What Size Solar System Do I Need? Read Your Bill in 5 Minutes
By UVR Green Energies Team · Published · 6 min read · Cost & Savings
A five-minute, pen-and-paper method to size rooftop solar from your own electricity bill, with worked examples for 150 to 900 units a month.
The short answer
To size rooftop solar, take your average monthly units from a year of bills and divide by 126, UVR's estimate of monthly units per kW. A 400-unit home needs about 3.2 kW. Then check roof space at roughly 70 sq ft per kW, round to whole 550 W panels, and avoid sizing far above your use, since Gujarat buys surplus at only ₹2.25 a unit.
Key takeaways
- Size = average monthly units ÷ 126 (UVR's planning estimate of units per kW per month).
- Use 12 months of bills, not just your worst summer bill.
- Allow about 70 sq ft of shadow-free roof per kW and round to whole 550 W panels.
- Subsidy follows the panels' DC capacity, not the inverter size, and caps at ₹78,000 from 3 kW.
- Oversizing wastes money in Gujarat because surplus earns only ₹2.25 a unit for five years.
You already have the most important document
Before you call any installer, find one thing: your electricity bill. Better still, find the last twelve of them. Everything you need to size rooftop solar is printed on those pages.
This walkthrough takes about five minutes with a pen and a calculator. By the end, you will know roughly how many kW you need, how many panels that means and how much roof it takes.
The core rule: your system size in kW ≈ average monthly units ÷ 126.
Step 1: Find your average monthly units
Look for "units consumed" or "net units" on each bill. Write down the units for every billing period you can find across a full year.
Add them up and divide by the number of months covered. If your bills are for two-month periods, divide each one by two first.
Step 2: Divide by 126
UVR's planning figure is that each kW of rooftop solar produces about 126 units a month on average across the year. It is an estimate for sizing, not a guarantee. Shadows, panel tilt, dust and weather all move the real number.
Step 3: Round to real panels
Solar panels come in fixed sizes. UVR's calculator assumes 550 W panels, so your system grows in steps of 0.55 kW.
| Panels (550 W) | System size | Estimated monthly units |
|---|---|---|
| 2 | 1.1 kWp | about 139 |
| 4 | 2.2 kWp | about 277 |
| 5 | 2.75 kWp | about 347 |
| 6 | 3.3 kWp | about 416 |
| 9 | 4.95 kWp | about 624 |
| 13 | 7.15 kWp | about 901 |
The Anand home would pick six panels (3.3 kWp), which comes close to its 383-unit average.
Step 4: Check your roof
UVR plans for about 70 sq ft of shadow-free roof per kW. Measure the clear part of your terrace, not the total area.
| System size | Clear roof needed (approx.) |
|---|---|
| 1 kW | 70 sq ft |
| 2 kW | 140 sq ft |
| 3 kW | 210 sq ft |
| 5 kW | 350 sq ft |
| 7 kW | 490 sq ft |
Water tanks, stair cabins, parapet walls and the neighbour's taller building all throw shadows, which move through the day and the seasons. Try the rooftop area calculator to turn your terrace measurements into a rough kW figure.
Step 5: Sanity-check against Gujarat's rules
Two Gujarat rules should shape your final number.
First, do not chase surplus. Under the state's residential net metering agreement, your exports first cancel your imports within each billing cycle. Any leftover surplus is bought at just ₹2.25 a unit for the first five years. You save much more by covering your own use.
Second, watch the 6 kW line. GERC's 2024 amendment lets DISCOMs recover system strengthening charges where a premises' aggregate rooftop capacity goes above 6 kW. That does not stop you, but it should be in your quote.
Worked examples at a glance
Here is the whole method applied to five hypothetical homes. All figures are estimates using UVR's planning assumptions.
| Avg monthly units | Units ÷ 126 | Suggested size | Clear roof (approx.) | Central subsidy |
|---|---|---|---|---|
| 150 | 1.2 kW | 1.1 kWp (2 panels) | 77 sq ft | ₹33,000 |
| 250 | 2.0 kW | 2.2 kWp (4 panels) | 154 sq ft | ₹63,600 |
| 400 | 3.2 kW | 3.3 kWp (6 panels) | 231 sq ft | ₹78,000 |
| 600 | 4.8 kW | 4.95 kWp (9 panels) | 347 sq ft | ₹78,000 |
| 900 | 7.1 kW | 7.15 kWp (13 panels) | 501 sq ft | ₹78,000 |
These line up with MNRE's own guidance for the scheme: 0–150 units a month suits 1–2 kW, 150–300 units suits 2–3 kW, and above 300 units suits more than 3 kW.
Step 6: Think about the next five years
Your bill tells you about the past. A good size also accounts for what is coming.
- An electric vehicle charged at home can add a lot of units every month.
- More air conditioners as children grow or rooms get added.
- Moving the geyser, pump or washing machine to daytime raises the share of solar you use directly.
- A family member moving out or a planned shift to another house lowers it.
If a big new load is certain, size a little higher now. Adding panels later is possible, but it means fresh paperwork and possibly a new structure.
Special cases that change the maths
A few homes need a slightly different approach. If one of these sounds like you, mention it during your survey.
- You already have solar. If your existing system received subsidy, any expansion can claim subsidy only for the balance up to 3 kW in total. Size the addition to the gap between your use and your current generation.
- Two meters in one house. The subsidy is tagged to a residential connection, so each connection is looked at separately. Work out each meter's average on its own.
- Very small use. If your average is under about 100 units, even a 1 kW system will often overproduce. Look carefully at whether the surplus is worth it at ₹2.25 a unit.
- Elevated structure. If you want to keep using the terrace below the panels, the structure gets taller and the price changes, but the size calculation stays the same.
Five sizing mistakes we see often
- Using the single highest bill instead of a yearly average.
- Counting total roof area instead of shadow-free area.
- Ignoring the 6 kW line in Gujarat and being surprised by system strengthening charges.
- Chasing the "bill zero" dream by oversizing, then exporting surplus at ₹2.25.
- Forgetting planned loads like an EV, then wanting to add panels a year later.
Your 5-minute checklist
- Collected 12 months of bills
- Worked out average monthly units
- Divided by 126 to get kW
- Rounded to whole 550 W panels
- Checked clear roof area at about 70 sq ft per kW
- Made sure the size does not create large regular surplus
- Noted any planned loads like an EV
Want to see the rupees? Our price guide for Vadodara shows the estimated cost for every size from 1 kW to 10 kW.
Next step
Your paper estimate is a great start. A free site survey turns it into a firm design: an engineer measures shadow-free area, checks the structure and reviews your electrical setup.
Call +91 95375 66799 or book through the contact page. For more on home systems, visit residential rooftop solar.
Frequently asked questions
How do I calculate what size solar system I need?
Add up the units from your last 12 electricity bills and find the monthly average. Divide that number by 126, which is UVR's planning estimate of units produced per kW per month. The answer is your approximate system size in kW. Then check that you have about 70 sq ft of shadow-free roof for each kW.
What size solar system do I need for 300 units per month?
Dividing 300 by 126 gives about 2.4 kW. In practice, that becomes a system of four or five 550 W panels, which is 2.2 to 2.75 kW. MNRE's own sizing guide for PM Surya Ghar suggests 2 to 3 kW for homes using 150 to 300 units a month. A site survey confirms the final size.
How many solar panels do I need for a 3 kW system?
With 550 W panels, you need about six panels, which adds up to 3.3 kWp. Five panels give 2.75 kWp. The PM Surya Ghar subsidy is calculated on the rated DC capacity of the panels, so a 3.3 kWp system qualifies for the full ₹78,000 cap. Panel wattage can vary by brand and quotation.
How much roof space do I need for rooftop solar?
UVR plans for about 70 sq ft of shadow-free roof per kW. So 2 kW needs about 140 sq ft, 3 kW about 210 sq ft, and 5 kW about 350 sq ft. Water tanks, stair cabins, parapets and neighbouring buildings can cast shadows, which is why a site survey measures usable area rather than total roof size.
Should I install a bigger system than I need?
Usually not in Gujarat. Exports first cancel your imports within each billing cycle, and any leftover surplus is bought at only ₹2.25 a unit for the first five years. You save far more by covering your own consumption. The exception is a planned load increase, such as an electric vehicle or extra air conditioners.