Solar System Size by Monthly Consumption
| Monthly Units | Typical Bill | Recommended Size | Panels (550W) |
|---|
| 100 – 150 | ₹700 – ₹1,200 | 1 – 1.5 kW | 2 – 3 |
| 200 – 300 | ₹1,500 – ₹2,500 | 2 – 3 kW | 4 – 6 |
| 300 – 450 | ₹2,500 – ₹4,000 | 3 – 4.5 kW | 6 – 9 |
| 500 – 700 | ₹4,000 – ₹6,500 | 5 – 7 kW | 9 – 13 |
| 900 – 1,200 | ₹8,000 – ₹11,000 | 9 – 12 kW | 17 – 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.
Worked Example: Sizing a Chennai Home From 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.
How This System Size Calculator Works
System kW = Monthly units ÷ (Peak sun hours × 30 × 0.8)
Off-grid: System kW × 1.25 (battery charging headroom)
Assumptions
- 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.
