The quickest way to size a home solar system is to start from the units on your electricity bill, not from a round number someone suggests. Take your average monthly units, divide by about 120 to 135 (what 1 kW of panels makes in a month in most of Gujarat), and you have a good first estimate in kW. Then check that your roof has room for it.
Step 1: Collect a full year of bills
Your electricity use changes a lot through the year in Gujarat. Summer months with ACs and coolers running can be double what you use in winter. If you size from one winter bill, the system will be too small. If you size from one May bill, it may be bigger than you need.
So gather the last 12 months of bills. If you have not kept them, check your DISCOM's website or app (UGVCL, MGVCL, DGVCL, PGVCL or Torrent Power) for past bills using your consumer number, or ask at the local office. Look for "units consumed" on each bill.
Also check the billing period printed on the bill. Some bills cover one month and some cover two, so make sure you know how many months each figure represents.
Step 2: Find your average monthly units
Add up the units for the whole year and divide by 12. That is your average monthly consumption.
For example, if your bills add up to 3,600 units over the year, your average is 300 units a month.
If you have a big change coming, adjust now. Planning to buy an electric car, add an AC, or have family move in? Add a reasonable estimate for that. If you just moved into the house and have no history, a recent few months plus a careful guess for summer will do.
Step 3: Divide by what 1 kW produces
A rooftop system in most of Gujarat makes roughly 4 to 4.5 units per kW per day averaged across the year. Kutch and Saurashtra do a little better, south Gujarat a little less, and every region dips in the monsoon. The Ministry of New and Renewable Energy's rooftop solar FAQ says 1 kW can make 4 to 5.5 units on a clear sunny day.
Over a month, that is about 120 to 135 units per kW. So:
System size (kW) = average monthly units ÷ 120 to 135
Here is what that looks like for common consumption levels:
| Average monthly units | Rough size needed | Practical choice |
|---|---|---|
| 100 | 0.75 to 0.85 kW | 1 kW |
| 150 | 1.1 to 1.25 kW | 1 to 1.5 kW |
| 200 | 1.5 to 1.7 kW | 2 kW |
| 300 | 2.2 to 2.5 kW | 2.5 to 3 kW |
| 400 | 3 to 3.3 kW | 3 to 3.5 kW |
| 500 | 3.7 to 4.2 kW | 4 kW |
| 600 | 4.4 to 5 kW | 5 kW |
These are estimates, not exact figures. Real output depends on shading, which way the panels face, how clean they are kept and the equipment used. Keeping panels clean in Gujarat's dust makes a real difference, as we explain in our cleaning and maintenance guide.
How this compares with the PM Surya Ghar guide
The national PM Surya Ghar scheme publishes a simple guide that matches this method. As shown on the national portal, pmsuryaghar.gov.in:
| Average monthly units | Suitable rooftop solar capacity |
|---|---|
| 0 to 150 | 1 to 2 kW |
| 150 to 300 | 2 to 3 kW |
| Above 300 | Above 3 kW |
The working above gives you a more precise number inside these bands.
Step 4: Check your roof space
Once you know the size, see if it fits. The same MNRE FAQ says about 10 to 12 square metres of shadow-free area is needed per kW. In square feet, that is roughly 110 to 130 per kW.
| System size | Shadow-free roof area (approx.) |
|---|---|
| 1 kW | 10 to 12 sq m |
| 2 kW | 20 to 24 sq m |
| 3 kW | 30 to 36 sq m |
| 5 kW | 50 to 60 sq m |
"Shadow-free" matters. Water tanks, staircase rooms, parapet walls, neighbouring buildings and trees can all throw shade at some point in the day, and even partial shade on a panel cuts its output a lot. A good installer will check this at the site visit. If your roof is smaller than the size you need, a slightly smaller system is still worth it. It will cover part of your bill instead of all of it.
Why the subsidy should not decide your size
Under PM Surya Ghar: Muft Bijli Yojana, homes get ₹30,000 per kW for the first 2 kW and ₹18,000 for the third kW, up to a maximum of ₹78,000. Systems larger than 3 kW still get ₹78,000, no more. Read the full details on our PM Surya Ghar subsidy page.
This leads people into two mistakes:
- Going to 3 kW only because the subsidy maxes out there, even when they use 150 units a month. They end up making far more units than they use.
- Stopping at 3 kW when they use 500 units a month, because extra capacity gets no extra subsidy. They end up still paying a large bill.
Size the system to your consumption. The subsidy is a bonus on top of a sensible decision, not the reason for it.
What happens to units you make but do not use depends on net metering rules set by the Gujarat regulator, so it is worth understanding before you go bigger than your needs. Our post on net metering in Gujarat explains how it works.
Things that can change the final number
Panel size comes in steps. Systems are built from whole panels, so the final size is often slightly above your round figure. For example, six panels of 550 W make 3.3 kW. That is normal.
Future plans. An EV, a new AC or a growing family can add a lot of units. It is often simpler to size for that now than to add panels later.
Daytime vs night use. If most of your use is at night, panels still help through net metering, but it is worth understanding how your DISCOM settles units before deciding.
Backup in power cuts. A plain on-grid system switches off when the grid fails. If you need power during cuts, a hybrid system with batteries is sized for your essential loads as well as your bill; see on-grid vs hybrid vs off-grid solar.
Your location. A home in Kutch or Rajkot will make a bit more per kW than one in Valsad or the Dang, so the same bill may need a slightly different size.
A worked example
A family in Rajkot has bills that add up to 3,600 units over the past year, so they average 300 units a month. Summer bills are about 450 units and winter bills about 180.
- 300 ÷ 135 = 2.2 kW, and 300 ÷ 120 = 2.5 kW.
- They plan to add one more AC next year, so they go to the upper end.
- A 3 kW system needs about 30 to 36 sq m of shadow-free roof, and their terrace has enough space.
- 3 kW also gets the maximum ₹78,000 subsidy, so in their case need and subsidy line up neatly.
If they lived in a flat with only 20 sq m of usable roof, a 2 kW system would still cover a large part of their bill.
Next step
Open your DISCOM's website or app tonight, note the units for the last 12 months, and work out your average. With that one number you can have a far more useful conversation with any installer, and you will know straight away if someone is pushing a size that does not fit. You can also browse solar installation across Gujarat's districts. If you want, we can connect you with one checked installer near you.
This guide is general information, not advice. Scheme rules and amounts change, so check the official portal or your DISCOM before you decide.