Understanding morning versus afternoon shading
Why the same shadow can cost different amounts at different times
Not exactly. For a true south-facing array, afternoon shading typically costs slightly more than the same number of morning shaded hours, since many climates see slightly more total production in the afternoon than the morning. This gap is usually small for a south-facing array but grows for east or west biased orientations. A NLR field study of partially shaded arrays found a shadow can cut output by more than 30 times its physical size on a series-wired string, which is why the timing of a shadow matters as much as its size.
How orientation shifts which window matters more
A southeast-facing array produces more of its daily energy in the morning, so morning shading costs it more, while a southwest-facing array produces more in the afternoon and is more vulnerable to afternoon shade. If you're deciding between two roof sections with different shade patterns, this orientation effect is worth factoring in. A peer-reviewed study published in Royal Society Open Science found that shifting a fixed panel's orientation to account for a morning-versus-afternoon cloudiness pattern typically gains no more than about 5% in total annual energy, a useful reminder that orientation timing matters most for shading decisions rather than as a major redesign lever on its own.
Using this to prioritize which shade source to address
Yes. If you have limited budget to address multiple shade sources, prioritize the one that shades your array during its highest-production time window based on your specific orientation, rather than assuming morning and afternoon shade cost the same. A tree that shades the array for two afternoon hours might be worth removing before one that shades it for two morning hours on a southwest-facing roof. Before committing to removal, the vegetation trimming maintenance cost tool can help weigh whether a trim now and then is enough versus full removal. Check the neighboring building calculator too if a large structure is part of what you're weighing.
What this comparison doesn't cover
No, this tool compares a fixed number of daily shaded hours year-round. Check the seasonal shading variation calculator separately if your shade source changes meaningfully with the sun's seasonal angle, since a shadow's timing and duration often shift across the year even from a fixed object. The time of use rate savings calculator shows what that loss is worth under rates that change through the day.
What to do with this number next
Once you know which window costs you more, run the tree shading calculator or chimney shading calculator for a full loss estimate specific to your actual shade source.