Bill of materials mistakes that eat a signed job's margin

Ask an EPC owner where the margin went on a job that went wrong and you will rarely hear that the price was too low. You hear a list: the structure had to be taller, the cable run was longer than anyone measured, the customer wanted a walkway, the crane cost more than expected.

None of those are pricing failures. They are bill of materials failures, and they happen after the customer has signed, when your negotiating position is at its weakest.

The items that go missing on residential jobs

1. Structure height. Quoted at standard height, built taller because the parapet shades the first row or the customer wants to use the terrace. Every extra 300 mm is steel, foundations and wind load.

2. The DC run from a split array. Two roof faces mean two runs, and the second one is always longer than the first estimate. Cable is cheap per metre and expensive per hundred metres.

3. The AC run to the meter board. The inverter goes where the shade is. The meter board is where the builder put it, often four floors away.

4. Earthing. Pits, chemical compound, strips and the excavation. Rocky ground turns a routine item into a day of labour.

5. Lightning protection and surge devices. Frequently discussed, occasionally installed, rarely priced.

6. Conduit, trays and UV rated fittings. The difference between a job that looks right in year five and one that does not, and it never appears in a competitor's cheap quotation either.

7. Access and handling. Crane, hoist, scaffolding, or six people carrying modules up a staircase. On a narrow plot in a dense colony this can be the largest single surprise.

8. Walkways and maintenance access. Not optional on a dense array, and a customer who cannot reach their modules will not clean them.

9. Water line for cleaning. If there is no tap on the terrace, someone is carrying buckets, and generation suffers within a season.

10. DISCOM charges and the net meter. Application fees, meter cost, testing charges, and in some states a security deposit. State clearly whether your price includes them.

11. Load enhancement. If the system exceeds sanctioned load, the customer needs an enhancement first, with its own fee and its own timeline. Catch this from the bill, as described in sizing a system from the customer's electricity bill.

12. Connectivity. With daily generation reporting now required under PM Surya Ghar, the SIM and its recurring cost are part of the system. Say who pays for it in year two.

13. Transport, unloading and storage. Including what happens if the site is not ready and the material sits somewhere for a fortnight.

14. Wastage and consumables. Cable offcuts, fasteners, sealant, MC4 connectors, spare fuses. Individually trivial, collectively a percentage point.

The extra ones on commercial jobs

Structural assessment. An older industrial roof may need an engineer's opinion before anything is fastened to it, and sometimes reinforcement after it.

Sheet fixing and waterproofing. Standing seam clamps, self drilling fasteners, sealing, and the warranty conversation with whoever installed the roof. A leak traced to your fixings is an argument you lose even when you are right.

Shutdown windows. Tie in work on a running plant happens on a Sunday or at night, at a different labour rate.

Storage. In Maharashtra above 100 kW, a battery is now part of the scope by rule, which is a very large line item to discover late. See Maharashtra wants a battery on every job above 100 kW.

The two clauses that protect the rest

Even a complete bill of materials does not survive a market that moves twice a year. Two clauses do most of the work.

An exclusions list. Short, specific and on the same page as the price. Civil work beyond X, structure beyond Y metres, DISCOM charges, load enhancement, anything behind the meter board. A customer who reads it before signing cannot be surprised by it afterwards, and most will not even question it.

A price validity and variation clause. A stated validity period, and a defined mechanism if module or cell prices move beyond a threshold. This has gone from prudent to necessary: GST moved down in 2025, cell costs moved up when ALMM List-II took effect in June 2026, and both landed inside a single sales cycle. That story is in ALMM List-II is live.

Why this keeps happening

Because the bill of materials usually lives in one person's head or in a spreadsheet they built and never revisit, and because the cost of getting it wrong arrives weeks later, attached to a different job number.

The fix is not discipline, it is structure. A standard template with every item above present, forced to be either priced or explicitly excluded, and a design that produces its own quantities: cable lengths from the actual layout, structure from the actual roof, module count from the actual array.

When the quantities come out of the design rather than out of memory, the surprises mostly stop. The residual ones, access and the state of the roof, are the ones you catch by walking the site properly.

The short version

• Margin is usually lost after signature, in items that were never in the bill of materials.
• The usual residential misses: structure height, split array DC runs, the AC run to the board, earthing, surge protection, conduit, access and handling, walkways, a water line, DISCOM charges, load enhancement, connectivity, transport, and wastage.
• Commercial adds structural assessment, sheet fixing and waterproofing, shutdown windows, and in Maharashtra a mandatory battery above 100 kW.
• Carry a specific exclusions list on the price page.
• Carry a validity period and a price variation mechanism. Two rule changes moved prices in opposite directions inside one year.
• Generate quantities from the design, not from memory.