On-Site Renewables

Recall the honest catch from Module 0: electrification only decarbonises a building as fast as the grid cleans. On a coal-heavy grid, an all-electric building's near-term carbon win can be modest, because it is drawing largely coal-fired electricity through the wire. On-site renewables cut straight through that problem. When a building generates its own solar power and uses it directly, those electrons never come from a coal plant at all - the building sidesteps the dirty grid rather than waiting for it to clean up. Electrification sets a building up to ride grid decarbonisation; on-site solar lets it decarbonise a slice of its own supply immediately, without waiting for the utility.

The pairing is tighter than 'both are green'. An all-electric building runs everything - heating, cooling, hot water, cooking, appliances - on a single currency: electricity. That is exactly the currency solar produces. A building that still burned gas for heat could only ever solar-power part of itself; an all-electric building can, in principle, be powered substantially by its own roof. And the biggest all-electric load in India - cooling - happens to peak when the sun is strongest. Solar generation and air-conditioning demand rise and fall together through the hot part of the day, a natural (if imperfect) match that makes on-site solar unusually valuable in a cooling-led climate.

There is a hard-nosed economic layer too. A unit of solar you make and use yourself displaces a unit you would have bought at the full retail tariff, transmission losses and all. A unit you generate but cannot use, and export, is usually worth far less. So on-site generation is worth most when the building can actually consume it - which is why generation, efficiency and flexibility belong in the same conversation. An efficient building needs less; a flexible building can move its demand to when the sun is up; both raise the value of every panel. This is 'efficiency first, then electrify, then flex' seen from the generation side: the array is worth more on a building that is already lean and already electric.

One boundary, as always: how many panels a roof can carry, how they connect, what the building's electrical system and the local grid can accept, and every carbon or payback figure are engineering and regulatory questions - for a qualified electrical engineer, the installer, and the DISCOM under the net-metering rules. The design idea here is the strategy; the sizing and interconnection are theirs.

Once a building generates its own power, every unit it makes goes to one of two places: it is self-consumed (used on the spot by a load that happens to be running) or it is exported to the grid. The split between them - the self-consumption ratio - quietly governs the whole economics and much of the carbon case of on-site solar, and it is the single most useful concept in this lesson.

Here is why it matters. A self-consumed unit displaces a unit you would otherwise have imported at the full retail price. An exported unit earns only whatever the export arrangement pays - and that varies enormously. Under generous net metering, exports can be credited against imports at close to retail value, so the distinction softens. Under a low feed-in tariff or a net-billing scheme that pays wholesale rates, an exported unit might be worth a fraction of a self-consumed one. In some places export is capped, discouraged, or not paid at all. So the same solar array can be a brilliant investment or a mediocre one depending on how much of its output the building uses itself versus dumps onto the grid - and on the local rules, which are set by the state regulator and the DISCOM, not by the designer.

This reframes design. It is not enough to maximise how much a roof generates; you want to maximise how much of that generation the building can actually absorb. A building whose loads run in the daytime - offices, schools, shops, daytime cooling - self-consumes solar naturally and is a superb solar host. A building whose demand is mostly in the evening, after the sun has gone, exports its midday surplus and then re-imports expensively at night: a poor natural match. The gap between generation and demand is precisely the opening for storage and flexibility (the rest of this module): a battery stores the midday surplus for the evening; a flexible load (pre-cooling, water heating, EV charging) is *moved* into the sunny hours so it can be self-consumed rather than exported.

So 'how much solar should we put on?' is the wrong first question. The better ones are: how much can this building use directly, how much can we shift to use directly, and what will the rest be worth when exported under our local rules? Answer those and the array sizes itself around real value - though the binding sizing, the interconnection and the tariff arithmetic still go to the engineer, the installer and the DISCOM.

On-site solar has one inescapable property: it is variable and intermittent. It generates only when the sun is up and strong - nothing at night, little in early morning and late evening, and much less under heavy monsoon cloud. A rooftop array does not produce a steady trickle you can rely on around the clock; it produces a bell-shaped burst in the middle of the day and then stops. This is not a defect to be engineered away - it is the fundamental nature of the resource, and a designer who pretends otherwise will build a disappointing system.

The consequence is the evening gap. In most buildings, and especially in Indian homes, a big chunk of demand - cooking, lighting, the heaviest cooling, everyone home and appliances running - lands in the evening, exactly when solar output has collapsed to nothing. So a solar-only building generates a surplus it cannot use at midday and faces its largest demand when its panels are dark. Solar covers the daytime beautifully and the evening not at all. This is the building-scale echo of the grid's 'duck curve': solar floods the middle of the day and leaves a steep ramp into the evening peak.

This is why on-site generation is never the whole story and why this module is titled 'generation and storage'. To make solar genuinely useful across the day you need one or more of three things: storage, to move midday surplus into the evening (batteries, next lesson; thermal storage, the one after); flexibility, to move the loads themselves into the sunny hours (pre-cooling, heating water, charging the EV at noon rather than 8pm); and the grid, which where reliable acts as a giant shared battery - you export surplus and draw it back later, letting the wider system do the balancing. Most real buildings use a blend of all three. On-site generation, storage and flexibility are three faces of one design problem: matching a variable supply to a demand that has its own rhythm.

Two honest riders. First, on a still-coal grid, exporting your surplus and re-importing at night means your night-time electricity is still whatever the grid is burning - solar does not clean your evenings by itself. Second, the array's own manufacture carries embodied carbon and material impacts; it pays that back over its life, but 'on-site solar' is not cost-free or impact-free, only far better than the fossil alternative. Clear-eyed, not starry-eyed.

On-site renewables are not bolted on at the end; the best results come from decisions made at design stage, most of them architectural. The first is simply roof and facade real estate: an unshaded, well-oriented roof of the right pitch, kept clear of clutter, water tanks and haphazard services, is a solar asset, and one designed around from the start is worth far more than one fought for later. In dense Indian cities where roofs double as terraces, water storage and services, protecting solar-ready area is a real design act.

Second is orientation and shading. In the northern hemisphere a south-facing tilt maximises annual yield, but that is not the only goal. West-leaning panels generate later into the afternoon, better matching an evening cooling peak; splitting orientation can flatten the generation curve to fit the load. Shading is the silent killer - a single vent, parapet or neighbouring tower shadow can slice output - so the array must be planned against the real shadow study, a genuinely architectural analysis. This is also where building-integrated photovoltaics (BIPV) enter: PV that *is* the facade, canopy, skylight or roof rather than sitting on top of it, turning the envelope itself into a generator. BIPV is a rich design field in its own right, covered in the dedicated BIPV and Solar Architecture course; here it is enough to know it exists and that the earlier it is considered, the better it integrates.

Third is designing for self-consumption, the theme of this lesson made physical: locate and schedule the big electric loads so they can run on solar. Daytime cooling, water heating on a timer, and EV charging that can be nudged to midday all raise self-consumption without a single extra panel. This is where generation quietly hands off to flexibility and storage, and where the electrified building's loads become an asset rather than just a bill.

The firm boundary holds throughout. The building designer owns the strategy - protect the roof, orient for the load, design for self-consumption, leave room for storage and future BIPV. The binding results - array sizing, structural loading of the roof, electrical design, inverter and protection selection, interconnection approval and the net-metering paperwork, and every yield, carbon and payback figure - belong to the structural and electrical engineers, the solar installer, and the DISCOM under the governing codes (in India, the relevant IS standards, CEA regulations and state net-metering rules). Design brilliantly for solar; let the specialists make it real and safe.