 hours of daylight might only accumulate 5.5 PSH of actual energy. This is why Phoenix with 14 hours of summer daylight gets 8.8 PSH in June, while London with similar daylight duration gets only 5.2 PSH — the sun angle and atmospheric clarity matter as much as hours above the horizon.

Q: How many peak sun hours does my US state get?

A: Peak sun hours vary significantly across the United States based on latitude, altitude, and local climate. Arizona leads with 6.0–7.0 PSH annually in most cities. Nevada and New Mexico average 5.8–6.5 PSH. California ranges from 4.5 PSH in foggy San Francisco to 6.5 PSH in the Inland Empire and desert communities. Texas averages 5.0–5.5 PSH.

Colorado reaches 5.0–5.5 PSH despite its northern latitude thanks to high altitude reducing atmospheric scatter. The Midwest averages 4.0–4.8 PSH: Chicago 4.2, Denver 5.0, Kansas City 4.8. The Northeast delivers 3.8–4.2 PSH: New York 4.0, Boston 4.0, Philadelphia 4.2. The Pacific Northwest is the weakest US market: Seattle 3.5, Portland 3.8, with heavy winter deficits.

Q: Why do peak sun hours matter so much for solar panel sizing?

A: Peak sun hours are the multiplier that converts your panel’s rated wattage into actual kilowatt-hours of electricity. A 400W panel in Phoenix at 6.5 PSH produces approximately 400 × 6.5 = 2,600 watt-hours (2.6 kWh) per day before losses. The same 400W panel in Seattle at 3.5 PSH produces only 400 × 3.5 = 1,400 Wh (1.4 kWh) per day — 46% less from an identical panel.

This is why a solar system in Arizona can be significantly smaller than one in Washington state while still meeting the same household’s electricity needs.

Using the wrong PSH figure when sizing a system leads to either an undersized array that doesn’t cover your bills or an oversized one that wastes money on unnecessary panel capacity.

Q: What is the best free tool to find exact peak sun hours for my address?

A: NREL’s PVWatts Calculator (pvwatts.nrel.gov) is the gold standard for US locations. It uses TMY3 data — 30 years of hourly weather records from over 1,000 US stations — to calculate location-specific annual and monthly PSH for any US address. Enter your address, set your system parameters, and PVWatts returns monthly kWh production estimates you can back-calculate to PSH.

The NASA POWER database (power.larc.nasa.gov) provides satellite-derived global irradiance data at 0.5-degree resolution for locations between weather stations. Both tools are free. The peak sun hours calculator on this page uses curated regional averages from these same data sources — ideal for quick estimates, while PVWatts is recommended for final system sizing and financial projections.

Q: How does shading affect peak sun hours and what should I do about it?

A: Shading reduces effective PSH proportionally — 20% shading penalty reduces a 5.5 PSH location to 4.4 effective PSH, costing the equivalent of over an hour of peak sun every day. More critically, in string inverter systems, partial shading of even one panel can reduce output across the entire string by 30–50% during shaded hours — the actual impact is often worse than the shading percentage alone suggests.

Physical mitigation options include trimming or removing trees, relocating the array to an unshaded roof section, or splitting the array across multiple roof faces. Technology mitigation includes power optimisers or microinverters, which isolate each panel’s output and prevent one shaded panel from dragging down the entire string.

Before adding panels to compensate for shading, calculate whether removing the shading obstacle is more cost-effective.

Q: How do I use peak sun hours to size my solar battery storage?

A: For grid-tied systems, use your annual average PSH for system sizing. For off-grid systems, always design to the winter minimum PSH — not the annual average — to ensure you can cover your energy needs during the least productive months. Find your winter low PSH from the monthly chart, then calculate: Battery Capacity (kWh) = Daily Load (kWh) ÷ Depth of Discharge × Autonomy Days.

If your Chicago winter drops to 1.6 PSH in December and your daily load is 10 kWh, your 8 kW system produces only 8 × 1.6 = 12.8 kWh/day before losses — leaving very little margin after the derate factor. Off-grid designers in northern states typically need significantly larger arrays and battery banks than the annual average PSH would suggest.

Q: What peak sun hours should I use for sizing an RV or van solar system?

A: For a mobile system that will travel across the US, use a conservative figure that reflects your typical travel range and season. If you spend winters in the Southwest and summers in the Pacific Northwest, averaging the two regions’ PSH gives a reasonable planning figure — approximately 4.5–5.0 PSH for that travel corridor. If you primarily stay in one region, use that region’s annual average.

RV systems are better designed around winter minimums than annual averages because RV users often rely on solar as their primary or only power source. A system that covers your needs at 3.5 PSH will work comfortably anywhere from Yuma, Arizona to southern Canada in summer.

Q: Why is Chicago’s peak sun hours similar to New York but both are much less than Phoenix?

A: Latitude explains the difference between the US Southwest and Northeast — Phoenix sits at 33°N while Chicago is at 42°N and New York at 40°N, meaning the sun is consistently higher in the sky in Phoenix, reducing atmospheric path length and boosting irradiance.

The similarity between Chicago (4.2 PSH) and New York (4.0 PSH) despite their similar latitudes comes down to local climate patterns — both cities have significant cloud cover and winter snow, with Chicago’s inland continental climate producing slightly more clear-sky days than New York’s coastal-influenced weather.

Denver (5.0 PSH) sits at 39°N — nearly the same latitude as New York — but achieves far higher PSH because its mile-high altitude places it above much of the atmospheric moisture that scatters and absorbs sunlight at sea level.