Start with the units on your bill, not the rupee total
This site’s calculator assumes each kW of rooftop solar generates about 120 units a month. Divide your average monthly units by 120 and round up to get a suggested size.
A home using 720 units over a two-month bill averages 360 units a month, which suggests 3 kW. A home using 1100 units over two months averages 550 units a month, which suggests 5 kW. Your own bill, roof and daytime use decide the real answer.
What 1, 2, 3 and 5 kW look like
| Capacity | Modelled generation | Modelled roof space | Budget before subsidy |
|---|---|---|---|
| 1 kW | about 120 units a month | about 110 sq ft | ₹70,000–₹85,000 |
| 2 kW | about 240 units a month | about 220 sq ft | ₹1.30 lakh–₹1.55 lakh |
| 3 kW | about 360 units a month | about 330 sq ft | ₹1.80 lakh–₹2.20 lakh |
| 5 kW | about 600 units a month | about 550 sq ft | ₹2.75 lakh–₹3.50 lakh |
The 1–3 kW budgets follow the Tamil Nadu solar portal. Larger sizes use an illustrative allowance of ₹55,000–₹70,000 per kW. These are planning ranges, not an Arc Solar quotation.
Is 3 kW enough for a house?
In this model a 3 kW system generates about 360 units a month. That matches a household using about 360 units a month, or 720 units on a two-month bill. If your bill is higher, 3 kW covers part of it. If it is lower, 3 kW may be more than you need.
Roof space is a separate limit. The model allows about 330 sq ft for 3 kW, and a site assessment confirms what your roof can hold.
When is 5 kW the better fit?
A 5 kW system generates about 600 units a month in this model, which suits usage near 600 units a month (1200 over two months). It needs about 550 sq ft of usable roof. Larger systems cost more upfront, and the household subsidies described in the subsidy guide are limited by system size, so check the dated conditions before counting on them.
You can choose a smaller size
The suggestion is a starting point. You can choose any whole-kW size from 1 to 30 kW, and the calculator recalculates generation and budget for your choice. A smaller system can fit a tight budget or a small roof, and it will cover a smaller share of your bill.
What the number does not tell you
- Shade, roof direction and access can reduce what a roof produces.
- The model assumes 70% of generation is used directly in the building, so daytime use matters.
- A household already inside its free-unit allowance may see little energy-charge saving.
- Businesses use a commercial tariff and do not qualify for the household subsidies.
Read the cost guide for what a written quotation should include, and check scheme conditions on the Tamil Nadu solar portal and the PM Surya Ghar portal.
Try your own numbers
Enter the units from your bill in the estimate calculator, choose a size and download the budget PDF, or contact Arc Solar to talk it through with the owner. A site assessment and written quotation follow before any final size is agreed.