Solar takeoff and estimating: from array to interconnection.

The takeoff chain: module to grid

Think of a solar takeoff as following electricity from the module to the utility. Each link has its own quantities and its own scope questions:

Link	Take off	Typical source
Modules	Count by model and wattage	Array layout, module datasheet
Racking	Rails or trays by row, attachments or ballast, clamps, end caps	Layout, racking engineering, attachment details
Module-level electronics	Optimizers, microinverters or rapid shutdown devices	Single-line, equipment schedule
DC wiring	Homeruns by string, wire management, combiners, DC disconnects	String plan, single-line
Inverters	String or central inverters, pads, mounting	Equipment schedule
AC collection	AC panels, transformers, feeders, conduit, trench	Single-line, site plan
Interconnection	Disconnects, meters, switchgear work, protection	Single-line, utility requirements
Grounding, monitoring, labels	Equipment grounding, DAS, required signage	Single-line, notes

DOE's cost benchmarks break a PV system into the same logic: modules, inverters, storage, structural balance of system, electrical balance of system, fieldwork, office work and other costs. Organizing your estimate that way makes it easier to compare against benchmarks and your own history.

Quick math (and where it breaks)

modules  = system DC size (W) ÷ module rating (W)
strings  = modules ÷ modules per string   (from the electrical design)
DC wire  ≈ strings × average homerun length × 2 conductors + module leads and jumpers

Example (illustrative): a 250 kWdc rooftop system with 500 W modules is 500 modules. With 8 string inverters, the string count and length come from the designer's string plan, not from you. The estimator's job is to price the design, not to size strings. If there's no string plan yet, that's a stated assumption.

These shortcuts break when the fire code pathways, setbacks or rooftop equipment reduce the usable roof area. Count modules from the layout, not from the system size in the narrative, and check that they match.

NEC items that hit the estimate

• Article 690 (PV systems). Rapid shutdown in 690.12 for PV systems on buildings usually means module-level devices or a listed rapid shutdown system, plus an initiation device and labels. Price the devices and the labor per module.
• Article 705 (interconnected power sources). Load-side connections are limited by busbar and conductor ampacity rules (the familiar "120% rule" is one option). If the existing panel can't accept the backfeed, the design moves to a supply-side connection or a service upgrade, which is a very different price. Read the single-line carefully.
• Disconnects, overcurrent protection and labeling at each point the code and the utility require.

Price to the NEC edition the jurisdiction has adopted, and read local amendments.

Roof-mounted arrays

Roof work adds:

• Attachments and flashing, counted from the layout and the racking engineer's attachment spacing, which tightens in roof edge and corner wind zones. Counting from a typical detail at uniform spacing undercounts.
• Roofing coordination. Many roofing warranties require the roof manufacturer or a certified roofer to flash penetrations. That can be a separate subcontract.
• Ballasted systems trade penetrations for weight. Ballast block count comes from the racking engineering and usually needs a structural check.
• Fire code access pathways and setbacks (International Fire Code Chapter 12, as adopted locally), which affect module count and layout.
• Material handling: crane or lift days, roof protection and staging.
• Structural review of the existing roof, and any reinforcement it finds.

Ground-mounted arrays

Ground mount moves cost into civil and structural:

• Geotechnical data, pile type and length (driven, helical or ground screw) and pull testing.
• Fixed-tilt tables or single-axis trackers, with their own install rates.
• Trenching for DC and AC runs, by depth and conduit count. This is often one of the largest lines.
• Fencing, gates, access roads, erosion control and vegetation management during construction.
• Equipment pads, transformers and medium-voltage work on larger systems.

Labor and soft costs

Most solar contractors track mechanical labor per module or per watt (racking and module setting) separately from electrical labor (DC homeruns, inverters, AC feeders, terminations) and commissioning. Rates vary a lot with roof type, height, racking system and crew experience. Use your own production history.

Price these as separate lines rather than burying them:

• Engineering and permit drawings, if in your scope.
• Permits, inspections and utility interconnection application fees.
• Utility-required equipment and witness testing.
• Commissioning, monitoring setup and owner training.
• Warranty and O&M obligations, if the contract includes them.

Sanity check against benchmarks

DOE's Q1 2025 benchmarks (2024 dollars, PV only) give a reference point:

Segment	Benchmark system	Min. sustainable price	Modeled market price
Residential	8 kWdc rooftop	$2.78/Wdc	$2.95/Wdc
Commercial	250 kWdc rooftop (school)	$1.95/Wdc	$1.98/Wdc
Utility	100 MWdc single-axis tracking	$1.07/Wdc	$1.12/Wdc

These are modeled national figures for specific reference systems, not a price list. A small commercial rooftop with a service upgrade will cost more per watt; a repetitive large ground mount may cost less. If your number is far from the benchmark, find out why before bid day.

Checks before you submit

• Module count matches the layout and the system size in the narrative.
• Attachment count follows the racking engineering, including edge and corner zones.
• Rapid shutdown devices and labels are priced.
• Interconnection method (load side, supply side, service upgrade) is clear.
• Utility-required meters, disconnects and switchgear work are included or excluded in writing.
• Trench and conduit runs from array to inverters to the point of interconnection are measured.
• Roof warranty requirements are known.

Common solar takeoff misses

• Roof attachments taken from a typical detail instead of the actual layout and wind zones.
• Trenching and conduit between array, inverters and the point of interconnection.
• Utility-required production meters, disconnects and switchgear modifications.
• Rapid shutdown devices and initiation equipment.
• Equipment pads, fencing and signage on larger systems.
• Lifts, cranes, storage and site security.

Note for homeowners

If you're a homeowner asking what solar will cost for your roof, installers estimate from roof area, orientation, shading, your electricity use and local incentives. A site visit and a utility bill review produce a real quote. This guide is written for contractors preparing bids.

Where RenderDraw fits

RenderDraw's AI takeoff proposes counts and lengths from plan sets, with the source sheet on every row and uncertain items in a review queue. On a solar set, that means you check the extracted quantities instead of counting modules and attachments one by one. String design, interconnection method and pricing remain your call.