Sizing is where the margin in a rooftop job is made or lost. The customer asks for "a solar system"; your job is to convert that into a kilowatt figure they will still be happy with in year eight.
Start with units, not with roof space
Take twelve months of KSEB bills if you can get them, or at minimum a bill from the current season and one from the opposite season. What you want is average monthly units consumed, not the rupee amount, because the tariff is slabbed and the rupee figure hides the actual consumption.
As a working rule for Kerala, 1 kW of well-oriented rooftop solar generates roughly 100 to 120 units per month averaged across the year. Kerala's generous monsoon means the annual average sits below what the same array would produce in Rajasthan or Gujarat, and any calculator built on north-Indian assumptions will overpromise.
So a household consuming 300 units a month is broadly a 3 kW conversation. One consuming 500 units is a 4 to 5 kW conversation, with the phase question from the net metering rules immediately in play.
Then check the roof will take it
Budget 80 to 100 sq ft of genuinely shade-free area per kW. "Genuinely" is doing a lot of work in that sentence. Shade that arrives at 3 pm in December still costs generation, and a single shaded panel in a string drags down more than its own share of output.
- Walk the roof and look for what will shade it later in the day, not just now.
- Coconut palms grow. A clearance that works this year may not in five.
- Water tanks, parapets and satellite dishes cast longer shadows in winter than installers expect.
- On tin roofs, check the purlin spacing and condition before promising a mounting solution.
Matching panels to inverter
Once capacity is settled, the BOM follows. With modern high-wattage modules the panel count is lower than customers expect:
| System | Approx. modules at 540–550 W | Typical inverter |
|---|---|---|
| 3 kW | 5–6 panels | 3 kW single-phase |
| 5 kW | 9–10 panels | 5 kW three-phase |
| 10 kW | 18–19 panels | 10 kW three-phase |
Size the inverter against the array with a sensible DC-to-AC ratio rather than matching them exactly. A slight DC overbuild captures more energy in Kerala's diffuse, cloudy light, where the array rarely hits nameplate output anyway. Check the inverter's MPPT voltage window against the string configuration you actually intend to build — not against the one in the brochure.
What the customer actually pays
Turnkey residential pricing in the Indian market has broadly settled around the following, before subsidy:
| System size | Indicative installed cost | Central subsidy | Net to customer |
|---|---|---|---|
| 1 kW | ≈ ₹65,000 | ₹30,000 | ≈ ₹35,000 |
| 2 kW | ≈ ₹1,20,000 | ₹60,000 | ≈ ₹60,000 |
| 3 kW | ≈ ₹1,91,000 | ₹78,000 | ≈ ₹1,13,000 |
These are indicative national figures and will move with component pricing, structure requirements and site difficulty. Treat them as a sanity check on your own costing rather than as a price list.
The sizing conversation worth having
Because banked export settles below the retail tariff, a system that covers daytime consumption is usually worth more to the customer than one sized to maximise export. Ask when the household actually uses power. A family where both adults work and the house is empty until evening has a genuinely different optimum from one running a home business on daytime load.
That question takes ninety seconds and is the difference between a system that performs as promised and one that quietly underdelivers on the customer's bill.
Common questions
How many units does 1 kW of rooftop solar generate per month in Kerala?
Roughly 100 to 120 units per month averaged across the year for a well-oriented, unshaded array. Kerala's monsoon means annual output sits below that of drier northern and western states.
How much roof area is needed per kW of solar in India?
Approximately 80 to 100 sq ft of shade-free roof area per kW. A 3 kW system therefore needs around 300 sq ft that remains unshaded through the day.
How many solar panels are needed for a 3 kW system?
With modern 540–550 W modules, about five to six panels. Older or lower-wattage modules will need more.