How many kW of rooftop solar do I need? Sizing from your bill and roof

A quick first answer: divide your average monthly electricity use in units by about 120 to get the system size in kW. A home using 360 units a month would start at about 3 kW. That divisor is an illustrative thumb rule based on 1 kW producing roughly 110–130 units a month in much of India; it is lower in cloudier or more shaded places and higher in sunny, dry regions. Then check two limits: your shade-free roof area (roughly 8–10 m², or about 90–110 sq ft, per kW for typical modules) and your sanctioned load, because many state rules cap rooftop capacity at it. Finally, consider the PM Surya Ghar subsidy, which stops at 3 kW. This guide gives the full method, with worked examples, and links to the other sizing and checklist guides in this section.

Step 1: Find your real consumption

Use 12 months of bills, not one. Indian household consumption swings with the seasons: air conditioners and coolers in summer, geysers in winter in the north, pumps in some homes.

Write down the units for each billing period. If your DISCOM bills every two months, divide by two for monthly figures.
Add them up for the year and divide by 12 for the average.
Note the highest and lowest months.

Under net metering, what matters most is your annual consumption, because surplus in sunny months can offset imports in cloudy ones if your state carries credits forward. Where surplus is settled monthly at a low rate, size closer to your lower months.

Step 2: Convert units to kW

Average monthly units	First estimate (÷ 120)	Typical choice
120	1.0 kW	1 kW
240	2.0 kW	2 kW
300	2.5 kW	2.5–3 kW
360	3.0 kW	3 kW
480	4.0 kW	3–4 kW
600	5.0 kW	4–5 kW

The figures in the middle column are arithmetic on an illustrative assumption. A vendor's site survey, using local solar data, should refine them. Ask the vendor for the expected annual generation in writing and how they calculated it.

Step 3: Check your roof

Area: allow roughly 8–10 m² of shade-free roof per kW. A 3 kW system needs about 24–30 m² (260–320 sq ft), plus space for walking and cleaning.
Orientation: in India, south-facing is generally best; east-west layouts work with some loss.
Shade: walk the roof at 9 am, noon and 3 pm. Water tanks, staircase rooms (mumty), parapets, dish antennas, trees and taller buildings next door cast moving shadows. A shadow on one module can reduce the output of a whole string.
Structure: a flat RCC roof usually needs a ballasted or anchored mounting structure; sloping tin or tile roofs need different fixings. Raised structures can leave the terrace usable but cost more.
Condition: if the roof needs waterproofing within a few years, do it before installing panels.

Step 4: Check your sanctioned load and DISCOM limits

Your bill shows the sanctioned or contract load in kW. Many states cap a rooftop system's capacity at the sanctioned load, and the Electricity (Rights of Consumers) Rules allow net metering up to the sanctioned load or 500 kW, whichever is lower. If your sanctioned load is 2 kW and you want 3 kW of solar, you may need a load enhancement, which can raise your fixed charges. Ask your DISCOM what applies. See net metering vs gross metering.

Step 5: Weigh the subsidy and the surplus rate

Under PM Surya Ghar the CFA is ₹30,000 per kW for the first 2 kW and ₹18,000 for the third, with nothing extra above 3 kW. That means:

the subsidy share of the price is highest for 1–2 kW systems;
the cost per kW is often lower for larger systems, because fixed costs are spread;
above your own consumption, extra kW mostly produce surplus that may earn a low rate.

For most households, the sweet spot is a system that covers most of their annual use without large surplus. See PM Surya Ghar subsidy explained.

Worked examples (illustrative)

Flat-roof house, 280 units a month. 280 ÷ 120 ≈ 2.3 kW. Shade-free area 30 m². Sanctioned load 3 kW. A 2–2.5 kW system fits. At 2.5 kW, expected generation ≈ 2.5 × 1,300 = 3,250 units a year versus use of 3,360: almost no surplus. CFA = ₹60,000 + 0.5 × ₹18,000 = ₹69,000 (portal calculation is final).

Two-storey house, 520 units a month, water tank on the roof. 520 ÷ 120 ≈ 4.3 kW. Usable area after the tank's shadow 32 m², enough for about 3.5 kW. Sanctioned load 5 kW. A 3–3.5 kW system on a raised structure; CFA ₹78,000.

Bungalow, 900 units a month in summer, 400 in winter. Average 620, so ≈ 5 kW. In a state with annual settlement, 5 kW is reasonable; in a state paying little for monthly surplus, 4 kW may give a better return per rupee.

Use the system cost estimate by kW for a rough price, then the payback calculator.

Batteries change the sizing question

For an on-grid system, you size the panels to your units. For a hybrid or off-grid system, you also size the battery in kWh to the loads you want to run during outages and for how long. See on-grid vs hybrid vs off-grid and home batteries: lithium vs lead-acid.

Other guides in this section

Frequently asked questions

Should I size for a future EV or air conditioner?

If the purchase is firm and soon, yes, within your roof and load limits. Otherwise remember that adding capacity later is possible but costs more per kW and is outside the subsidy.

Is a 1 kW system worth it?

It gets the highest subsidy share, but fixed costs make the price per kW high. It suits low-consumption homes or small roofs.

Why does my vendor suggest a much bigger system?

Larger systems mean a larger sale. Ask for generation and payback calculations for two sizes and compare.

What is a kWp?

Kilowatt-peak: the module capacity under standard test conditions. Real output is usually lower.

Official sources

PM Surya Ghar national portal — subsidy calculator and capacity rules
Ministry of New and Renewable Energy — technical specifications
Your DISCOM's website — sanctioned load and rooftop capacity limits