Insolation peaks at solar noon and depends on latitude
Solar irradiance (power per unit area) depends on the sun's angle above the horizon. At solar noon (when the sun is highest), irradiance is near its daily maximum, typically 1,000 watts per square meter on a clear day at sea level. As the sun sets, the angle decreases, light travels through more atmosphere, and irradiance drops. At latitude 40 degrees (northern US, southern Europe), the sun never climbs above 53 degrees even on the summer solstice; at the equator, it reaches 90 degrees on equinoxes, delivering maximum insolation year-round. At latitude 60 degrees (Scandinavia, Alaska), winters bring minimal irradiance, summer brings 24-hour twilight.
For solar panels, the irradiance curve directly determines daily output. A 10 kW array produces almost nothing before dawn, ramps to 8-9 kW around noon (if cloudless), and falls to nearly zero by dusk. This is why solar output and electricity demand are mismatched: peak solar (midday) and peak demand (morning and evening) seldom align.
Seasonal variation and cloud cover uncertainty
Winter solstice at 40 degrees latitude sees peak irradiance of 400-500 W/m2 at solar noon (half the summer value), because the sun is lower and light travels through more atmosphere. Summer solstice reaches 950+ W/m2. This 2-fold seasonal swing drives energy storage requirements in temperate zones: a winter day's solar output is one-third of summer's, so grids need seasonal storage or fossil backup.
Cloud cover reduces peak irradiance instantly and dramatically. A scattered cloud casts the site into shadow, dropping output to 20-30 percent of clear-sky value within seconds. Convective storms bring irradiance to near-zero. This variability on 1-minute timescales is why solar grids require fast-response batteries and reserve capacity; solar's smoothed daily curve hides enormous sub-hourly noise.