Solar Design: The Plan Set Behind Your Permit

What solar design means in plain terms

Think of solar design as the blueprint for your system. A sales quote says "7 kW on your roof". The design says which 16 panels go on which roof face, how they bolt to which rafters, how the wires run and where they join your home's power.

The DOE notes that panels are only one part of a PV system. Mounts, inverters and other parts must be in place for the power to be useful ⁴. Design picks each of those parts and fits them together.

The result is written down in a plan set, a short stack of drawings and documents. Your local building department reviews it before work starts. That is why design is sometimes called "permit drawings".

Where design sits in the project

1. Site assessment → sizing → design and plan set
2. → permit review → installation → inspection → permission to operate

How design works: the decisions a designer makes

• Which roof faces to use, from orientation, tilt and shade data.
• Panel layout around vents, skylights and required clearances.
• Attachment spacing to rafters, which sets the number of roof penetrations.
• Inverter type and how strings are wired. The DOE notes a single inverter is generally cheaper and easier to service, while microinverters let each panel work on its own ⁴.
• Whether the main panel can take the new breaker or needs an upgrade.
• Where the inverter, disconnects and any battery will be mounted.

Types of documents in a residential plan set

The single-line diagram, in plain English

A single-line (or one-line) diagram draws the whole electrical path with one line per circuit. It shows the panels, any optimizers or microinverters, the inverter, disconnects, the rapid shutdown system, the main panel and the meter. Wire sizes, breaker ratings and the connection method are labeled on it.

It is the page the electrical inspector reads most closely. Charleston County lists an electrical layout and schematic tying into the existing service or a battery system as a permit requirement.

Sources: [1] [2]

Example: from usage to a design in Charleston, SC

This example uses real inputs: the EIA's 2022 US average home use of 10,791 kWh a year ⁷, a PVWatts V8 run for Charleston on 7 October 2026 (7 kW DC, standard module, roof mount, 20° tilt, due south, 14% losses, 10,179 kWh a year) ⁸, and a 440 W panel ⁹.

The layout is illustrative.

A real design replaces each assumption with measured data from your roof. Two installers can reach different, valid layouts from the same roof.

Where the design shows up for you

Before you sign, you may see a layout picture in the proposal: panels drawn on a photo of your roof, with the system size and yearly kWh. That is a sketch of the design.

After you sign, the full plan set goes to the building department. Ask for a copy. It is the clearest record of what was promised: panel model, count, inverter, roof faces and how the system connects.

On install day and at inspection, the crew and inspector work from the approved plan set. Keep a copy with your solar paperwork for the life of the system. You will want it if you sell the home or need repairs.

What good design gives you, and its limits

Limits: design on paper versus the roof

Some first designs are drawn from satellite images before anyone visits. They are useful for a first quote, but the final design should follow a site survey. Rafters, panel boxes and new tree growth do not show up from above.

Design also cannot change the rules. Utility size caps, fire-code walkways and setbacks limit what fits, however clever the layout.

How design choices drive cost and lifespan

We do not quote dollar prices here. Design is where most of the price is set. See solar cost and cost by system size for current figures.

• Size and panel model. Total kW and the panel chosen set most of the price.
• Electronics. Panel-level electronics add parts but help on shaded or multi-face roofs.
• Roof faces. Each extra face adds racking, wiring and labor.
• Electrical work. A main panel upgrade, if needed, is a separate line item.
• Engineering and permits. Structural letters and permit fees are part of the job.
• Lifespan. The DOE expects inverters to need replacing at least once in a 25-year array life ⁴, and says a system should produce for at least 30 years ⁵. Good design leaves room to swap the inverter.

The design and permit process, step by step

A licensed installer and its designer or engineer do this work. You review and approve.

1. The installer surveys the site and collects your usage.
2. The designer sizes the system and lays out panels on the roof faces.
3. The designer models yearly kWh for that layout.
4. An engineer checks the roof or ground structure and writes the structural letter, where required.
5. The designer draws the single-line diagram and gathers spec sheets.
6. The installer submits the plan set and permit applications to the building department.
7. The installer applies to the utility to connect. See solar interconnection.
8. After install, inspectors check the work against the plan set.

Keeping the design up to date

A design is a living record. If you add panels or a battery, the plan set is updated and goes back for permits. Our guide to system upgrades covers what that involves.

Keep the as-built plan set with your home papers. Repair crews use it to find strings and disconnects. The DOE notes there are no routine maintenance costs for a typical roof system ⁵, so the paperwork is often the main thing you look after.

Red flags: what to check before you sign

Ask questions, or get a second quote, if any of these apply:

• No layout drawing, or one that does not match the panel count and size on the contract.
• Modelled production based on an ideal layout, not the designed one.
• No word on who signs the structural letter, or whether they saw the site.
• A different inverter or panel on install day than in the approved plan set.
• No check of your main electrical panel.

Safety

Rules the design must meet

The design must meet the building and electrical codes your local building department has adopted. In the US, the electrical rules come from the National Electrical Code, NFPA 70, as adopted locally ⁶.

South Carolina has the 2021 building codes in force, and the 2024 International Residential Code is scheduled for local use from 1 January 2027 ³.

Local departments set their own submittal lists. The City of Charleston requires a building permit and an electrical permit, a roof plan with access and pathways, a sealed structural letter, a single-line diagram and spec sheets ¹.

Charleston County asks for a structural analysis of the roof, a wind and seismic design analysis, a layout showing clearances, a fastening design and an electrical schematic ².

Licensing shapes who builds the design. City of Charleston guidance says a contractor licensed only as a Solar Panel Installer may install panels and the electrical system but not roof mounting or framing ¹.

Solar design in SC, GA and VA

Solar design compared with related steps

Common misconceptions

Myth The sales layout is the design.

Reality A sales sketch is a starting point. The permit plan set, after a site survey, is the real design.

Myth Design is just paperwork.

Reality It sets your yearly kWh, the holes in your roof and whether your panel needs an upgrade.

Myth Any installer can swap parts after approval.

Reality Changes to the approved plan set can mean a revision and can fail inspection.

Myth Every town wants the same drawings.

Reality Charleston city and county ask for different documents for the same kind of job.

When the design matters to you

Use these rules:

• If you are comparing quotes, ask each for a layout drawing and yearly kWh for that layout.
• If your roof has shade or several faces, ask how the design handles it.
• If your home is in a historic district, ask about the extra review before signing.
• If parts change after you sign, ask for the revised plan set.
• Next step: check the size with the system size calculator, then review quotes with the proposal analyzer.

Related terms

Solar Site Assessment

A solar site assessment is an inspection of a property to decide whether, where and how solar panels can be installed, covering sunlight, shade, roof direction and angle, roof condition and structure, and the home's electrical service.

Solar System Sizing

Solar system sizing is the process of choosing how many kilowatts (kW) of solar panels a property needs, by dividing the electricity it uses in a year by what one kW of panels produces there.

Roof Penetration

A roof penetration is any hole made through the roof covering, and on a solar install it usually means each point where a mount is bolted through the shingles into a rafter to hold the racking.

Solar Permit

A solar permit is the approval a local building department, the authority having jurisdiction, issues before a solar system can be installed.

Solar Inspection

A solar inspection is the check of a finished solar installation against the approved permit and the adopted codes, carried out by the local building department's inspector.

Rapid Shutdown

Rapid shutdown is a safety function required by the National Electrical Code (NEC 690.12) that drops the voltage on a building-mounted solar array's wiring to a controlled level within 30 seconds of someone starting shutdown, so firefighters are not exposed to live DC wires on the roof.

Solar Mounting System

A solar mounting system, also called racking, is the set of attachments, rails, clamps and bonding hardware that secures solar panels to a roof or the ground and holds them at a fixed angle against wind and snow.

Electrical Panel

An electrical panel, also called the main service panel, breaker box or load center, is the metal cabinet where utility power enters a home and is split into branch circuits, each protected by a circuit breaker.

Also referenced from: Solar Site Assessment, Solar System Sizing, Solar Panel Orientation, Solar Panel Tilt, Solar Shading, Solar Access, Roof Penetration, Solar Permit