Solar PV Potential

A photovoltaic array turns part of the incident solar radiation into electricity - typically 18-22 percent of it for a modern silicon module, the rest lost as heat and reflection. So the cumulative radiation map from Lesson 6.1 is already half the PV study: the high-kWh/m2 surfaces are the good sites, and the roof, facing the whole sky all day, almost always tops the list.

Rooftop PV is the workhorse. A flat roof lets you tilt panels freely toward the sun and space rows to avoid self-shading; a pitched roof takes panels flush at whatever tilt and orientation the roof already has, which is simpler but ties yield to the roof geometry. Facade PV / BIPV (building-integrated photovoltaics) puts modules on vertical walls or as cladding - architecturally powerful and useful on tall towers with little roof per floor, but a vertical surface collects markedly less annual radiation than a well-tilted roof, so yield per panel is lower.

How much capacity fits is a rough area sum: a modern module produces on the order of 190-210 watts per square metre of panel at standard test conditions, and after walkways, plant, shading set-backs and tilt spacing you can usually populate perhaps half to two-thirds of a flat roof. As a planning number, useful rooftop area divided by roughly 7-10 square metres per kilowatt gives an installable kWp. That capacity, not the raw roof area, is what drives the yield estimate.

Two geometric choices set how much a fixed array collects: the tilt (angle from horizontal) and the orientation/azimuth (compass direction it faces). The physics is the same cosine effect that governed shading - a panel collects most when it faces the sun squarely across the year.

The classic rule of thumb is tilt roughly equal to the site latitude, facing the equator - south in India and the rest of the northern hemisphere. At a latitude of, say, 20 degrees, a panel tilted near 20 degrees and facing south collects close to the annual maximum. Tilt it flatter and you favour summer and lose winter; steeper and you favour winter. Crucially, the peak is broad and forgiving - within roughly 15 degrees of the optimum tilt, and within maybe 30-45 degrees of due south, yield typically falls only a few percent. This is why so many real installs simply lie flush on an existing pitched roof and still perform well.

A few practical wrinkles. In hot, dusty Indian conditions a slightly steeper tilt sheds dust and rain better, which protects yield between cleanings. On a flat roof, tilting rows means spacing them so a front row does not shade the one behind in winter - a shadow-study question straight out of Lesson 6.2. And east/west 'butterfly' layouts, though below the south-facing peak, spread generation across the morning and evening, which can suit a load that is not midday-heavy. The right tilt is the one that serves the building's demand, not only the one that maximises the annual kWh.

Capacity in kWp is a nameplate; yield in kWh/year is what the building actually gets, and it is always less than the naive product of capacity and sunshine because of real-world losses. The bridge between them is the performance ratio (PR) - the fraction of the theoretically available energy the system actually delivers after losses from heat, wiring, inverter conversion, soiling, shading and downtime. A decent rooftop system runs a PR around 0.75-0.80.

The cleanest way to express results is specific yield - kWh generated per kWp installed per year. For much of sunny India that lands roughly in the 1,300-1,600 kWh/kWp/yr range; cooler or cloudier regions less, the clear high-altitude northwest more. A worked estimate: a 5 kWp rooftop array at a specific yield of 1,500 gives about 5 x 1,500 = 7,500 kWh/year - enough to cover a large share of a typical urban home's consumption. Note the temperature catch: PV output falls as cells heat up, so a scorching site delivers a bit less per installed watt than its raw radiation suggests, which the PR and specific-yield figures already fold in.

The free tool of record is PVGIS (the European Commission's Photovoltaic Geographical Information System), which covers India and most of the world: enter location, capacity, tilt, orientation and a loss assumption, and it returns monthly and annual yield from its own solar database. Ladybug's PV components do the same inside Grasshopper and add proper geometric shading from your model. Use both - PVGIS for a fast, credible headline number, Ladybug when self-shading and context matter.

Generating the units is only half the economics; what happens to them decides the value, and it hinges on the mismatch between when the sun shines and when the building uses power.

Self-consumption is the share of generation the building uses on site, instantly, offsetting electricity it would otherwise buy - so each self-consumed unit is worth the full retail tariff. Export is the surplus pushed to the grid, usually at midday when generation peaks but many buildings (especially homes) use little. Under net metering - the common Indian rooftop arrangement - exported units spin the meter backwards and offset later imports, so export still has value; under gross metering or a low feed-in tariff, exported units are worth far less than self-consumed ones. Either way, the daily picture is a tall midday generation hump against a load that is often higher morning and evening - the classic 'duck' mismatch.

This is why self-consumption is a design lever, not just an accounting term. Shifting flexible loads into the sunny middle of the day - running the water heater, pool pump, EV charger or pre-cooling on a timer - raises self-use without any extra hardware. A battery stores midday surplus for the evening peak, pushing self-consumption higher, at a capital cost you weigh against the export tariff. For an architect the takeaway is that PV is not only a roof-area question: the more the building's demand can be nudged to coincide with generation, the more every panel is worth.

India has among the world's strongest solar resources and a policy push to match, which is why PV potential is increasingly part of an architect's brief. National programmes - most recently the large residential rooftop scheme branded PM Surya Ghar - subsidise home rooftop systems, and state DISCOMs run net-metering rules that set how export is credited. Green-building frameworks reinforce it: GRIHA and IGBC award on-site renewable generation, and net-zero-energy targets make rooftop PV the usual first move once demand has been cut. The exact subsidy amounts, capacity caps and metering rules change and vary by state, so treat specific figures as things to confirm with the current DISCOM and scheme - not to quote from memory.

To simulate a building's potential, the workflow chains the module together. Start with the cumulative radiation map to find the good surfaces and quantify self-shading (a stair core or water tank shading part of a roof genuinely costs yield). Size an installable capacity from the usable area. Push location, tilt, orientation and losses through PVGIS for a headline annual yield, or through Ladybug's PV component when the model's own shadows matter. Then set that generation against an hourly load profile to read self-consumption versus export, and test whether load-shifting or a battery is worth it.

The honest framing is the same as everywhere in this course: this is a credible early estimate to guide the decision - roof-area reservation, orientation, whether BIPV earns its cost - not a guaranteed generation figure or a substitute for a MNRE-empanelled installer's detailed design and the DISCOM's sanction. Simulate to decide where and how much; leave the certified sizing and grid approval to the specialist.