The quickest way to size a rooftop system is to work backwards from your bill. Divide your monthly bill by your tariff to get units consumed, divide that by thirty for daily units, and divide again by about 3.6 to get the kW you need. On a Delhi NCR tariff near Rs 8 per unit, a Rs 3,000 monthly bill is roughly 375 units, about 12.5 units a day, which points to a system somewhere around 3 kW. Here is the same calculation across common bill sizes, assuming a Rs 8 tariff and typical Delhi NCR generation. A Rs 1,500 bill suggests roughly 1.5 kW. A Rs 3,000 bill suggests around 3 kW. A Rs 5,000 bill points to about 5 kW, and a Rs 8,000 bill to somewhere near 8 kW. These are starting points for a conversation, not final answers, because your roof and your usage pattern both shift the number. Why 3 kW is the number you will hear most often. The central subsidy caps at Rs 78,000, and you reach that cap at 3 kW. A 5 kW system receives exactly the same subsidy as a 3 kW one, so the extra two kilowatts are paid entirely by you at full price. That does not make larger systems wrong, but it does mean the economics change sharply above 3 kW. If your bill genuinely justifies 5 kW, you are still likely to save money overall, just with a longer payback on the unsubsidised portion. Check your roof will actually hold it. Budget roughly 80 to 100 square feet of clear, shadow-free roof per kW. A 3 kW system therefore wants around 250 to 300 square feet with genuinely open sky access. Count only the area that stays sunlit through the middle of the day. Space beside a water tank that falls into shadow by two in the afternoon is not usable area, and installers who count it anyway are setting you up for disappointing generation. Check your sanctioned load. Net metering is generally allowed up to your sanctioned load, so a system substantially larger than your sanctioned connection can run into approval problems. Your sanctioned load is on your electricity bill. If you are planning something large relative to it, raise this with your DISCOM early rather than after installation. Think about where your usage is heading, not just where it is. If you are likely to add an air conditioner, or buy an electric two-wheeler or car in the next couple of years, your consumption will rise and a system sized purely on today's bill will end up undersized. Solar is a twenty-five year asset, so sizing for the household you will be in three years is reasonable. Just weigh that against the subsidy cliff at 3 kW. Seasonal variation is normal and does not mean something is broken. Delhi NCR generation drops noticeably during the monsoon and during heavy winter smog, and peaks in the clear months of spring and autumn. The annual average is what matters for payback. A system that underperforms in a smoggy January is behaving exactly as expected, which is also why panel cleaning matters more here than in cleaner-air cities. What oversizing and undersizing actually cost you. Undersize and you keep paying a meaningful bill, though you still save. Oversize well beyond your consumption and the surplus you export earns credit at settlement rates that are usually less generous than what you pay for imported power, so the last few kilowatts pay back slowly. Sizing close to your annual consumption, rather than well above it, is generally the efficient choice. Two tools on this site will do this arithmetic for you. The rooftop solar calculator sizes a system from your bill and tariff and shows subsidy, net cost and payback. The roof area tool works the other way, from available space to installable capacity. Running both takes about three minutes and gives you two independent sanity checks before any installer visits, which is the best position to negotiate from.
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Last updated 24 July 2026 · Reviewed by the RenSetu team