Ten 550 Wp Modules, an 8 m × 6 m Roof and a Water Tank in the Corner
A homeowner in Ludhiana (latitude about 30.9° N) wants the largest possible grid-tied system on a flat RCC roof measuring 8 m north–south by 6 m east–west. A 2 m × 2 m water tank stands in the north-east corner and a 1.2 m high parapet runs along the southern edge; the east and west edges are open. The customer has already bought ten 550 Wp modules (2.28 m × 1.13 m, Voc 49.6 V, Isc 14.0 A) and an 8 kW string inverter with two MPPTs. The DISCOM has sanctioned a 5 kW residential connection with net metering. A neighbour has told him that a 15° tilt 'catches more sun', and he wants the frames fixed that way. For the site, winter noon sun elevation is about 36° and summer noon elevation about 82°. The subsidy file and the net-metering application must be submitted during the same visit, and the installation has to be finished before the monsoon.
1. Work out how many modules can actually be laid out without shading, showing the shadow, gap and pitch calculations. State your verdict on the ten-module plan.
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Shadow and pitch at 15° tilt
- tan(36°) = 0.73, so the parapet shadow at winter noon is 1.2 / 0.73 ≈ 1.65 m. No module may start closer than this to the south parapet.
- Module rise = 2.28 × sin 15° = 0.59 m; horizontal projection = 2.28 × cos 15° = 2.20 m.
- Inter-row gap = 0.59 / 0.73 = 0.81 m, so pitch (front edge to front edge) = 2.20 + 0.81 = 3.01 m.
- Depth needed for two rows = 3.01 + 2.20 = 5.21 m. Depth available = 8.00 − 1.65 = 6.35 m → two rows fit with 1.14 m spare.
- Width: 6.00 / 1.13 = 5.3 → five modules per row (5.65 m); six would need 6.78 m.
The 2 m × 2 m tank in the north-east corner removes roughly two positions from the north row, leaving 5 + 3 = 8 modules (4.4 kWp) shade-free.
Verdict: do not force ten modules in. Either relocate the tank, or accept 8 modules and renegotiate the inverter (see next task). Shading one module of a series string drags the whole string, so a '10-module' layout that shades is worse than a clean 8-module one. Confirm the tank shadow with a shadow meter at 9 am and 3 pm before signing the layout drawing.
2. Evaluate the 15° tilt and the choice of an 8 kW inverter for this array, and state what you would recommend instead.
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Tilt: the energy-optimal fixed tilt for 30.9° N is roughly 25–30°. At 25°: rise = 2.28 × sin 25° = 0.96 m, gap = 0.96 / 0.73 = 1.32 m, pitch = 2.07 + 1.32 = 3.39 m, two rows = 3.39 + 2.07 = 5.46 m - still inside the 6.35 m available. So the space argument for 15° is weak; 25° recovers roughly 3–4% annual yield and self-cleans dust better. Keep the tilt at 15° only if the customer needs walkway clearance, and then accept the energy penalty. Azimuth must be true south; magnetic declination at this longitude is under 1°, so a compass reading is close enough but note it in the survey.
Inverter: with 8 modules the array is 4.4 kWp on an 8 kW inverter → DC:AC = 0.55. The inverter runs most of the day at low load where part-load efficiency and start-up thresholds are poorest, and the customer has paid for capacity he can never use. A 5 kW inverter gives 0.88, which is a healthy ratio.
If the tank is relocated and all ten modules go up (5.5 kWp), 5.5 / 5 = 1.1 - still fine, and the 8 kW unit is still unnecessary. Either way, exchange the inverter before the mounting work starts.
3. List what you must record during this site survey so the subsidy and net-metering file survives scrutiny, and flag anything that could stall the application.
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Record on site (with photographs dated and geo-tagged):
- Address, GPS coordinates, DISCOM consumer number, sanctioned load (5 kW), meter type and serial number, existing single-line diagram.
- Roof type, dimensions, parapet and tank positions, shadow-free area marked on a scaled layout drawing with row lines and pitch dimensioned.
- Structure fixing detail: pedestal/chemical anchors that do not puncture the waterproofing, base plate size, and the basic wind speed for the site taken from IS 875 (Part 3) used for the anchor design.
- Cable route length (roof to inverter to ACDB to meter), proposed DC and AC cable sizes with the voltage-drop calculation.
- Earthing pit location and proposed resistance target, lightning arrester position and its zone of protection.
- Module and inverter make, model, serial numbers, ALMM listing and datasheet.
Risks to flag now: the sanctioned load must be at least equal to the inverter AC capacity for net metering - 5 kW sanctioned against a proposed 5 kW inverter is at the edge, so confirm the DISCOM's exact rule in writing. Also confirm the meter is a net meter or bi-directional capable, and that the roof is owned by the applicant (tenant installations need owner consent). File the survey photos with the application; missing roof-ownership proof is the most common cause of rejection.