A solar Single Line Diagram (SLD) is a simplified electrical drawing that shows how every part of a solar system connects — from the panels to the inverter, the protection devices, the meters and the grid — using one line to represent each circuit.
For homeowners, the SLD is one of the documents in the DISCOM file. For EPCs, it is far more: the drawing the installation crew wires to, the document the DISCOM inspector checks against, and the record anyone servicing the system will rely on for 25 years. This guide explains what an SLD shows, walks through each component from DC to AC, shows a simplified example for a rooftop system, and lists the mistakes that delay approvals.
Key points
- An SLD shows the components, their ratings, how they connect, where protection and isolation sit, and how the system is earthed.
- A typical grid-connected rooftop runs: PV array → DC distribution box → inverter → AC distribution box → meters → grid, with earthing on both sides.
- DISCOMs commonly ask for an SLD with the net-metering application and check the installation against it at inspection.
- The most common problems are missing protection devices, unstated ratings, unclear earthing and an SLD that doesn't match what was installed.
What “single line” means
A real electrical circuit has two or more conductors — positive and negative on the DC side, phase and neutral (or three phases) on the AC side. An SLD draws each circuit as a single line so the whole system fits on one page and can be read at a glance. The ratings, device types and connection points are written next to each symbol. Detailed wiring diagrams, cable schedules and string layouts sit alongside it for the installation team.
Why the SLD matters
| Who uses it | What for |
|---|---|
| DISCOM | To check the connection, protection and metering before approving net metering |
| Installation crew | To wire the system exactly as designed |
| Inspector | To verify that the installed devices match the approved design |
| Service technician | To understand the system years later when something needs fixing |
| Customer | As part of the system's documentation for warranties and resale |
A simplified rooftop SLD
The diagram below shows the main blocks of a typical single-phase, grid-connected rooftop system. Real SLDs use standard electrical symbols and state the rating of every device.
The components, from DC to AC
| Component | What it does | What the SLD should state |
|---|---|---|
| PV array | Converts sunlight into DC power | Panel model and wattage, panels per string, number of strings |
| DC cables | Carry DC power from the array | Cable type and size |
| DC distribution box (DCDB) | Protects and isolates the DC side | Fuse ratings, DC surge protection device (SPD), DC isolator rating |
| Inverter | Converts DC into grid-synchronised AC; shuts down if the grid fails | Make, model, AC rating, number of MPPTs, phase |
| AC distribution box (ACDB) | Protects and isolates the AC output | MCB or MCCB rating, AC SPD, RCCB/ELCB rating |
| Solar generation meter | Measures total solar generation, where required | Meter type and position |
| Bidirectional net meter | Records units imported from and exported to the grid | Position at the supply point |
| Earthing and lightning protection | Carries fault and surge currents safely to ground | Earth pits for the DC side, AC side and lightning arrester, as specified |
“The SLD is the promise the installation has to keep. If the two don't match, the inspection won't pass.”
How to read an SLD in five steps
- Start at the array — count the strings and panels per string, and check the panel rating.
- Follow the DC side — find the fuses, the surge protection device and the isolator that lets the array be disconnected safely.
- Check the inverter — its AC rating should suit the array, and the number of strings should match its MPPT inputs.
- Follow the AC side — find the breaker, AC surge protection and residual-current protection between the inverter and the building's supply.
- Finish at the meters and earthing — confirm where generation and net metering are measured and how each side is earthed.
What DISCOMs and inspectors look for
- The system capacity on the SLD matches the application and the installation.
- Protection devices are present and suitably rated on both the DC and AC sides.
- There are clear isolation points so the system can be switched off safely.
- Earthing is shown and installed as drawn.
- Meters are in the positions the DISCOM requires.
- Equipment on site carries labels matching the SLD.
Requirements differ between DISCOMs, so always check the local format and any specific devices they ask for. Our DISCOM approval guide covers the full process.
Common SLD mistakes
| Mistake | Why it causes trouble |
|---|---|
| Generic SLD reused from another job | Ratings and string counts don't match the installation |
| Missing surge protection or isolator | Safety gap; often flagged at inspection |
| Device ratings not stated | The inspector can't verify the protection |
| Earthing not shown clearly | Questions about safety and lightning protection |
| Capacity differs from the application | Approval and subsidy delays |
| Changes on site not reflected | The as-built system no longer matches the drawing |
SLDs for other system types
- Hybrid systems add a battery and usually a separate backup (essential-load) circuit from the hybrid inverter. See on-grid vs off-grid vs hybrid.
- Three-phase and commercial systems may have several inverters feeding an LT panel, larger breakers and different metering arrangements. See commercial vs residential solar.
- Microinverter systems move the conversion to each panel, so the DC side shrinks and the AC side carries the protection. See string vs micro inverters.
Generating the SLD from the design
An SLD drawn by hand for every job is slow and easy to get wrong. When it is generated from the same design that produced the layout and the proposal, the capacity, strings and equipment always match. SolarDraft's Solar 3D Design produces a Single Line Diagram, PV array and string route drawings and a Structure BOM from the 3D model, and the proposal carries the same capacity — so the DISCOM file, the crew's drawings and the customer's proposal all tell the same story.
New to the documentation side of solar? See what a solar EPC does and common mistakes new EPCs make.
FAQ
Is an SLD required for net metering?
Most DISCOMs ask for an SLD with the application or the work completion documents. Check your DISCOM's current checklist.
Who prepares the SLD?
The EPC or its design engineer prepares it from the system design, and it should be updated if anything changes on site.
What is the difference between an SLD and a wiring diagram?
An SLD shows the system on one page with one line per circuit and the key ratings. A wiring diagram shows every conductor and terminal in detail for the installers.
Why are the DC and AC sides earthed separately?
Separate earthing for the DC side, the AC side and the lightning arrester is common practice on Indian rooftops and is often specified by DISCOMs and scheme guidelines. Follow the local requirement and show it on the SLD.
