Here is how to calculate solar panel size from electricity bill data in one line: take your average monthly units (kWh), not the rupee amount, and divide by 120. That gives the kW of rooftop solar that roughly covers your use in most of India, because 1 kW makes about 4 units a day, or 120 units a month.
That shortcut covers most homes. The rest of this guide explains where the number comes from, when to adjust it, and how to check it against roof area, panel count and your DISCOM's limits. Want the answer without doing the maths? Enter your units in the free rooftop solar size and subsidy calculator and it will suggest a size, subsidy and savings.
Step 1: Read units on the bill, not rupees
The rupee total on a bill is a poor guide to system size. It includes fixed charges, electricity duty, fuel adjustment charges and slab-wise tariffs, and all of these vary by state and DISCOM. Two homes paying ₹3,000 a month in different states can be using very different amounts of electricity.
What you need is the units consumed, usually printed as "Units", "kWh", "Consumption" or "Net units" next to the previous and current meter readings.
- Collect 12 months of bills. Most DISCOM apps and portals show billing history. Summer bills with ACs running can be double the winter ones, so one month on its own will mislead you.
- Work out the monthly average. Add up the 12 months of units and divide by 12. If your DISCOM bills every two months, add up all six bills and still divide the yearly total by 12.
- Note your sanctioned (connected) load. It is printed on the bill in kW. You will need it in Step 5.
- Add planned load. If an EV, another AC or an electric cooktop is coming in the next year or two, add a realistic estimate of its monthly units now.
Step 2: Use the right units per kW per day
The key input in any solar load calculation is how many units 1 kW of panels produces per day at your site. MNRE's rooftop FAQ says a 1 kWp plant can generate 4 to 5.5 units on a clear sunny day. MNRE also notes that most of India receives 4 to 7 kWh of solar radiation per square metre per day.
A clear day is the best case. Over a full year you also get monsoon weeks, hazy winter mornings in the north, dust and high summer temperatures, all of which pull output down. That is why planners use a lower annual average:
- About 4 units/kW/day (120 units/kW/month): a sensible default for an unshaded, south-facing system in most of India. It is consistent with the government's own figure that a 3 kW PM Surya Ghar system generates more than 300 units a month on average.
- About 3.5 units/kW/day (105 units/kW/month): a conservative figure for cloudier regions, partly shaded roofs, heavy dust or east-west layouts.
- Above 4 units/kW/day: possible in high-sunshine areas such as parts of Rajasthan and Gujarat. Use it only if you have local generation data to back it up.
How to calculate solar panel size from electricity bill units: the formula
Once you have average monthly units and a daily generation figure, the rooftop solar size calculation is:
System size (kW) = Average monthly units ÷ (30 × units per kW per day)
At 4 units/kW/day the bottom half of the formula is 120, so the shortcut is monthly units ÷ 120. At 3.5 units/kW/day, divide by 105. Round the result to the nearest practical size (usually 0.5 kW steps) and then check it against the roof and panel count.
Quick-reference table: monthly units to kW solar system for home
| Average monthly units | kW at 4 units/kW/day | kW at 3.5 units/kW/day | Suggested size | Approx. shadow-free roof area |
|---|---|---|---|---|
| 100 | 0.83 | 0.95 | 1 kW | 108–129 sq ft |
| 150 | 1.25 | 1.43 | 1.5 kW | 161–194 sq ft |
| 200 | 1.67 | 1.90 | 2 kW | 215–258 sq ft |
| 250 | 2.08 | 2.38 | 2–2.5 kW | 215–323 sq ft |
| 300 | 2.50 | 2.86 | 2.5–3 kW | 269–388 sq ft |
| 400 | 3.33 | 3.81 | 3.5–4 kW | 377–517 sq ft |
| 500 | 4.17 | 4.76 | 4–5 kW | 431–646 sq ft |
| 600 | 5.00 | 5.71 | 5–6 kW | 538–775 sq ft |
| 800 | 6.67 | 7.62 | 7–8 kW | 753–1,033 sq ft |
| 1,000 | 8.33 | 9.52 | 8.5–10 kW | 915–1,292 sq ft |
The area column uses MNRE's guidance of 10–12 sq m of shadow-free area per kWp (about 108–129 sq ft per kW). The figures are indicative only. The PM Surya Ghar guidance matches this range: 1–2 kW for homes using up to 150 units a month, 2–3 kW for 150–300 units, and above 3 kW beyond that.
Step 3: Check roof area
MNRE says you need roughly 10–12 square metres of shadow-free area per kWp. "Shadow-free" is the important word. Water tanks, stair rooms (mumty), parapet walls, neighbouring buildings and trees all cast shadows that move through the day and change with the season. Shaded space does not count.
If the usable roof is smaller than the table suggests, the roof sets your limit, not the bill. Higher-wattage modules and elevated structures can recover a little space, but they cannot change the basic ratio very much.
Step 4: Work out how many solar panels you need
Panel count is simple division. Always round up:
Number of panels = Target size in watts ÷ module wattage (Wp)
Take the wattage from the datasheet of the module you are actually quoting, because it changes the count. For example, a 3 kW target with 540 Wp modules is 3,000 ÷ 540 = 5.56, which rounds up to 6 panels, or 3.24 kWp installed. Rounding up gives you slightly more capacity than the target. That is normal, as long as it stays within your DISCOM's approved capacity and the subsidy terms.
Worked examples
These are illustrative examples, not real customer cases. The arithmetic uses the formulas above.
Example 1: A 250-unit home in a sunny city
- 12-month total: 3,000 units, so the average is 3,000 ÷ 12 = 250 units/month.
- Size at 4 units/kW/day: 250 ÷ 120 = 2.08 kW.
- Panels with 540 Wp modules: 2,000 ÷ 540 = 3.7, rounded up to 4 panels = 2.16 kWp.
- Expected generation: 2.16 × 4 × 30 ≈ 259 units/month, which covers the average use.
- Roof needed: 2.16 × 10–12 sq m = 21.6–25.9 sq m, or about 232–279 sq ft.
Example 2: A 400-unit home in North India with big summer bills
- Bills swing from about 250 units in winter to 600 in peak summer. The 12-month total is 4,800 units, so the average is 400 units/month.
- The site has some winter haze, so we use the conservative 3.5 units/kW/day: 400 ÷ 105 = 3.81 kW. At 4 units/kW/day it would be 3.33 kW.
- Panels with 575 Wp modules: 4,000 ÷ 575 = 6.96, rounded up to 7 panels = 4.03 kWp.
- Expected generation: 4.03 × 3.5 × 30 ≈ 423 units/month on the conservative figure.
- Sanctioned load on the bill is 3 kW. Before quoting 4 kW, check whether the DISCOM needs a load enhancement (see Step 5).
In the 600-unit summer months this home will still draw some power from the grid. Under net metering, surplus from the lighter months can offset it, depending on how your state handles banking and settlement.
Example 3: A 600-unit home with a small roof
- Average use: 600 units/month, so the bill points to 600 ÷ 120 = 5 kW.
- Usable shadow-free roof: 350 sq ft ≈ 32.5 sq m. At 10–12 sq m per kW, that fits only about 2.7–3.25 kW.
- A 3 kW system at 4 units/kW/day gives 3 × 4 × 30 = 360 units/month, or 60% of this home's use.
- In this case the roof decides the size. Tell the customer upfront that solar will cut the bill substantially but will not bring it to zero.
Step 5: Sanctioned load and DISCOM limits
The bill tells you how much solar you need. Your DISCOM decides how much you are allowed to connect. Many state rooftop solar regulations link the maximum system size to your sanctioned or contract load, and the exact rules differ from state to state.
National rules have made this easier. The Electricity (Rights of Consumers) Amendment Rules notified in February 2024 say that:
- complete applications for systems up to 10 kW are deemed technically feasible, with no feasibility study needed;
- the DISCOM handles any matching increase in sanctioned load for these systems;
- distribution upgrades needed for systems up to 5 kW are made at the DISCOM's cost.
As of September 2026, confirm the exact capacity cap and any load-enhancement fee on your DISCOM's portal before you finalise the quote. Also check whether the DISCOM measures capacity by panel (DC) or inverter (AC) rating.
Oversizing vs undersizing: where the money is
With net metering, surplus units you export are credited against the units you import. The net metering process explains how this works. The limit shows up at settlement time. Surplus left at the end of the settlement period is usually paid at a much lower rate than the tariff you pay, and in some states it can lapse. The rules vary by state.
- Oversizing beyond your yearly consumption adds cost, but the extra units earn little. It only makes sense if you are confident about future load, such as an EV.
- Undersizing is not a disaster. On slab tariffs, solar removes your costliest units first, so the first few kW deliver the biggest rupee savings per kW.
- Subsidy matters too. Under PM Surya Ghar, central assistance is ₹30,000 for 1 kW, ₹60,000 for 2 kW and ₹78,000 for 3 kW or more, capped at 3 kW. See the PM Surya Ghar subsidy guide for eligibility details.
Seasonal variation: set expectations early
Generation is not flat through the year. Clear pre-monsoon months usually give the highest output. Monsoon cloud and, in the north, winter fog and haze bring it down. Consumption often moves the opposite way, peaking in summer when AC use is high.
This is why you size on the 12-month average and let net metering even out the months, rather than sizing on your worst summer bill. Show customers a month-by-month estimate, not just one annual figure, so a low August bill-saving does not come as a surprise.
Putting it into a proposal
For installers, the sizing sheet belongs inside the quote: average units, the units-per-kW figure you assumed, roof area, panel count and expected monthly generation. Customers trust a number they can check against their own bill. Our solar quotation format shows where each item goes. For pricing by size, see rooftop solar cost per kW. Tools like PitchSun can pull these figures from a customer's bill and roof into a proposal you can share on WhatsApp.
Frequently asked questions
How many units does a 1 kW solar system generate per day in India?
MNRE says a 1 kWp rooftop plant can generate 4 to 5.5 units on a clear sunny day. Averaged over a full year, including cloudy and monsoon days, planning with about 3.5 to 4 units per kW per day is safer for most of India, and lower in cloudier regions.
How many solar panels do I need for 300 units per month?
At about 4 units per kW per day, 300 units a month needs roughly 2.5 kW, or 3 kW for a sunnier margin. With 540 Wp modules, a 3 kW target means 6 panels, which works out to 3.24 kWp.
What size solar system do I need for a Rs 3,000 electricity bill?
You cannot size a system from the rupee amount, because tariffs, slabs, fixed charges and duties differ by state and DISCOM. Look for the units (kWh) consumed on the same bill and size from that number instead.
How much roof area is needed for a 3 kW rooftop solar system?
MNRE guidance is about 10 to 12 square metres of shadow-free area per kWp, so a 3 kW system needs roughly 30 to 36 square metres, or about 320 to 390 sq ft.
Can I install a solar system bigger than my sanctioned load?
It depends on your state's rooftop solar regulations, and many link system size to sanctioned or contract load. Since February 2024, national rules let the DISCOM carry out any needed load enhancement for systems up to 10 kW, but confirm the exact limit with your DISCOM before you finalise the size.
Is a bigger solar system always better if I have net metering?
No. Units you export beyond your own yearly use are usually settled at a much lower rate than your tariff, and in some states unused credit can lapse. Sizing close to your 12-month average consumption usually gives the best payback.
Sources
- PIB Research Unit (MNRE) — Frequently Asked Questions: Solar Rooftop System (28 April 2022)
- Ministry of New and Renewable Energy — Solar Overview
- PIB — Cabinet approves PM-Surya Ghar: Muft Bijli Yojana for installing rooftop solar in One Crore households (29 Feb 2024)
- PIB — PM Surya Ghar: Muft Bijli Yojana, Redefining Solar Power and Energy Access (12 Aug 2024)
- PIB — Government amends Electricity (Rights of Consumers) Rules (23 Feb 2024)
- PM Surya Ghar: Muft Bijli Yojana — National Portal