Peak sun hours (sometimes called equivalent sun hours or daily peak solar insolation) tell you how much usable solar energy hits your site in one day, expressed as a number of hours at 1,000 watts per square meter (W/m²). That 1,000 W/m² benchmark is the same intensity labs use for panel nameplate ratings. A long summer day might have 14 hours of daylight but only 5–6 peak sun hours, because morning and evening light is weaker. You need that single daily number—not raw daylight hours—to size panels and batteries without winter failures.
For US zip-level lookups and state tables, see Peak Sun Hours by Zip Code. Once you have your worst-month value, plug daily watt-hours into the WattSizing Calculator.
What Are Peak Sun Hours?
Peak sun hours compress a full day of changing sunlight into one equivalent number of "perfect sun" hours at 1,000 W/m².
Solar panels are rated under Standard Test Conditions (STC), which include 1,000 W/m² irradiance, 25°C cell temperature, and a fixed spectrum. In the field, the sun rarely sits at that angle and intensity all day. Rather than integrate every minute of production, planners add up the day's energy and divide by 1,000 W/m².
Example: If your roof receives the same total energy as four hours of perfect 1,000 W/m² sun, your site has 4 peak sun hours that day—regardless of whether the real sun was up for 10 or 14 clock hours.
Equivalent Sun Hours and Peak Solar Insolation (W/m²)
You will see the same idea under different names:
| Term | What it means |
|---|---|
| Peak sun hours | Daily solar energy ÷ 1,000 W/m², in hours |
| Equivalent sun hours | Same metric—hours of 1,000 W/m² equivalent |
| Peak solar insolation | Instantaneous or averaged irradiance, often quoted in W/m² or kWh/m²/day |
| PSH / PSH/day | Abbreviation for peak sun hours per day |
Peak solar insolation at 1,000 W/m² is the reference line. When maps or tools report 5.2 kWh/m²/day for a month, that is roughly 5.2 peak sun hours for that period (same energy, different units).
Off-grid sizing almost always uses the worst month PSH, not the annual average, so December (Northern Hemisphere) or June (Southern Hemisphere) numbers drive array size.
Peak Sun Hours vs Peak Hours of Sun (Daylight)
These are not interchangeable:
- Daylight / peak hours of sun (colloquial): Clock time from sunrise to sunset, or informal "sunny part of the day."
- Peak sun hours (technical): Energy-based. A cloudy 12-hour day might still show 12 hours of daylight but only 2–3 peak sun hours.
Sizing with daylight hours will undersize winter off-grid systems. Always use irradiance-based peak sun hours from a map or calculator.
The Sizing Formula You Actually Need
Array Watts = Daily Energy Need (Wh) ÷ Peak Sun Hours ÷ System Efficiency
Typical real-world system efficiency (charge controller, wiring, heat, soiling): 0.75 to 0.80.
Illustrative example (Ohio cabin, December):
- Daily load: 2,400 Wh
- Worst-month peak sun hours: 2.5
- Efficiency: 0.75
2,400 ÷ 2.5 = 960 W(raw array before losses)960 ÷ 0.75 = **1,280 W**(four 320 W panels)
Using summer's ~5.5 peak sun hours instead would suggest only ~581 W—the system would fail from November through February. See Winter and Low-Sun Sizing: Design for Your Worst Month for worst-month planning.
Peak Sun Hours in My Area: Maps and Lookup Tools
Searchers often ask for a peak solar hours map or peak sun hours in my area. Reliable options:
| Tool | Best for | What to read |
|---|---|---|
| NREL PVWatts | US addresses | Monthly Solar Radiation (kWh/m²/day) ≈ peak sun hours |
| Global Solar Atlas | Worldwide | Global Horizontal Irradiation (GHI) maps and site data |
| Peak Sun Hours by Zip Code | US quick reference | State/zip context and sizing tie-in |
How to pull the right number:
- Enter your address or coordinates.
- Open the monthly table (not annual average alone).
- Note the lowest month's value—that is your off-grid design peak sun hour.
- Match tilt and azimuth to your real mount (flat RV roof ≠optimal tilt on the map).
Maps assume average weather history. Micro-shading from trees or chimneys can cut your real peak sun hours below the map.
Typical Peak Sun Hours by Region and Season
Regional averages help sanity-check your map lookup:
| Region | Summer average | Winter average | Annual average |
|---|---|---|---|
| Southwest USA (e.g., Arizona) | 6.5–7.5 h | 4.0–5.0 h | 5.5–6.5 h |
| Northeast USA (e.g., New York) | 5.0–6.0 h | 2.0–3.0 h | 3.5–4.5 h |
| Australia (e.g., Queensland) | 6.0–7.0 h | 4.5–5.5 h | 5.0–6.0 h |
| Northern Europe (e.g., UK) | 4.5–5.5 h | 1.0–2.0 h | 2.5–3.5 h |
Illustrative ranges only. Your site, tilt, and shading change the real peak sun hour.
What Most Guides Skip
- Worst-month rule for off-grid: Size on the month with the fewest peak sun hours, not the annual average.
- System derate: Heat, cable loss, MPPT conversion, and dust often cost 20–25%—use 0.75–0.80 in the formula.
- Mounting angle: Tables assume optimal tilt toward the equator. Flat mounts on vans lose winter peak sun hours when the sun stays low.
- Load vs sun data: "How to check load" for solar means two steps—(a) measure or estimate daily Wh load, (b) pair it with site peak sun hours from a map. Do not confuse electrical load (watts) with solar resource (peak sun hours).
Practical Checklist
- Get site data: PVWatts (US) or Global Solar Atlas (global).
- Pick worst month: Lowest kWh/m²/day in the monthly table.
- Match your mount: Enter real tilt/azimuth if the tool allows.
- List daily Wh: Add appliances and autonomy days you need.
- Run the formula or use the WattSizing Calculator with your worst-month peak sun hours.
FAQs
What are peak sun hours in simple terms?
They are the number of hours per day of full-strength sun at 1,000 W/m² that would deliver the same total energy your location actually receives. One peak sun hour equals one hour at that reference intensity.
Are equivalent sun hours the same as peak sun hours?
Yes. Equivalent sun hours and peak sun hours describe the same daily energy metric, just different wording in datasheets and maps.
What is peak solar insolation in W/m²?
Solar insolation is power per area (W/m²). Peak refers to the STC reference of 1,000 W/m². Daily maps often report energy (kWh/m²/day), which converts directly to peak sun hours.
How do I find peak sun hours in my area?
Use PVWatts for US sites or the Global Solar Atlas elsewhere. Read the monthly solar radiation column, or use our for US context.
Is there a peak solar hours map I can trust?
Yes—government and multilateral tools (NREL, World Bank Global Solar Atlas) publish peak sun hour and GHI maps based on long-term weather models. Avoid random blog maps without a cited data source.
Are peak sun hours the same as daylight hours?
No. Daylight is clock time; peak sun hours measure energy. A 14-hour summer day might yield only 6 peak sun hours.
Do panels work outside peak sun hours?
Yes. Panels produce whenever the sun is up, but output is lower in morning and evening. That lower production is already baked into the daily peak sun hour total.
Should I use summer or winter peak sun hours?
Off-grid / year-round RV: use winter (worst month). Grid-tied with net metering: annual average may be enough for bill modeling, but winter backup loads still need worst-month checks.
How do clouds change peak sun hours?
Heavy clouds can drop a 5 peak-sun-hour clear day to 1–2 hours equivalent. That is why worst-month historical data matters for reliability.
Does panel temperature change peak sun hours?
No—peak sun hours describe sunlight on the site, not panel temperature. Hot cells do reduce panel efficiency, which is why you still apply a 0.75–0.80 system efficiency factor in sizing math.
Sources and Further Reading
- National Renewable Energy Laboratory (NREL) – PVWatts Calculator
- World Bank Group – Global Solar Atlas
- U.S. Department of Energy – Planning a Home Solar Electric System
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