What size solar system do I need?
Divide your average monthly units by roughly 110 to 115, the number of units a kilowatt of rooftop solar generates in a month on Verde Solaire's published planning yield after a derate for dust and monsoon, and round to the nearest published capacity. A 300-unit household lands close to 3 kW, and most homes using 300 to 400 units a month settle on 3 kW. The rest of this guide works the formula through worked examples, sets out roof area and panel count per kilowatt, and covers the DISCOM limits that can change the answer.
Getting the size wrong costs money in both directions. A system too small still leaves a chunk of the bill on the grid at the most expensive slab, so the saving is smaller than the roof could have delivered. A system too large past the subsidised 3 kW step pays full price for the extra kilowatts and spends part of the year exporting more than it uses, credited at the DISCOM's own rate rather than what you would have paid to buy it. The right answer sits at the average of your bill, not at the smallest system that fits the roof or the largest one the roof can hold.
Reading your bill for the right number
The figure to divide is units, sometimes printed as kWh, on the consumption section of your electricity bill, not the rupee total. Add up the units on your last twelve bills and divide by twelve; a single month understates winter and overstates a summer peak, and either one on its own points to the wrong size. If your board bills every two months, halve the two-monthly figure before averaging. A household moving into a new connection with no bill history can use a comparable neighbour's bill or the solar calculator, which asks for the same average and returns a size, generation and net-price estimate together.
The formula, and why it uses a derate
Verde Solaire's commercial planning pages give a yield of about 1,450 units per kW a year in Delhi and 1,500 in Noida before losses. This guide applies a further 10% derate for dust and monsoon shading, which is Verde Solaire's own editorial assumption rather than a published figure, and divides by twelve for a monthly average. Per kilowatt, that works out to about 109 units a month in Delhi and about 113 units a month in Noida, or call it 3.5 to 3.7 units a kilowatt a day. Divide your monthly average by that per-kilowatt figure and you have the capacity that would offset it over a year; the system size calculator runs the same arithmetic on your own bill.
0–150 units a month: 1–2 kW · 150–300 units: 2–3 kW · above 300 units: above 3 kW is the scheme's own shorthand, and it agrees with the arithmetic above at every band.
Sizing ladder: capacity, generation, roof and price
The table below turns the formula into a lookup. Pick the row closest to your twelve-month average from the generation columns, and read off the roof area and the published price for that capacity; if your average falls between two rows, the worked examples further down show how Verde Solaire's own customers usually decide.
| Capacity | Generates in Noida | Generates in Delhi | Roof area (approx.) | Published price |
|---|---|---|---|---|
| 1 kW | about 113 units a month | about 109 units a month | 100 to 117 sq ft | ₹70,000 to ₹80,000 |
| 2 kW | about 225 units a month | about 218 units a month | 200 to 233 sq ft | ₹1,35,000 to ₹1,50,000 |
| 3 kW | about 338 units a month | about 326 units a month | 300 to 350 sq ft | ₹1,99,000 to ₹2,10,000 |
| 5 kW | about 563 units a month | about 544 units a month | 500 to 585 sq ft | ₹3,25,000 to ₹3,50,000 |
| 7 kW | about 788 units a month | about 761 units a month | 700 to 820 sq ft | quoted per site |
Roof area scales from the 300 to 350 square feet Verde Solaire publishes for a 3 kW system, which uses six panels of about 540 Wp, the module size on its recent Noida and Delhi installations; a 5 kW system uses ten, and a 7 kW system about thirteen. Prices beyond the published 1, 2, 3 and 5 kW steps, including 7 kW above, are quoted per site after a survey, and the 10 kW price page carries the next published step.
Worked examples for 200, 400 and 700 units a month
About 200 units a month. By the formula, roughly 1.8 kW; round up to the published 2 kW step. Roof area is about 200 to 233 square feet, and the 2 kW price page has the published range. This is a small flat or a one- or two-person household with modest air-conditioner use.
About 400 units a month. By the formula, roughly 3.5 to 3.7 kW, on the boundary between the published 3 kW step and the next one up. A 3 kW system generates about 338 units a month in Noida and about 326 units a month in Delhi, which covers most but not all of a 400-unit household and leaves a smaller residual bill; because the subsidy is capped at 3 kW, every kilowatt bought beyond that is unsubsidised, so some households buy 3 kW and accept the residual, and others step up to 5 kW for headroom. The 3 kW vs 5 kW guide sets out that trade-off with the same figures used here.
About 700 units a month. By the formula, roughly 6.2 to 6.4 kW, above the published ladder. A system in this range is quoted per site after a roof survey; a 5 kW system alone generates about 563 units a month in Noida, which would still leave a meaningful residual for a 700-unit household, so 6 to 7 kW is the more realistic target, subject to the roof area and the sanctioned load below.
System size calculatorEnter your average monthly units and get a recommended capacity, roof area and generation estimate for your city.Open the toolSanctioned load and DISCOM limits
The kilowatt figure above is what your roof and your bill call for; what the electricity board allows is a separate check. In Delhi, under the DERC amendment of July 2024, no feasibility study is needed up to 10 kW on the same supply type, and the DISCOM handles any load increase or transformer upgrade, so a 5 or 7 kW system on a smaller existing connection is not refused outright, and Delhi's rule applies up to 10 kW regardless of what you are sanctioned for today. In Uttar Pradesh, under the UPERC rooftop solar regulations of 2019, residential users may choose net or gross metering, and the DISCOM allows capacity within transformer limits, which is where a large request on a crowded local transformer, more than a formula on this page can predict, may be trimmed by the DISCOM. Either way, the feasibility check happens after you apply, not before, so size on your bill first and let the survey and the DISCOM confirm the rest.
Do not size on your highest bill
Twelve months, not one. A home that reads 250 units in a winter month and 800 in June averages around 450, and the formula above works from that average, not the peak. A system bought to offset the June air-conditioner load alone spends the cooler months generating more than the house draws, and in Delhi any surplus at year end is settled at the DISCOM's own rate rather than the retail tariff you would have paid for it. The summer AC bill guide and the seasonal generation guide both work from the same twelve-month view.
Questions people ask
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