Solar panels for your home: how to size a system
Size rooftop solar from your monthly units, not your roof. A simple method, the usual kW ranges by consumption, and what to ask installers.
Key takeaways
- Start from your monthly electricity units, not from how much roof you have.
- Indian state guidance links usage to size: 0-150 units a month suggests 1-2 kW, 150-300 suggests 2-3 kW, above 300 suggests more than 3 kW.
- Bigger is not always better: the PM Surya Ghar subsidy is capped at Rs 78,000, so extra kW beyond 3 add cost without extra subsidy.
In this article
Most people size solar backwards. They look at the roof and ask how many panels will fit. Start with your electricity bill instead.
Step 1: find your real usage
Pull out 12 months of bills and average the monthly units (kWh). If your use swings with summer cooling, note the highest and lowest months too, because the average hides them.
Step 2: match usage to a size range
The Delhi government’s solar portal, which publishes PM Surya Ghar guidance, suggests:
| Average monthly units | Suggested size |
|---|---|
| 0-150 | 1-2 kW |
| 150-300 | 2-3 kW |
| Above 300 | Above 3 kW |
Treat this as a starting point. Sun hours, shade, panel direction and your goal (bring the bill close to zero, or just trim it) all change the final number.
Step 3: sanity-check with a rough output figure
A quick way to test a quote: multiply the system size by the units one kW makes per day, then by 30.
For example, if an installer in your city says 1 kW gives about 4 units a day (an assumption here, ask for your own), then:
- 2 kW gives about 240 units a month.
- 3 kW gives about 360 units a month.
If your bill shows 250 units a month, a 2 kW system covers most of it, while 3 kW would produce more than you use. Whether the extra is worth it depends on how your utility credits exported power.
Step 4: look at the subsidy shape
The central subsidy is Rs 30,000 per kW for the first 2 kW, Rs 18,000 for the third kW, and capped at Rs 78,000 above that. The first 3 kW get the most help per rupee, and going bigger earns no extra subsidy. Confirm current terms on the official portal.
Step 5: check your roof
- Shadow-free space matters more than total area. A shaded corner loses output.
- Direction and tilt: in India, panels generally face south.
- Structure: the roof should carry the extra weight, and you need safe access for cleaning.
Step 6: ask installers the right questions
- What annual generation do you expect for my city, and how did you work it out?
- Which panel and inverter brands, and what are their warranties?
- What is included: wiring, net-meter paperwork, monitoring, cleaning advice?
- How is the subsidy claimed, and who handles it?
Get at least three quotes and compare the same kW and the same equipment, so you are comparing like with like.
Sources
Frequently asked questions
How many kW of solar do I need?
Look at your average monthly units on your bill. Guidance published on the Delhi government's solar portal suggests 1-2 kW for 0-150 units, 2-3 kW for 150-300 units and above 3 kW for more than 300 units. An installer should confirm using your roof and shade.
Does a bigger system get a bigger subsidy?
Not beyond 3 kW under PM Surya Ghar. The subsidy is Rs 30,000 per kW for the first 2 kW, Rs 18,000 for the next kW, and capped at Rs 78,000 for larger systems.
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