Peak Sun Hours Explained for Solar Street Light Sizing
Peak Sun Hours Explained for Solar Street Light Sizing
Peak sun hours (PSH) are the hours per day when solar irradiance — the power arriving from the sun per unit area — reaches at least 1,000 W/m², the standard test condition for rating a panel. A site with 5 PSH does not get 5 hours of "good sun" and 19 of darkness; it gets roughly 13 hours of daylight compressed into the energy equivalent of 5 full-power hours. Every solar street light sizing exercise starts with this number, because it converts a panel's watt rating into daily watt-hours.
Why PSH, not daylight hours, drives the design
Two sites can share the same daylight length and differ by 40% in PSH. The factors that move the number:
- Latitude — closer to the equator means more annual PSH.
- Climate — persistent cloud cover cuts it hard; desert and semi-arid sites win.
- Season — most regions swing 30–50% between summer and winter PSH, and it is the winter minimum, not the annual average, that sizes your battery.
- Altitude — thinner atmosphere means higher irradiance at the same sun angle.
- Shading — trees, buildings, and even the fixture's own pole can eat the 10 AM–4 PM window that produces most of the day's energy.
The annual yield formula — with a correction
The textbook estimate is:
Energy (kWh/year) = PSH × Panel Watts × 365 × System Efficiency
Worked example, corrected: a 100 W panel at 5 PSH with 0.85 system efficiency gives 5 × 100 W × 365 × 0.85 = 155,125 Wh ≈ 155.1 kWh per year . (The original version of this page showed 155,375 Wh — an arithmetic slip; 155.1 kWh is the correct figure.)
Now the part sizing guides skip. That 0.85 factor fits grid-tied or directly-coupled loads. A battery-based street light never sees it. Between the panel and the LED you have a charge controller, a battery round-trip, an LED driver, and cable runs — each takes a cut. When we size a project, the panel-to-lamp derate we apply is well below 0.85, and the exact factor depends on the components in the BOM. Treat the formula's output as an upper bound, then discount for your actual architecture.
Typical PSH by region
The table below is the same reference data the original page published — annual averages, with the right-hand column showing ideal reference yield per kWp (kilowatt-peak, the panel's rated output at standard test conditions) before any system losses:
| Region | Peak Sun Hours | Reference yield (kWh/kWp/year) |
|---|---|---|
| North America (Average) | 3.5 – 5.5 | 1,278 – 1,980 |
| Europe (Average) | 2.5 – 4.5 | 913 – 1,643 |
| Asia (Average) | 3.0 – 6.0 | 1,095 – 2,190 |
| Australia | 4.5 – 6.5 | 1,643 – 2,373 |
| Middle East | 5.0 – 7.0 | 1,825 – 2,555 |
| South America (Average) | 4.0 – 6.5 | 1,460 – 2,373 |
Two warnings on this table. First, the averages hide enormous in-country spread — a coastal site and a high-desert site 300 km apart can differ by a full PSH. Second, the yield column is the ideal DC figure; it is not what a battery system stores. Use the winter-month PSH for your market, not the annual mean, when sizing autonomy. If you do not have site-specific data, our solar energy yield calculator walks the same arithmetic with derates you can set explicitly.
Placement rules that actually matter on site
- Orientation: true south in the Northern Hemisphere, true north in the Southern Hemisphere. A 15° error costs a few percent; a 90° error (facing east or west all day) costs far more than most people expect.
- Tilt: match the panel tilt to local latitude as the baseline. On fixed all-in-one fixtures the tilt is built in — check it against your latitude before ordering, because a fixture tuned for 30° performs poorly at 50°.
- Shading audit before pouring the foundation. Walk the site at 10 AM and 3 PM on the shortest day of the year. That 20-minute survey prevents the classic failure: a tree that was small at installation and full-grown at year five. We cover shading losses in detail in how shade affects solar panels.
What this page does not cover
This page defines PSH and shows the sizing arithmetic. It does not cover panel temperature effects, battery autonomy calculation, or lighting-design software output — for full photometric and autonomy modeling use our lighting simulation service or send us the site coordinates and we will run the yield study with you.