Shaded South Roof, Busbar Limit, and a Four-Year Payback Claim
A homeowner in a utility territory with 1:1 net metering, a monthly $12 minimum bill, and non-bypassable charges on imports asks for a proposal that offsets 100% of 10,800 kWh/year. The south roof plane can hold 24 modules, but a chimney shades the lower-left corner until 10 a.m. from November through February; a plumbing vent shades one module mid-day. An east-west roof can hold 18 modules with no shade. The main panel is a 200 A busbar with a 200 A main breaker; the utility limits PV breaker backfeed to the busbar rating minus the main breaker. The roof is 15-year-old asphalt shingle with an expected 8–10 years remaining. The customer wants the fastest payback and asks you to "fill the south roof and skip the expensive electronics." Your first draft is 8.4 kWdc with a 7.6 kW string inverter and 40 A backfeed.
1. How should you evaluate the south-roof shade and equipment choice? Show the production and risk reasoning you would use before recommending a string inverter or module-level electronics.
Read the worked answer
Start with a shade model, not a rule of thumb. The chimney shadow is worst in winter mornings when the sun angle is low, so it can cover several modules even if the chimney is small. A mid-day vent shadow is a hard shadow on one module. If the south array is split into two strings on a string inverter, any shaded module limits the current of its entire string; if the shade is concentrated on one string and each string has an independent MPPT, the loss is reduced but not eliminated. Module-level electronics reduce mismatch and give per-module monitoring, but add cost and failure points.
Estimate production: an 8.4 kW south array at about 1,350 kWh/kW would make roughly 11.3 MWh before shade. An 8–15% annual shade loss leaves about 9.6–10.4 MWh, so 100% offset of 10.8 MWh is not assured. The east-west plane at about 6.3 kW and 1,150 kWh/kW makes about 7.2 MWh, with lower winter output but no chimney shade. Model both layouts at hourly resolution before pricing.
Roof age matters: with 8–10 years of shingle life left, removing and reinstalling the array later could cost $2,000–$4,000 and risk roof warranty issues. Present two priced options: south-heavy with module-level electronics on shaded sections, or a hybrid south/east-west layout with a string inverter. Do not promise "skip the electronics" until the shade model shows the affected modules are isolated on a dedicated MPPT and the customer accepts the production risk.
2. The 40 A PV breaker with a 200 A main breaker on a 200 A busbar is rejected. What interconnection options and code/sales points should you present?
Read the worked answer
Under the common busbar sum rule, the main breaker plus the PV breaker cannot exceed the busbar rating: 200 A + 40 A = 240 A on a 200 A busbar, so the load-side breaker is not acceptable. The 40 A breaker is already the minimum for a 7.6 kW inverter after continuous-duty sizing (7,600 W ÷ 240 V = 31.7 A; ×1.25 = 39.6 A), so simply shrinking the breaker is not a practical fix.
Options to present:
- Supply-side or line-side connection ahead of the main breaker, with a utility-approved lockable disconnect and overcurrent protection. This often avoids a service upgrade but needs utility and AHJ approval.
- Main-breaker derate to 150 A if a load calculation shows the existing service can carry the home on 150 A: 150 A + 40 A = 190 A, which is within 200 A. A 175 A main would still be 215 A, so it does not solve the problem with a 40 A PV breaker.
- Service or panel upgrade to a larger busbar, but check the arithmetic carefully: a 225 A busbar with a 200 A main still allows only 25 A of PV breaker under the sum rule. A 400 A service may be needed for the full array.
- Reduce the AC size or split the system, but that changes production and may not meet the customer’s offset goal.
Sales point: do not promise the cheapest option before the utility and AHJ respond. The line-side tap may be faster and less expensive than a service upgrade, but it changes the customer’s equipment and approval path. Give a written scope and schedule for each option.
3. The customer's payback estimate is 4 years based on 100% offset and no roof work. What corrections and sales-language should you make?
Read the worked answer
Rebuild the calculation with the customer’s assumptions exposed. If the installed cost is about $2.80/W, 8.4 kW is roughly $23,520. A 30% tax credit is about $7,056, leaving $16,464 net. At 100% offset and $0.16/kWh, annual savings would be about $1,728, giving a simple payback near 9.5 years-not 4. If the customer used $0.25/kWh and ignored shade, the 4-year figure may be an artifact of optimistic inputs.
Correct for shade: 9,800 kWh of production on a 10,800 kWh load is about 91% offset, not 100%. Non-bypassable charges, the $12 minimum bill, and time-of-use mismatch reduce bill savings further. Add roof work: if the roof is replaced now, the PV reinstall or mounting change may add $2,000–$8,000; if the roof is replaced later, the customer faces a removal/reinstall cost. Add inverter replacement at year 12–15, module degradation, and financing cost if applicable.
Use conservative P50/P90 production and show a sensitivity table: lower production, higher shade, and a change from net metering to net billing. Say "simple payback under current tariffs and modeled production," not "payback" as a guarantee. Explain that utility rules can change and that the customer should compare the proposal against a roof-first plan. This protects both the customer and the salesperson from an unrealistic 4-year promise.