Rooftop solar has quietly become one of the best returns available to an Indian household. With the PM Surya Ghar subsidy launched in 2024, a typical family can recover the cost in three to five years from panels that last twenty-five. But the numbers only work if the system is sized properly. Here is how to do that without trusting a salesperson's guess.
How a rooftop system turns sunlight into savings
Solar panels are made of silicon cells. When sunlight hits a cell, it knocks electrons loose and creates a direct current. An inverter converts that DC into the AC your home uses. With an on-grid system and net metering, any electricity you do not use flows into the grid, and the utility credits it against what you draw at night. You need no batteries, which keeps cost and maintenance low.
Step 1: read your electricity bill
Look at units consumed over the last twelve months and take the average. Summer months with air conditioning can be double the winter figure, so a single bill is misleading. Suppose your average is 300 units a month, or 10 units a day.
Step 2: know your sunlight
The figure that matters is peak sun hours, the equivalent number of hours of full-strength sunlight per day averaged across the year. Most of India receives between 4 and 5.5. Western and central states sit around 4.5 to 5.5, while cloudier and more northern regions are closer to 4.
Step 3: apply the real-world losses
A panel rated at 540 W produces that only under laboratory conditions. On a real roof, heat reduces output, dust blocks light, and the wiring and inverter lose a few percent each. Combined, you can expect about 75% of the rated output. So:
System size (kW) = daily units ÷ (peak sun hours × 0.75)
For 10 units a day at 4.5 sun hours: 10 ÷ 3.375 = 2.96 kW. Rounded to whole panels, that is six 540 W panels, or 3.24 kW.
Step 4: check the money
| Item | 3.24 kW example |
|---|---|
| System cost (about ₹55,000 per kW) | ₹1.78 lakh |
| PM Surya Ghar subsidy (2024 rates) | ₹78,000 |
| Your net cost | about ₹1 lakh |
| Yearly units generated | about 3,990 |
| Yearly saving at ₹8 per unit | about ₹28,800 |
| Payback | about 3.5 years |
After payback, the electricity is effectively free for the remaining two decades of panel life, minus cleaning and one inverter replacement along the way.
What decides whether solar works for you
- Shadow-free roof. You need roughly 100 sq ft per kW with no shade from water tanks, trees or neighbouring buildings between 9 am and 4 pm.
- Tariff slab. The higher your per-unit rate, the faster the payback. Households in the top slabs benefit most.
- Roof life. If the roof needs waterproofing in the next few years, do it before installation.
- Vendor quality. Choose an empanelled vendor, ask for panel and inverter datasheets, and get the warranty terms in writing.
On-grid, off-grid or hybrid?
On-grid systems are cheapest and qualify for net metering, but they switch off during a power cut for the safety of line workers. Off-grid systems use batteries and work without the grid, but cost much more and the batteries need replacement. Hybrid systems combine both. For most city homes with reliable supply, on-grid with net metering gives the best return.
Frequently asked questions
Does solar work in the monsoon?
Yes, but output drops on overcast days, often to a third or less. The yearly calculation already averages this out.
How long do solar panels last?
Quality panels usually carry a 25-year performance warranty and lose about half a percent of output per year. Inverters typically last 8 to 12 years.
Can I install solar in a flat?
Individual flats usually cannot, but housing societies can install a common system for shared areas such as lifts, pumps and lighting, which cuts maintenance charges.