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Rooftop Solar Planner
How many kW your roof needs, what it costs after the PM Surya Ghar subsidy, and how fast it pays back.
Quick Answer
In most of India, 1 kW of rooftop solar makes about 100 to 130 units a month on a yearly average, so a home using 300 units a month needs a 2.5 to 3 kW system, and 3 kW takes about 325 sq ft of shadow-free roof. Under PM Surya Ghar, the central subsidy is ₹30,000 per kW for the first 2 kW and ₹18,000 for the third kW, so it tops out at ₹78,000 for 3 kW or more (₹85,800 in the special category states). At MNRE's benchmark cost of ₹50,000 per kW, a 3 kW system costs ₹1,50,000, or ₹72,000 after the subsidy. Enter your bill units, state and roof area below for your own numbers.
Figures checked Oct 2026. Source: MNRE: Guidelines for PM-Surya Ghar: Muft Bijli Yojana, CFA to Residential Consumers, para 5 (CFA, benchmark cost, DCR), pp. 5 to 8 (copy on the Delhi Government solar portal).
100–130units per kW per month, yearly average
107.64 sq ftshadow-free roof per kW
0.78loss factor, from NREL PVWatts
35states and UTs in the table
Part of the system: Inverter backup for a 2BHK, Plan rooftop solar for a 3BHK. Steps, checks and a free printable page.
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The Formula
System size = monthly units ÷ (sunshine × 0.78 × 365 ÷ 12), rounded up to 0.5 kW and capped by roof area ÷ 107.64 sq ft per kW; payback = (kW × cost per kW − PM Surya Ghar subsidy) ÷ (units offset × 12 × your rate per unit)
Sunshine is the 20-year average solar energy reaching the ground in your state, from NASA's POWER dataset. The 0.78 factor turns that into units per kW of panels after heat, wiring, inverter and dust losses. We set it from NREL's PVWatts model, run at eight of these cities. PVWatts gave 0.78 to 0.91, and we use the bottom of that range so the estimate errs low.
Assumptions
- Generation is a yearly average. A system makes more in March to May and less in the monsoon, so one month's bill can look better or worse than this.
- Panels face roughly south, tilted near your latitude, with no shade through the middle of the day. Shade from a water tank or the next building can cut output a lot.
- Roof area: 10 m² (107.64 sq ft) of shadow-free roof per kW, the low end of MNRE's 10 to 12 m². Leave extra for walkways and the tank.
- Subsidy as in MNRE's PM Surya Ghar guidelines: ₹30,000/kW for the first 2 kW, ₹18,000 for the third kW and nothing beyond 3 kW. In Uttarakhand, Himachal Pradesh, Jammu and Kashmir, Ladakh, the North East including Sikkim, Andaman and Nicobar and Lakshadweep it is ₹33,000 and ₹19,800.
- The subsidy needs a residential connection, a vendor registered on the national portal, and modules made in India from Indian cells (DCR). Some states add their own subsidy on top; this calculator does not include it.
- Savings count only the units you would otherwise buy from the DISCOM, at your own average rate. Extra units sent to the grid are not counted, because what DISCOMs pay for them differs by state.
- Payback is simple payback. It ignores panel ageing (output falls slowly over the 25-year life), tariff increases, loan interest and inverter replacement.
Solar generation per kW, state by state
These are the figures the calculator uses. Each state is represented by one city, usually its capital, so a district far from it may differ a little.
| State / UT | Reference city | Sunshine (kWh/m²/day) | Units per kW per day | Units per kW per month | Subsidy rate |
|---|---|---|---|---|---|
| Andaman and Nicobar Islands | Port Blair | 5.04 | 3.93 | 120 | Special category |
| Andhra Pradesh | Amaravati | 5.12 | 3.99 | 121 | Standard |
| Arunachal Pradesh | Itanagar | 3.65 | 2.85 | 87 | Special category |
| Assam | Guwahati | 4.39 | 3.42 | 104 | Special category |
| Bihar | Patna | 4.78 | 3.73 | 113 | Standard |
| Chandigarh | Chandigarh | 5.22 | 4.07 | 124 | Standard |
| Chhattisgarh | Raipur | 5.16 | 4.02 | 122 | Standard |
| Delhi | New Delhi | 4.89 | 3.81 | 116 | Standard |
| Goa | Panaji | 5.32 | 4.15 | 126 | Standard |
| Gujarat | Ahmedabad | 5.53 | 4.31 | 131 | Standard |
| Haryana | Gurugram | 4.89 | 3.81 | 116 | Standard |
| Himachal Pradesh | Shimla | 4.70 | 3.67 | 112 | Special category |
| Jammu and Kashmir | Jammu | 5.15 | 4.02 | 122 | Special category |
| Jharkhand | Ranchi | 4.75 | 3.71 | 113 | Standard |
| Karnataka | Bengaluru | 5.48 | 4.27 | 130 | Standard |
| Kerala | Thiruvananthapuram | 5.40 | 4.21 | 128 | Standard |
| Ladakh | Leh | 5.08 | 3.96 | 120 | Special category |
| Lakshadweep | Kavaratti | 5.33 | 4.16 | 127 | Special category |
| Madhya Pradesh | Bhopal | 5.22 | 4.07 | 124 | Standard |
| Maharashtra | Mumbai | 5.29 | 4.13 | 126 | Standard |
| Manipur | Imphal | 4.58 | 3.57 | 109 | Special category |
| Meghalaya | Shillong | 4.29 | 3.35 | 102 | Special category |
| Mizoram | Aizawl | 4.72 | 3.68 | 112 | Special category |
| Nagaland | Kohima | 4.31 | 3.36 | 102 | Special category |
| Odisha | Bhubaneswar | 4.84 | 3.78 | 115 | Standard |
| Puducherry | Puducherry | 5.40 | 4.21 | 128 | Standard |
| Punjab | Ludhiana | 5.03 | 3.92 | 119 | Standard |
| Rajasthan | Jaipur | 5.44 | 4.24 | 129 | Standard |
| Sikkim | Gangtok | 4.07 | 3.17 | 96 | Special category |
| Tamil Nadu | Chennai | 5.22 | 4.07 | 124 | Standard |
| Telangana | Hyderabad | 5.36 | 4.18 | 127 | Standard |
| Tripura | Agartala | 4.73 | 3.69 | 112 | Special category |
| Uttar Pradesh | Lucknow | 4.88 | 3.81 | 116 | Standard |
| Uttarakhand | Dehradun | 5.00 | 3.90 | 119 | Special category |
| West Bengal | Kolkata | 4.57 | 3.56 | 108 | Standard |
Sunshine: NASA POWER, all-sky surface irradiance, 2001 to 2020 average, fetched 2 October 2026. Units per kW = sunshine × 0.78. Subsidy rate class from MNRE's PM Surya Ghar guidelines.
Worked Examples
A 300-unit home in Kolkata
- West Bengal sunshine 4.57 kWh/m²/day × 0.78 = 3.56 units per kW per day, or 108.3 a month
- 300 units ÷ 108.3 = 2.77 kW, rounded up to 3 kW. The 400 sq ft roof fits 3.5 kW, so the plan stays at 3 kW
- 3 kW makes about 325 units a month and costs 3 × ₹50,000 = ₹1,50,000
- Subsidy: ₹30,000 × 2 + ₹18,000 × 1 = ₹78,000, so you pay ₹72,000
- Saving: 300 units × 12 × ₹7 = ₹25,200 a year, so payback is about 2.9 years
A 150-unit home in Dehradun (special category rate)
- Uttarakhand sunshine 5.00 × 0.78 = 3.90 units per kW per day, or 118.6 a month
- 150 ÷ 118.6 = 1.26 kW, rounded up to 1.5 kW
- Cost at ₹55,000 per kW (the special category benchmark): ₹82,500
- Subsidy at the special category rate: ₹33,000 × 1.5 = ₹49,500, so you pay ₹33,000
- Saving: 150 × 12 × ₹6 = ₹10,800 a year, payback about 3.1 years
A 600-unit home in Ahmedabad with a small roof
- Gujarat sunshine 5.53 × 0.78 = 4.31 units per kW per day, or 131.1 a month
- Use alone needs 600 ÷ 131.1 = 4.58 kW, rounded up to 5 kW
- But 250 sq ft of roof fits only 2.32 kW, rounded down to 2 kW
- 2 kW makes about 262 units a month, roughly 44% of the bill, and costs ₹1,00,000 less ₹60,000 subsidy = ₹40,000
- Saving: 262 × 12 × ₹8 = ₹25,170 a year, payback about 1.6 years
How to Use This Calculator
- Find your monthly units. Take the units (kWh) from your last few bills and average them. A summer bill alone will oversize the system.
- Pick your state. The state sets how much sun the panels get and whether the special category subsidy applies.
- Measure the open roof. Count only the part with no shade through the middle of the day. Leave out the water tank, stair room and a walkway along the edge.
- Enter your rate and a quote. Your rate per unit is the bill amount divided by the units. For cost per kW, the default is MNRE's ₹50,000 benchmark; replace it with your installer's quote.
- Read the working. Every step is printed under the result, so you can check an installer's proposal against it line by line.
Tips and tricks
- Go on the roof at 10 am, noon and 3 pm and note where the shadows of tanks, parapets and next-door buildings fall. Panels need shadow-free area.
- Size from 12 months of bills. A single summer bill can mislead.
- Leave roof space for the water tank, a walkway and anything you'll add later.
- Compare quotes on the same items: panel make and wattage, inverter make, structure height, warranty years.
- Ask for the expected generation in writing and compare it with this page's per-kW figure for your state.
What we would do
Size from your average monthly units over a year, and go bigger only if the roof allows it and you know your use will grow (an extra AC, an EV). Get three quotes on identical specs. We don't advise on subsidy eligibility; check the official portal for that.
Frequently Asked Questions
How many units does a 1 kW solar panel system produce in India?
On a yearly average, about 2.9 to 4.3 units a day depending on the state, or 87 to 131 units a month. MNRE says 4 to 5.5 units on a clear sunny day; cloudy and monsoon days pull the average down. Gujarat, Karnataka and Rajasthan sit at the top of our table, and Arunachal Pradesh and Sikkim at the bottom.
What is the PM Surya Ghar subsidy for a 3 kW system?
How much roof area do I need for a 3 kW solar system?
What size solar system do I need for 300 units a month?
Does the subsidy apply to flats and housing societies?
Why is my installer's quote higher than ₹50,000 per kW?
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Sources & Limitations
- MNRE: Guidelines for PM-Surya Ghar: Muft Bijli Yojana, CFA to Residential Consumers, para 5 (CFA, benchmark cost, DCR), pp. 5 to 8 (copy on the Delhi Government solar portal)(accessed Oct 2026)
- PIB: PM Surya Ghar, India's Solar Revolution (consumption-to-capacity table), March 2025(accessed Oct 2026)
- PIB: Cabinet approves PM-Surya Ghar: Muft Bijli Yojana (a 3 kW system makes more than 300 units a month)(accessed Oct 2026)
- MNRE: Frequently Asked Questions, Solar Rooftop System, 28 April 2022 (10 to 12 m² per kWp; 4 to 5.5 units a day)(accessed Oct 2026)
- NASA POWER Climatology API: all-sky surface irradiance (ALLSKY_SFC_SW_DWN), 2001 to 2020(accessed Oct 2026)
- NREL PVWatts v8 (yield factor check at 8 cities)(accessed Oct 2026)
What this calculator does not model
- It uses one city per state. Hill districts, coastal fog and local haze can change generation.
- It does not model shade, roof direction or tilt. An installer's site survey should.
- It leaves out state top-up subsidies, the value of exported units and loan interest.
- The subsidy figures are from MNRE's guidelines for the scheme, which runs until 31 March 2027. Check the national portal before you sign.