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Solar System Size Calculator (kW)

Work out exactly how many kW of solar your home needs. Enter your monthly units or bill and this solar system sizing calculator gives the recommended capacity, with panels, roof area and indicative cost for your city.

System size (kW) = monthly units ÷ (peak sun hours × 30 × 0.8), rounded up to the next 0.5 kW. At 4.8 sun hours:

Monthly unitsSolar system size
1001 kW
2002 kW
3003 kW
4003.5 kW
5004.5 kW
1,0009 kW

Find Your System Size in kW

₹
Off-grid systems are sized 25% larger to cover battery charging losses.

Your Recommended Size

–
recommended solar capacity
Monthly consumption–
Panels needed (540 – 620W)–
Roof area needed–
Estimated monthly generation–
Indicative cost after subsidy–
Sizing estimate: based on average sun hours and a planning tariff for your city. Your roof, shading and sanctioned load decide the final size, so have a local solar contractor confirm it along with a current price.

Built and maintained by Solar Calculators Pro · The sun-hour data and rounding rules behind this size

Solar System Size Chart by Monthly Consumption

Monthly UnitsTypical BillRecommended SizePanels (550W)
100 – 150₹700 – ₹1,2001 – 1.5 kW2 – 3
200 – 300₹1,500 – ₹2,5002 – 3 kW4 – 6
300 – 450₹2,500 – ₹4,0003 – 4.5 kW6 – 9
500 – 700₹4,000 – ₹6,5005 – 7 kW9 – 13
900 – 1,200₹8,000 – ₹11,0009 – 12 kW17 – 22

A useful rule of thumb for India: about 1 kW of solar for every 110 to 130 units of monthly consumption, or for every ₹1,000 of monthly bill in an average-tariff state.

Why City Sun Hours Matter for Sizing

The same 300-unit home needs about 2.5 kW in Jodhpur (5.8 peak sun hours) but 3.5 kW in Shillong (3.5 peak sun hours). This calculator holds real peak-sun-hour data for 90+ Indian cities so the capacity matches your location rather than a national average. Off-grid homes should add 25%, which the off-grid option applies automatically, and then size batteries separately with the solar battery calculator.

Bill or Units: Which Input Gives the Better Size

Units win every time you have them. A rupee bill must be converted through tariff slabs, and two homes with identical ₹3,000 bills can differ by 80 units depending on their DISCOM's slab structure. Your bill prints the exact kWh figure, usually labelled "units consumed": average the last 12 values and enter that. Use the bill amount only when the bills are not at hand, and treat the result as ±10%. Whichever input you use, pick the system type deliberately: on-grid for stable-grid homes, hybrid where cuts are routine, and off-grid only where no connection exists, since it forces a 25% larger array plus batteries.

Sizing a Chennai Home From Its Units

A Chennai family averages 420 units a month across the year. At the city's 4.8 peak sun hours, the maths gives 420 ÷ (4.8 × 30 × 0.8) = 3.6, rounded up to a 4 kW system. That is 7 to 8 panels, 320 to 400 sq. ft. of roof, and about 461 units a month of generation, a sensible 10% over consumption. Net cost after the ₹78,000 subsidy: roughly ₹1.54 lakh. The rounding-up rule matters here: a 3.5 kW system would leave the family buying 30 to 40 summer units at top-slab prices every month.

From kW Recommendation to Purchase Decision

  • Price the size: feed the kW into the solar panel cost calculator for gross, subsidy and net figures worth comparing with quotes.
  • Verify the roof: the roof area calculator tells you whether the recommended size physically fits before any installer visit.
  • Sanity-check with load: if your usage is about to change (new AC, EV, home office), size from appliances in the solar load calculator instead of history.
  • Confirm sanctioned load: systems above your sanctioned connection capacity need a load enhancement from the DISCOM first; start that paperwork early.

System Sizing Mistakes That Cost for 25 Years

  • Sizing from the cheapest month. The winter-bill trap: a system that covers December leaves April to June exposed at the highest slab rates.
  • Rounding down to save money. The half kilowatt you shave saves ₹25,000 once and costs ₹200 to ₹400 every summer month for decades.
  • Copying a neighbour's size. Same house, different habits: AC hours, geysers and EVs swing consumption 2x between identical homes.
  • Ignoring the off-grid multiplier. Off-grid homes sized like on-grid ones run flat in the first cloudy week; the 25% uplift is not optional.

Inside the Sizing Formula

System kW = Monthly units ÷ (Peak sun hours × 30 × 0.8)
Off-grid: System kW × 1.25 (battery charging headroom)

Sizing inputs

  • Performance ratio: 0.8, covering soiling, temperature, wiring and inverter losses
  • Rounding: up to the nearest 0.5 kW so the system fully covers consumption
  • Bill conversion: city planning tariff, slab-aware where DISCOM rules are verified

Next steps: price the recommended size with the solar panel cost calculator, and confirm your roof can take it with the roof area calculator.

How Many kW Do I Need? Common Questions

What size solar system do I need for a monthly bill of ₹3,000?+
In an average Indian city, a ₹3,000 bill equals roughly 350 units a month, which needs a 3 to 3.5 kW system. In a high-tariff city the same bill means fewer units, so the system can be slightly smaller.
How do I calculate solar system size from units?+
Divide monthly units by (peak sun hours × 30 × 0.8). For 300 units in a 4.8 sun-hour city: 300 ÷ 115 ≈ 2.6, rounded up to 3 kW. This calculator does the same maths with your city's exact sun data.
What is the best solar system size for a home in India?+
The most common residential sizes are 3 kW and 5 kW. 3 kW suits homes using 300 to 380 units a month and captures the full ₹78,000 subsidy; 5 kW suits AC-heavy homes using 500 to 650 units.
Should I oversize my solar system?+
Mild oversizing (10 to 20%) makes sense if you expect a new AC or EV within a couple of years, since panels are cheap relative to revisiting the installation. Large oversizing wastes money where export rates are low.
Why does an off-grid system need to be bigger?+
All energy must pass through a battery, which loses 10 to 20% in charge-discharge, and the system must cover cloudy days without grid support. The standard practice, used here, is to size off-grid arrays about 25% larger.
What is solar capacity measured in: kW or kWh?+
System capacity is in kW (kilowatts of peak panel power). Energy generated is in kWh, also called units. A 1 kW array in India produces roughly 4 units a day, so capacity times sun hours gives your daily energy.
Is a 3kW or 5kW solar system better for my home?+
Match it to units: under 400 units a month, 3 kW covers you and captures the full subsidy cap; 450 to 650 units points to 5 kW. Between the two, price both in the cost calculator, since the subsidy is identical above 3 kW.
Can my solar system be bigger than my sanctioned load?+
Usually not for net metering: most DISCOMs cap system capacity at or near the sanctioned load of your connection. If the calculator recommends more, apply for a load enhancement before finalising the system.
How many kW of solar do I need for 500 units a month?+
About 4.5 kW in a typical Indian city with 4.8 peak sun hours: 500 ÷ (4.8 × 30 × 0.8) = 4.3, rounded up. That is 8 to 9 panels of 540 to 620 W.
What is the difference between solar panel size and solar system size?+
Solar system size is the total capacity of all your panels, in kW; it decides how many units you generate. Panel size is the rating and dimensions of one module: most home panels in India are 540 to 620 W and about 2.28 m × 1.13 m. A 3 kW system, for example, is 5 to 6 such panels.

Turn Your kW Figure Into a Plan