What each type of obstruction actually costs
Chimneys carry the biggest clearance penalty
A standard 2 by 2 foot masonry chimney with a 1-foot flashing and access buffer on every side takes up about 16 square feet once clearance is included, four times the chimney's own physical area. Your local rules can require a wider buffer, and that pushes the number higher. Start from a roof area that already has setbacks removed, which the usable roof area calculator handles, so the two losses do not overlap. This clearance cost is also separate from the shading a chimney casts, which the output loss from chimney shading calculator covers on its own.
Vents cost little space individually, but disrupt row layout
A typical plumbing vent pipe is only 3 to 4 inches across, and a roughly 1-foot radius around it is a practical clearance zone, which comes to about 4 square feet per vent. The raw area lost is small, but a vent sitting mid-row often forces an installer to skip an entire panel position rather than just the vent's own footprint. The Department of Energy and NREL's Solar Ready Buildings Planning Guide makes the same point from the design side, favoring large contiguous unobstructed roof areas and placing vents on the north side of the roof. On an existing house, ask the plumber whether a vent can move out of the array field, and use the multi-roof plane array splitter to see where displaced panels could go instead.
Skylights depend on whether code applies to your roof
This calculator treats a skylight like a chimney, with a 1-foot buffer around its curb, which is a practical allowance for flashing and access. Fire code is a different matter. On houses, the model fire code mostly asks for pathways from eave to ridge and setbacks at the ridge, while the 4-foot pathways to roof hatches and smoke ventilation openings in Section 1205 of the 2021 International Fire Code are aimed at larger non-residential roofs. Local fire departments sometimes add their own rules, so confirm what your Authority Having Jurisdiction actually enforces before finalizing a layout.
Why obstruction count alone is not the whole picture
Two roofs with the identical number of obstructions can lose very different amounts of usable space depending on where each obstruction sits relative to panel rows. An obstruction near a roof edge or corner often costs less real panel capacity than the same one sitting in the middle of an otherwise clean array field, similar to how the fire code setback calculator treats edge zones differently from interior array space. The example the Load example button fills in, one chimney, three vents, and one skylight on a 750 square foot roof, removes about 51 square feet and 3 panels, though a real layout can differ by a panel either way. If a chimney is no longer used, the chimney removal for solar cost estimator prices out taking it off entirely, which can cost less than designing around it for years.