Rooftop solar for a 3BHK: the 7-step system
Turn a year of electricity bills into a sized and quoted rooftop solar plan for a 3BHK, using our two solar calculators and one printable page.
By Bulan Sarkar · Updated
A 3BHK that uses 300 to 400 units a month needs about 3 to 4 kW of rooftop solar in most of India, and that takes roughly 320 to 430 sq ft of shadow-free roof (MNRE puts it at 10 to 12 m² per kW). One kW makes 100 to 130 units a month on a yearly average, depending on the state. At MNRE's benchmark, ₹50,000 per kW for the first 2 kW and ₹45,000 per kW after that, 3 kW comes to ₹1,45,000 before the central subsidy. Plan it in seven steps: average 12 months of bills, measure the roof, size the system, decide on backup, get three quotes, check them line by line, then keep a monthly generation record.
Source: MNRE: Frequently Asked Questions, Solar Rooftop System, 28 April 2022 (Q3 roof area 10 to 12 m² per kWp; Q5 4 to 5.5 units a day; Q14 net metering), PIB copy, checked Oct 2026.
The decision: plan rooftop solar yourself?
Yes, if you own the roof or the house, pay your own electricity bill and have an open terrace. Sizing, roof checks and comparing quotes need only your bills, a measuring tape and this page.
Skip it for now if you rent, or if a taller building or water tank shades most of the roof through the middle of the day. A shaded roof makes far less than these figures.
If the roof is old, cracked or leaking, ask a licensed structural engineer whether it can take the panels and their frame. A registered vendor and your electricity company handle installation, earthing and the grid connection. Our figures are for planning only.
The steps, in order
Average 12 months of bills
Have ready:last 12 electricity bills, or the online bill historyyour connection's sanctioned load
Add up the units on a year of bills and divide by 12. Size from that average and not from one summer bill, because a May bill with the air conditioners running can sit far above the yearly average. Also divide each bill's amount by its units to get what you really pay per unit, which you need in step 3.
Measure the shadow-free roof
Have ready:measuring tapesketch of the terracea midday look at shadows
Measure only the part of the roof that stays in sun from about 9 am to 4 pm. Leave out the water tank, the staircase head room, the parapet's shadow and a walkway to reach the tank. MNRE puts the need at 10 to 12 m² of shadow-free area per kW, which is about 108 to 130 sq ft.
Size the system
Have ready:average monthly unitsshadow-free area in sq ftyour state
Divide your monthly units by what 1 kW makes in your state in a month. The calculator takes this from 20 years of NASA sunshine data for 35 states and UTs. Round up to the next half kW, then check that the roof fits it. If the roof is smaller, the roof sets the size, and the system covers part of the bill instead of all of it.
Decide on backup
Have ready:list of what must run in a power cuthow many hours cuts usually last
A plain grid-connected system switches off when the grid goes down, so it is not a backup. If you get frequent cuts, list only the loads that must keep running (fans, lights, router, fridge) and size the inverter and battery for those alone. Leave the air conditioners and the geyser off that list.
Get three quotes against the benchmark
Have ready:your kW figureprinted quote sheetthe roof sketch
MNRE's benchmark cost is ₹50,000 per kW for the first 2 kW and ₹45,000 per kW after that, or ₹55,000 and ₹49,500 in the hill and north-eastern states and the island UTs. Ask three vendors to quote the same kW. The calculator also shows the central subsidy as MNRE publishes it; the scheme's own terms are on the official portal, pmsuryaghar.gov.in.
Check the quotes line by line
Have ready:the three quotesthe quote sheet with the same lines
You cannot check a quote that says only "3 kW". Ask each one for the panel make, wattage and count (count times wattage should come to the kW), the inverter make and size, the mounting frame material and height, earthing and lightning protection, the cable run, the net meter, and the warranty years for panels, inverter and workmanship. Compare those lines, not just the totals.
Keep a monthly generation record
Have ready:the inverter or generation meter readingthe expected units from step 3
Note the generation reading on the same date every month next to the units the calculator expected. Output rises from March to May and falls in the monsoon, so compare each month with the same month last year once you have one. If a month falls well short in clear weather, look for new shade, dust or a tripped inverter, and call the installer.
What goes wrong
Variants: what changes the steps
Flat in an apartment block
The terrace usually belongs to the society, so the society decides. One common system for the lifts, pumps and stair lights can be easier to agree on than a separate system for each flat.
Frequent power cuts
Add a battery and size it for the backup list only. The panels and the battery are two separate decisions, so step 3 stays the same.
Small or shaded roof
The roof sets the size. A 2 kW system on a small Ahmedabad roof still covers about 44% of a 600-unit bill, and that share is the number to plan on.
Hill and north-eastern states
Some of these states get less sun (Arunachal Pradesh about 87 units per kW a month), and the benchmark cost is higher at ₹55,000 per kW, so expect a bigger system for the same bill.
Tips and tricks
- Average all 12 bills before you size anything. A single May bill can make the system look much bigger than you need.
- Look at the roof around noon in December. Winter shadows are the longest of the year, so a spot that is clear at noon then is clear at noon all year.
- Ask every vendor for the panel count and wattage. Count times wattage should equal the kW on the quote.
- Keep a clear walkway to the water tank and to the panels, so cleaning and tank work do not mean stepping on the frames.
- Photograph the inverter and meter readings on the same date each month. A phone album is a good enough record.
What we would do
For a 3BHK we would size from the 12-month average, round to the nearest half kW the roof can take, and ask three vendors to quote that same size on our quote sheet, with panel make, wattage and count, inverter make and every warranty in years. We would add a battery only if power cuts come most weeks; otherwise a plain grid-connected system is simpler and costs less. After commissioning, we would note the reading every month.
Print the working copy
Rooftop solar: 7-step working copy, from the Rooftop Solar Kit. One A4 page with tick boxes and blanks; it prints in black and white.
- 1. Bills
- 2. Roof
- 3. Size
- 4. Backup
- 5 and 6. Quotes
- 7. Before you sign
Frequently asked questions
How many kW of solar does a 3BHK need?
How much roof does a 3 kW system need?
How many units does a 3 kW system make in a month?
Does rooftop solar work during a power cut?
The source we built this on
MNRE: Frequently Asked Questions, Solar Rooftop System, 28 April 2022 (Q3 roof area 10 to 12 m² per kWp; Q5 4 to 5.5 units a day; Q14 net metering), PIB copy. Checked Oct 2026.
Benchmark costs are from MNRE's PM Surya Ghar guidelines, para 5(g) (₹50,000/kW for the first 2 kW, ₹45,000 after; ₹55,000 and ₹49,500 in special category states), checked 2026-10. State units per kW come from the NASA POWER table on our rooftop solar planner. Numbers only, not advice.
Planning estimate only. Roof strength is a licensed structural engineer's call. A registered vendor and your electricity company handle installation, earthing and the grid connection. Do not work on panels or their wiring yourself.