Solar Panel Roof-Fit Calculator

70 sq ft per kW already counts the setbacks you were about to subtract

A roof-fit estimate is one division, repeated. Usable roof area divided by the roof space a single kilowatt needs — about 70 sq ft, or 6.5 sq m — gives the system size, and that rule of thumb already accounts for fire-code setbacks and the spacing between panel rows, so there is no second deduction to make. System size then drives everything downstream: multiplied by peak sun hours, by 365, and by a performance-ratio (derate) factor it becomes yearly generation in kWh; multiplied by a cost per kW it becomes the installed cost. Everything the page reports afterwards — panel count, monthly kWh, net cost, payback, EMI — is a restatement of those two products, which is why a wrong roof area is the one input that can be wrong everywhere at once.

The generation line is the one people most often want on its own: estimated yearly kWh = system size (kW) × peak sun hours per day × 365 × a performance ratio, typically 0.75-0.85, which absorbs inverter losses, wiring, dirt, and heat. The calculator runs it the moment it knows your system size and location, and reports a yearly figure alongside a monthly average, because the monthly number is the one your electricity bill is denominated in.

Usable area is not roof area, and 400W panels change only the count

Enter the area panels can actually occupy: exclude chimneys, vents, skylights, and any section that sits in shade for a meaningful part of the day. That number, not the footprint of the house, is what the division above consumes. In metric the same rule reads 6.5 sq m per kW, so a 100 sq m usable roof carries roughly 15.4 kW; the calculator takes either unit. Panel wattage works differently. The calculator turns system size into a panel count by dividing by the wattage you choose — 400W by default — so switching to higher-wattage panels lowers the panel count for the same roof without raising the kilowatts that roof can hold. More kW needs more usable area, a lower area-per-kW figure, or both, and the area-per-kW figure is editable for exactly that reason.

A pitched roof is larger than its footprint; how to calculate roofing squares shows how pitch changes the area you measure.

A 1,000 sq ft Indian roof: 36 panels, ₹7.79 lakh net, a 5.3-year payback

At 70 sq ft/kW, a 1,000 sq ft usable roof carries about a 14.3 kW system — roughly 36 panels at 400W each. At 5.0 peak sun hours and an 80% performance ratio, that is about 20,857 kWh a year, around 1,738 kWh a month. At ₹60,000/kW the gross cost is about ₹8.57 lakh; the PM Surya Ghar subsidy caps at ₹78,000 for a system this size, giving a net cost of about ₹7.79 lakh. At ₹7/kWh that is roughly ₹1.46 lakh a year in avoided electricity cost — a simple payback of about 5.3 years.

The subsidy line is formulaic, which is why the calculator applies it for you: PM Surya Ghar Muft Bijli Yojana pays ₹30,000 per kW for the first 2 kW and ₹18,000 for the third, capping at ₹78,000 for any system of 3 kW or more. A 3 kW roof and the 14.3 kW roof above draw the identical ₹78,000, so past 3 kW the subsidy stops growing while the cost keeps climbing, and the payback on a larger Indian system is carried almost entirely by the electricity it displaces. Select India, enter your usable roof area in sq ft or sq m, add a cost per kW and your own tariff, and the system size, panel count, net cost, and payback all arrive in rupees with no further input.

If you are borrowing, switch on “I’m financing this” in the Financing section: enter the share of the net cost you are borrowing, an annual interest rate, and a tenure in years, and it returns the monthly EMI and the total interest on the standard reducing-balance loan formula. The EMI sits beside the payback rather than inside it — financing changes what you pay each month, not the number of years the panels take to earn back their net cost.

Switching country changes four defaults, and only India carries an automatic subsidy

The country selector rewrites four starting figures: peak sun hours, cost per kW, electricity rate, and whether an incentive is applied for you. Only the last of the four is a scheme rule rather than a default you should overwrite.

India. The PM Surya Ghar central subsidy above is applied automatically. Special-category states can receive more, which this calculator does not model — check the official portal for your state. Tariffs are typically slab-based and vary widely by state, from roughly ₹3 to ₹10 per unit; the ₹7/kWh default is a starting point, not your bill. Installed cost before incentives typically runs ₹45,000-₹75,000 per kW.

USA. The 30% federal residential solar tax credit (Section 25D) expired for systems installed after December 31, 2025 under 2025 legislation, so no federal credit is assumed by default — put any state, utility, or other incentive you separately qualify for in the “Other incentive” field. Irradiance varies more here than anywhere else on the list, from roughly 3.2 peak sun hours in the cloudiest states up to 6.5 in the sunniest, so peak sun hours is the figure worth correcting first. Typical installed cost is $2,400-$3,300 per kW, or $2.40-$3.30 a watt.

UK. Residential installations are zero-rated for VAT (0%) under a relief currently extended through 31 March 2027, and that relief is normally already inside an installer’s quoted price, so it is not modelled as a separate deduction — treat the £1,400-£2,000 per kW range as VAT-inclusive already. If you plan to export surplus power, add your expected yearly Smart Export Guarantee (SEG) income as the optional export-income figure; SEG rates are set per supplier and commonly range from a few pence to around 20p per kWh.

Anywhere else, choose “Other / custom” and supply your own peak-sun-hours, cost-per-kW, and electricity-rate figures: the roof-area, generation, and payback arithmetic is identical everywhere, and only the pre-filled defaults and the automatic Indian subsidy are country-specific. Whichever country you pick, the cost-per-kW figure is a published starting range, and real quotes vary by equipment, roof type, and installer — replace it with yours the moment you have one.

That 5.3-year payback assumes nothing shades, ages, or reprices

Four limitations sit outside the estimate, and they are worth saying out loud before you quote a payback year from this page. This is a roof-area calculation, not a structural, shading, or electrical assessment: it does not judge whether your roof can carry the load, how it is oriented or tilted, or whether the wiring is adequate, so get a professional site survey before purchasing a system. The peak-sun-hours, cost-per-kW, and electricity-rate defaults are general starting points for a country, not your specific location or a current quote. India’s special-category-state subsidy top-up, above the standard ₹78,000 cap, is not modelled. And nothing here models panel output degradation over time, net-metering rules, or future changes to electricity rates or incentive schemes — 5.3 years is 5.3 years at today’s tariff, held flat forever.

It is worth being equally clear about what this page is. Calculator Matters built it independently; it is not affiliated with, endorsed by, or produced by Tata Power Solar or any other installer or brand. It performs the same roof-area-to-system-size and cost estimate installer calculators typically offer, but from published formulas and government scheme figures rather than any one company’s pricing — which means it cannot quote you, and it cannot stand in for the site visit that produces a quote.

Every number here rebuilds in a spreadsheet, subsidy formula included

There is no separate PDF version, and none is needed: every figure above rebuilds directly in a spreadsheet. Use =RoofArea/AreaPerKw for system size, =SystemSize*PeakSunHours*365*PerformanceRatio for yearly kWh, and =(SystemSize*CostPerKw)-Incentive for net cost, then divide that last result by your annual bill savings for payback. India’s subsidy is a formula too: =MIN(78000, 30000*MIN(Size,2) + 18000*MAX(0, MIN(Size-2,1))). This page is the full formula reference, and the calculator itself is free and runs entirely in your browser, on desktop or mobile — no signup, download, or app required.

Sources and methodology

Roof-area-to-system-size and generation assumptions follow commonly published rooftop-solar rules of thumb; India’s subsidy formula follows the published PM Surya Ghar Muft Bijli Yojana structure. Country defaults are general, sourced starting points, not a live data feed.

• PM Surya Ghar Muft Bijli Yojana — official government portal (India)
• IRS — Residential Clean Energy Credit (USA federal credit status)
• GOV.UK — VAT on energy-saving materials, Notice 708/6 (UK)
• Ofgem — Smart Export Guarantee (SEG) (UK)
• PVWatts Calculator (National Laboratory of the Rockies, formerly NREL) — solar generation estimation methodology reference

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