The 9.2 kW Quote on a 5.2 kW Roof
A 40 ft × 16 ft south-facing roof plane (dimensions taken on the roof surface) with a 4:12 pitch on a 1990s ranch is the only viable array location. The homeowner's signed quote, generated from a satellite design app, promises 9.2 kW using 400 W modules (78.5 in × 39.5 in). On site you find: a 30 in × 30 in masonry chimney near the west gable, a 2 in plumbing vent stack about 6 ft from the east gable, 2×4 rafters at 24 in on center under 18-year-old three-tab shingles, and a 40 ft oak to the southeast that shades the plane until roughly 9:15 a.m. from April through September. The AHJ enforces a 36 in clear access pathway along the eave and 18 in setbacks at the ridge and both gable ends. The utility's summer on-peak window is 4–9 p.m.
1. Determine the realistic maximum module count and array rating, and state what you do about the signed quote.
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Work in module dimensions, not nameplate watts.
- Module footprint: 78.5 in × 39.5 in = 6.54 ft × 3.29 ft = 21.5 ft².
- Usable band after setbacks: width 40 − 1.5 − 1.5 = 37 ft; slope height 16 − 3.0 (eave pathway) − 1.5 (ridge) = 11.5 ft.
- Landscape orientation (6.54 wide × 3.29 tall): 3 rows fit (3 × 3.29 + two 2 in gaps ≈ 10.2 ft ≤ 11.5 ft). 37 ÷ 6.54 = 5.6 → 5 columns → 15 positions.
- Deduct one position for the chimney footprint plus clearance and one for the vent stack → 13 modules.
- 13 × 400 W = 5.2 kW DC.
Cross-check: 9.2 kW requires 23 modules = 495 ft² of module area, but the entire usable band is only 37 × 11.5 = 425 ft² before any gaps or row spacing. The quote is not buildable on this plane - the error is a satellite tool that ignored setbacks and obstructions.
Action: stop and communicate in writing with a scaled layout drawing before permits or materials are ordered. Present the honest 5.2 kW option plus real alternatives (detached garage, ground mount) rather than letting the crew discover it on install day.
2. Should the morning shade drive module-level power electronics, a different string layout, or nothing? Show your reasoning.
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Price the shade before you buy hardware for it.
- When does the loss occur? Shade ends ~9:15 a.m. The energy lost is in the early-morning block, which on this tariff is off-peak. It does not touch the 4–9 p.m. expensive window at all. So the dollar value of the lost kWh is low even if the kWh count is not trivial.
- Quantify it. Measure with a shade tool at 8:00 a.m. and 4:00 p.m. on a clear day, and at the equinoxes. If the shadow only touches the lowest row for the shoulder seasons, annual loss is typically single-digit percent.
- Cheapest electrical fix first. With two MPPTs available, split the array so the routinely shaded modules sit on their own MPPT (e.g., 6 shaded / 7 unshaded). Each input tracks independently, so the unshaded string is not dragged down to the shaded string's current - this costs nothing but a jumper change.
- Only then consider optimizers. If measured shade is deep and persistent across the lower two rows, module-level electronics on those modules are defensible; on a single-plane array with a short morning window they usually do not pay back.
- Also check orientation value. Because the peak window is 4–9 p.m., ask whether any east or west plane exists - a west-facing sub-array at a slightly lower annual kWh can be worth more per kWh on this tariff.
3. The rafters are 2×4 at 24 in on center under 18-year-old three-tab shingles. What attachment and structural issues do you raise before install, and why?
Read the worked answer
Separate the dead-load question from the attachment question - they have different answers.
- Dead load: a flush-mounted array adds roughly 2.5–4 psf, which is small against a 25 psf snow load. The array weight is rarely the governing problem.
- Attachment: at 24 in o.c. the mounting feet must land on or be spanned to rafters. That spacing constrains rail orientation and foot layout, and it frequently pushes the design into a rail-over-rafter layout with longer rail spans - check the racking manufacturer's span table rather than improvising.
- Rafter capacity: 2×4 rafters at 24 in o.c. may be near their limit depending on span and whether collar ties or a ridge beam are present. Verify from the attic (measure the actual span, check for sag, notching, and prior repairs). If the span is long or the AHJ requires it, obtain a stamped structural letter - do not self-certify.
- Roof condition: 18-year-old three-tab may be near end of life. If the roof has under ~10 years left, tell the customer now that re-roofing later means a full array removal and reinstall. A tear-off before mounting is far cheaper than a mid-life R&R, and lifting brittle tabs to flash penetrations will crack them.
- Flashing: specify flashed mounts, never sealant-only lag bolts, and verify sheathing thickness so the lag has adequate embedment.