Array Underperforms Only After a Roof-Mounted AC Unit Was Added

Why this matters

When production drops the same week a contractor sets a rooftop condenser, a mini-split lineset, or a new vent on the roof, the cause is almost certainly that new object - but the loss mechanism could be shading, soiling, a damaged module, or a disturbed connection, and each is remediated differently. A new shadow on even a few modules can drag a whole string on a string inverter, while module-level systems localize it. Soiling from a fresh install (dust, sealant overspray, foot-traffic grit) mimics shading. And a careless rooftop trade can crack a module or unseat a connector. This tree ties the loss to the new work and names the mechanism.

Symptom presentation

Production falls off a clear-sky baseline starting on or just after the date a rooftop AC unit, lineset, condenser stand, or roof penetration was added. The loss is often time-of-day specific (a shadow that tracks the sun) or step-change (a module went down). On module-level monitoring, specific modules near the new equipment dip; on a string inverter, a whole MPPT input sags. The customer connects the timeline themselves: "It was fine until the HVAC crew was up there."

Quick checks

Overlay the post-install production against the pre-install clear-sky baseline and note WHEN in the day the loss appears:

• Loss tracks a specific clock window and migrates with the season: shading from the new object casting a moving shadow. Shading branch.
• Loss is uniform all day, lower flat curve: soiling deposited during the install, or a module/connection damaged during the work. Soiling/damage branch.
• One or more modules read zero or a string MPPT collapsed: physical damage or a disturbed connection from rooftop traffic. Damage branch.

Walk the roof (or use a shade study) at the loss window to see the new object's shadow position relative to the array, and inspect modules nearest the new equipment.

Isolation tree

Branch A - Shading from the new object. A rooftop condenser, lineset stand, or raised vent casts a shadow that sweeps across modules as the sun moves, notching production at a repeatable time. Confirm: module-level data shows the affected modules dipping in sequence as the shadow crosses them, or, on a string inverter, the whole string drops during that window. Even partial shading of one module forces its bypass diode and can knock out a third or more of a module and drag the series string. Remedy: relocate the new equipment, raise/re-rack the array clear of the shadow path, add an extension/standoff to clear the lineset, or add module-level electronics (optimizers/microinverters) to isolate the affected modules so one shadow no longer drags the string.

Branch B - Soiling from the install. Drilling dust, roofing grit, sealant or coating overspray, or tracked-in debris coats the glass and lowers output uniformly. Confirm: the loss is flat across the day (not a moving notch) and a visual shows film, dust, or spatter on the modules near the work. Remedy: clean the modules per the maintenance procedure and re-baseline; if overspray bonded to the glass, use the manufacturer-approved cleaning method.

Branch C - Physical module damage from rooftop traffic. A boot, a dropped tool, or a panel walked on can crack cells or the glass; microcracks reduce output and may worsen over time. Confirm: visual inspection (cracked glass, snail trails, hot spots on a thermal scan) and module-level data showing a specific module producing low or zero. Remedy: replace the damaged module(s) with matched units.

Branch D - Disturbed connection. Foot traffic or moved conduit can unseat an MC4 connector, loosen a bonding point, or damage a homerun, dropping a string. Confirm: a string or device that went fully offline coincident with the work, plus an inspection finding an unmated connector or damaged conductor near the new equipment. Remedy: re-seat/re-terminate with proper matched parts and torque, and verify continuity and grounding.

Branch E - Disturbed PV electrical work. If the AC work involved electrical (a new disconnect, conduit across the array, or a panel change), verify nothing altered the PV circuit, grounding, or rapid-shutdown wiring. Confirm rapid shutdown still functions and the equipment-grounding path is intact. Remedy: correct any disturbed PV electrical work to code.

Confirming diagnosis

The timeline plus the time-of-day signature is the discriminator. A moving notch that maps to the new object's shadow path confirms shading (Branch A) - a shade study or a roof walk at the loss window seals it. A flat all-day drop with visible film confirms soiling (Branch B) and is proven by cleaning and re-baselining. A step-change to zero on a specific module/string confirms damage or a disturbed connection (Branch C/D), confirmed by visual/thermal inspection and a connector check at the equipment. Module-level monitoring is the most direct confirmation because it pins the loss to specific modules and tells shading (sequential dips at a clock time) from a dead module (one device flat).

Remediation

For shading, relocate or raise the offending equipment, re-rack to clear the shadow, or add optimizers/microinverters to localize the loss. For soiling, clean to the maintenance standard and re-baseline. Replace any cracked or hot-spotting module with a matched unit. Re-seat or re-terminate disturbed connectors and restore bonding/grounding and rapid-shutdown integrity. After remediation, overlay a fresh clear-sky day on the pre-install baseline and confirm production recovers across the previously affected window; on module-level systems, confirm each affected module is back in family with its neighbors.

References

• NEC Article 690 - Solar Photovoltaic Systems (array conductors, grounding/bonding) and NEC 690.12 Rapid Shutdown of PV Systems on Buildings.
• NREL PVWatts / SAM and IEC 61853 / shade-analysis references (clear-sky baseline modeling and shading-loss quantification).
• Manufacturer module and inverter documentation (Enphase, SolarEdge) on bypass-diode behavior, module-level monitoring, and approved module cleaning.
• IEC 61215 / UL 1703 (module durability, hot-spot and microcrack context) for damaged-module assessment.