What Are Peak Sun Hours?
Peak sun hours are not the same as daylight hours. One peak sun hour equals one hour of sunlight at 1,000 watts per square meter — the intensity used to rate solar panels. A location with 5 peak sun hours might have 10 hours of daylight, but only 5 hours' worth of strong enough sun to match rated output.
Phoenix averages 6.5 peak sun hours. Seattle averages 3.5. London sits around 2.5-3. This single number is the biggest factor determining your system's actual output. You can look up your location on the NREL PVWatts tool or Global Solar Atlas for an accurate local figure.
Seasonal variation matters too. Phoenix drops from about 7.5 peak sun hours in June to 5.2 in December. If you are sizing an off-grid system, use the worst month — not the annual average — or you will run short in winter. Our solar panel angle calculator shows how tilt adjustments can help maximise winter capture.
Where the 15% System Loss Comes From
Solar panels never hit their rated output in real conditions. Temperature alone costs 5-10% — panels are rated at 25°C (77°F) but rooftop panels regularly reach 50-60°C in summer. Every degree above 25°C reduces output by about 0.35% for monocrystalline panels. On a 40°C day, surface temperatures can reach 65°C, costing you roughly 14% from temperature alone.
Wiring resistance loses another 2-3%. Dust and dirt reduce output 2-5% depending on cleaning frequency. Inverter conversion adds 3-5% loss. Shading is the wild card — even partial shade on one cell can cut an entire string's output by 30-50% if your system lacks optimisers or microinverters.
The 15% default is conservative for a clean, well-installed system. Increase it to 20-25% if panels sit under trees, collect heavy dust, or face anything less than due south (in the northern hemisphere). Flat-mounted panels on commercial roofs often need 20% to account for suboptimal tilt.
Worked Examples
Residential Rooftop in North Carolina
Context
Calculation
Daily output = Panel Wattage × Sun Hours × (1 − Loss%) × Panel Count
= 400W × 4.7h × 0.85 × 8
= 400 × 4.7 × 0.85 × 8 = 12,784Wh (12.8kWh per day)
Monthly output = 12,784 × 30 = 383,520Wh (383.5kWh/month)
Interpretation
This 3.2kW array produces roughly 12.8kWh per day, covering about 43% of the average US household's 30kWh daily consumption. In summer months with 6+ sun hours, output climbs to 16+ kWh/day. In December with 3.5 sun hours, it drops to around 9.5kWh/day.
Takeaway
An 8-panel array is a solid start for offsetting grid usage, though not full self-sufficiency. To store excess daytime production for evening use, you'll want to size a battery bank matched to your nighttime consumption.
Portable Camping Setup in Arizona
Context
Calculation
Daily output = 100W × 6h × 0.75 × 2 panels
= 100 × 6 × 0.75 × 2 = 900Wh (0.9kWh per day)
Interpretation
At 900Wh/day, this setup comfortably charges phones, runs LED lights, and keeps a small 12V cooler cold. It won't handle heavy loads like a microwave or hair dryer.
Takeaway
If 900Wh isn't enough for your camping loads, use our battery runtime calculator to figure out exactly which appliances fit within your daily energy budget before your next trip.
Frequently Asked Questions
Glossary
Peak Sun Hours
The number of hours per day when solar irradiance averages 1,000 W/m². A location with 5 PSH receives the energy equivalent of 5 hours at full-rated intensity, even though actual daylight may last 10-12 hours.
System Losses
The combined efficiency reductions from temperature, wiring resistance, dust, inverter conversion, and shading. A well-maintained system typically loses 12-18% of rated output to these factors.
Irradiance
The power of sunlight hitting a surface, measured in watts per square meter (W/m²). Standard test conditions use 1,000 W/m² as the reference for panel ratings.
Degradation Rate
The annual percentage reduction in a solar panel's output over its lifespan. Most modern panels degrade 0.3-0.5% per year, retaining 80-85% of original output after 25 years.
Wondering how long it takes to recharge your batteries from solar? Our battery charge time calculator gives you a clear answer. Try it now →
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Solar output varies daily and seasonally. The number this calculator gives you is a useful average — some days you will produce more, some less. For off-grid systems, size your panels and batteries for the worst month, not the average. For grid-tied setups, annual totals matter more than any single day. To evaluate the financial payback of your system, run your production figures through our solar ROI calculator.
Last updated:
Written and maintained by Dan Dadovic, Commercial Director at Ezoic Inc. & PhD Candidate in Information Sciences. He works professionally as Commercial Director at Ezoic Inc., leading revenue strategy across digital publishing.
Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.