Understanding Solar Irradiance and Peak Sun Hours: Location-Based Solar Math

What Are Peak Sun Hours?

A peak sun hour is defined as one hour of solar irradiance at 1,000 watts per square meter (1,000 W/m²) — the intensity of direct noon sun on a clear day. This is a standardized measurement, not the actual number of daylight hours. For example, a location might receive 6 hours of daylight but only 4 peak sun hours because morning and afternoon sun is less intense. Peak sun hours (PSH) represent the total daily solar energy received, expressed as equivalent hours of peak intensity. A location receiving 5,000 Wh/m² per day has 5 PSH (5,000 ÷ 1,000 = 5). This number is the foundation of all solar system sizing calculations.

Peak Sun Hours by US Region

Peak sun hours vary dramatically by latitude, climate, and season. Annual averages for major US cities: Phoenix, AZ: 6.5 PSH (summer 8.0, winter 4.5). Los Angeles, CA: 5.8 PSH. Denver, CO: 5.5 PSH. Miami, FL: 5.4 PSH. Dallas, TX: 5.3 PSH. Atlanta, GA: 5.0 PSH. Chicago, IL: 4.2 PSH. New York, NY: 4.1 PSH. Seattle, WA: 3.8 PSH. Portland, OR: 3.7 PSH. Boston, MA: 4.0 PSH. These are annual averages on a south-facing surface tilted at latitude angle. Actual production varies monthly — December values can be 40-60% below June values in northern climates. For off-grid system sizing, always use the worst-month PSH (typically December) rather than the annual average.

How to Find Your Exact Peak Sun Hours

Three reliable methods: (1) NREL PVWatts Calculator (pvwatts.nrel.gov) — Enter your address, system size (1 kW), and panel tilt. The tool outputs monthly and annual PSH values specific to your location with weather data from 20+ years of measurements. This is the gold standard and it is free. (2) Solar insolation maps — NREL publishes maps showing average daily solar radiation by region. Useful for rough estimates but less precise than PVWatts. (3) Local weather stations — Some airports and universities publish solar radiation data. Search '[your city] solar insolation data' for local resources. For power station sizing, you need the monthly PSH for your specific location to ensure adequate winter charging.

From Peak Sun Hours to Panel Wattage

The calculation is straightforward: Daily energy needed (Wh) ÷ Peak Sun Hours = Minimum panel wattage required. Example: You need to recharge a 1,000Wh power station daily in Portland, OR (3.7 PSH annual average, 2.1 PSH in December). For December reliability: 1,000Wh ÷ 2.1 PSH = 476W of panels minimum. Add 25% for real-world losses (panel temperature, dust, wiring): 476 x 1.25 = 595W. Round up to 600W — which conveniently matches the maximum solar input of the Anker C2000 Gen 2 and Jackery 2000 v2. In summer, those same 600W panels produce excess energy, fully charging the unit in under 2 hours. Size for winter, enjoy excess in summer.

Seasonal Variations and System Design

Seasonal variation is the biggest challenge in solar system design. In Phoenix, summer PSH (8.0) is only 78% higher than winter (4.5) — manageable variation. In Boston, summer PSH (5.8) is 190% higher than winter (2.0) — extreme variation requiring winter-specific planning. Strategies for high-variation climates: (1) Oversize panels by 2-3x versus summer needs to ensure winter adequacy. (2) Use adjustable tilt mounts — increase tilt angle to 60-70° in winter to capture low-angle sun. (3) Accept reduced winter autonomy — plan for partial charging and supplement with grid or generator. (4) Track the sun — portable panels can be repositioned throughout the day for maximum exposure. For power station users, portable folding panels offer the advantage of easy repositioning to follow the sun.

Angle, Orientation, and Shading Effects

Panel angle significantly affects production: Fixed panels should tilt at your latitude angle for year-round optimization. Winter optimization: tilt latitude + 15°. Summer optimization: tilt latitude - 15°. A panel lying flat (0° tilt) loses 10-15% production versus optimally tilted panels. Orientation: South-facing is ideal in the northern hemisphere. Southeast or southwest facing loses 5-10% versus south. East or west facing loses 15-20%. North-facing should be avoided if possible. Shading: Even partial shading dramatically reduces output. A shadow covering just 10% of a panel can reduce output by 30-50% due to how cells are wired in series. Keep panels completely unshaded during peak sun hours (10 AM - 2 PM) for maximum production.

Real-World Factors That Reduce Production

Several factors reduce real-world production below theoretical PSH values: Temperature — panels lose 0.3-0.5% efficiency per °C above 77°F. A hot summer panel at 120°F produces 10-15% less than rated. Dust and dirt — unwashed panels lose 5-15% output. Rain naturally cleans panels; clean monthly in dry climates. Panel age — panels degrade 0.5-0.8% per year. After 10 years, a 200W panel produces 185-190W. Wiring losses — 2-5% loss in cables and connections. Use short, thick cables. Inverter/charge controller losses — 5-10% conversion loss in the power station's MPPT controller. Combined, expect 70-80% of theoretical PSH production in real-world conditions. The 25% oversizing recommendation accounts for these cumulative losses.

FAQ

How many peak sun hours do I need to fully charge my power station?

Divide your power station's capacity by your panel wattage and divide by 0.75 (real-world efficiency factor). Example: 1,000Wh ÷ 400W ÷ 0.75 = 3.3 peak sun hours. If your location averages 4+ PSH, 400W of panels reliably recharges a 1,000Wh unit daily.

Can I charge my power station on cloudy days?

Yes, but at reduced speed. Clouds reduce solar irradiance to 10-25% of clear-sky levels. A 600W array might produce 60-150W in overcast conditions. This is enough to slow battery drain but not fully recharge a large unit. For reliable off-grid power, size your array for cloudy day production or have backup charging options.

Do portable solar panels produce the same as rigid panels?

Portable folding panels typically achieve 18-20% efficiency versus 20-22% for rigid glass panels. The difference is 5-10% in practice. The portability advantage often outweighs the modest efficiency loss for power station users. Rigid panels are better for permanent installations; portable panels excel for camping and emergency use.

How does daylight saving time affect solar charging?

Daylight saving time does not change the sun's position — it only shifts clock time. Solar noon (peak sun) occurs at approximately 12:30 PM during DST, not 1:30 PM. Position your panels for true solar noon, not clock noon. The number of peak sun hours is unaffected by DST.

Can I use peak sun hour data from a nearby city?

If the city is within 50 miles and at a similar elevation, PSH data is reasonably accurate. For locations more than 50 miles away or at significantly different elevations, use PVWatts with your specific address. Microclimates (coastal fog, mountain valleys) can create substantial differences even between nearby locations.

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