Tilt your solar panels to 39° facing south in Rock Falls. That is the most efficient fixed angle for this exact spot, measured from satellite records of its sunshine and cloud rather than worked out from latitude. The full breakdown, including seasonal and monthly adjustments, is below.
Today, the most efficient tilt for photovoltaic panels in Rock Falls is 39.0°, facing south. At solar noon the sun sits 51.0° above the horizon. Setting your panels to this angle keeps them as close to perpendicular to the sun as possible, which is where they generate the most electricity.
Your photovoltaic panels need to be angled facing south. Rock Falls is in the northern hemisphere, so panels catch the most sun facing south.
If you're mounting the photovoltaic panels at a stationary angle, such as on your roof, the most efficient angle is 39°.
If you're planning to change the angle of your photovoltaic panels twice per year, the most efficient angle is 19.2° facing south in summer months and 57° facing south in winter months.
When changing the angle of your photovoltaic panels each season, the most efficient angle is 15.5° facing south in summer months and 62.5° facing south in winter months, and 40° facing south in autumn and spring months.
A 400W panel at its best fixed angle in Rock Falls makes about 549 kWh a year, worth $75.28 at 13.72 cents per kWh, the 2025 residential average for Iowa. Per kilowatt of panels that is 1,372 kWh, or $188.21, a year.
A home in Iowa uses about 10,210 kWh of electricity a year, so covering all of it from the roof in Rock Falls takes roughly 19 panels of that size. Fewer panels cover proportionally less of it.
In Rock Falls, December is the worst month and generates 47% of what July does: about 68 kWh per kW of panels against 146. An array or battery sized for summer will come up short in December.
Averaged across the year, panels at the angle above see 4.7 peak sun hours a day. That is the figure to use when sizing an off-grid system, but size it for December rather than for the average, or it will run flat when you need it most.
If you adjust your photovoltaic panels every month, the table below gives the most efficient angle for Rock Falls throughout the year. The shallowest angle is 19.8° in June, and the steepest is 66.6° in December.
| Month | Optimal tilt | Sun angle at noon |
|---|---|---|
| January | 63.3° | 26.7° |
| February | 54.4° | 35.6° |
| March | 43.6° | 46.4° |
| April | 31.6° | 58.4° |
| May | 23.1° | 66.9° |
| June | 19.8° | 70.2° |
| July | 22.8° | 67.2° |
| August | 31.4° | 58.6° |
| September | 43.4° | 46.6° |
| October | 55.0° | 35.0° |
| November | 63.6° | 26.4° |
| December | 66.6° | 23.4° |
The sun angle at noon is how high the sun sits above the horizon at midday. Facing your panels south at the tilt shown for each month keeps them as close to perpendicular to the sun as possible, which is where they generate the most electricity.
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For Rock Falls it is measured, not calculated. PVGIS, the European Commission's solar radiation model, works through years of satellite records of the sunshine and cloud at this exact coordinate and finds the fixed angle that collects the most light over a whole year. That is 39°.
Latitude sets the starting point, because it sets how high the sun climbs. It cannot see the rest: how cloudy the winters are, how much of the light arrives diffuse rather than direct, and whether hills block the low sun. A rule based on latitude alone gives 38.9° here. Across every city PVGIS has measured, that kind of rule comes out too steep more than eight times in ten. The seasonal, monthly and daily angles above do still come from latitude, because those follow the sun's height through the year, which latitude does determine. How the figures are worked out.
The angle is settled. Here is what is worth checking on the panels themselves:
Fixed angle and generation measured with PVGIS at this location's own coordinates, © European Union, 2001-2026, CC BY 4.0. Electricity prices from the US Energy Information Administration.