What we check before a rooftop solar EPC proposal

The most common solar enquiry is "how much can our roof take?" — and the honest answer is that the roof is the third question. This is what our solar engineers check before a capacity is proposed, in the order the answers constrain it.

1. Twelve months of electricity bills

Consumption by month, the sanctioned load, the tariff and the demand charges. The plant is sized to consumption — to the daytime share of it, under net-metering — and the bills are the only record of what that is. A single bill sizes the plant to a season.

2. Sanctioned load and the metering model

Net-metering capacity is capped at or near the sanctioned load in most states; gross-metering, captive HT and open-access models have different rules. Which model applies, and what the cap is, decides the maximum before the roof is measured. The sanctioned-load check runs this first.

3. Load profile — day against night, weekday against weekend

A plant that runs three shifts uses daytime solar directly; a single-shift plant that shuts on Sundays exports on Sundays, at whatever the export tariff is. The profile decides how much of the generation displaces retail units and therefore what the savings are.

4. The roof — type, age, structure, usable area

RCC or metal sheet; its age and condition; what it can carry, from the structural drawings or a structural engineer's assessment; and the area that is actually usable once plant, parapets, skylights, vents and maintenance access are excluded. An old sheet roof is assessed before it is promised an array.

5. Shadow analysis

Adjacent buildings, the plant's own chimneys, water tanks and AC units, trees, and the sun's path through the year. The shadow-free area is what the array is laid out on; the rest of the roof is not solar roof.

6. DISCOM, connection voltage and the point of connection

Which DISCOM, LT or HT connection, where the LT panel is, whether it has a spare way, and the cable route from the roof. HT connection brings the CEIG into scope; the DISCOM's connection standard sets the protection the inverter must be configured to.

7. Approvals in the way

Net-metering feasibility, CEIG where applicable, a structural stability certificate, and any scheme requirements — ALMM modules, DCR — for a subsidised or government-linked project. Filed before installation, or the plant waits.

8. Storage, expansion and what the client actually wants

Backup for critical loads, load-shifting for peak tariffs, a future expansion the roof should leave room for. A plant is easier to design for these now than to retrofit.

What commonly changes the proposed capacity

• The sanctioned load is lower than the roof-limited capacity — the cap decides.
• Daytime consumption is a fraction of the total; sizing to the total over-exports.
• The roof cannot carry a standard structure without strengthening.
• Shading removes a large part of the apparent area.
• The LT panel has no spare way, or the point of connection is far from the roof.

What the checks produce

A proposal sized to consumption and sanctioned load, on a structure the roof can carry, with the approvals named and the yield stated with its assumptions — which is what the evaluation checklist tests for. The solar calculator frames the commercial case before the survey; the solar EPC page describes the delivery.