Most people start with the roof. They look at the terrace, guess how many panels will fit, and ask for that. We'd suggest starting somewhere else: the electricity bill sitting in your drawer.
The size of a home solar plant should come from how many units your family uses in a year. Not the biggest month, not the size of the terrace, and not what the neighbour put up. Here is how we work it out when someone walks into our Sevoke Road office with a bill in hand.
Step 1: find your yearly units
Look for "units consumed" or "billed units" on your WBSEDCL bill. Many home bills here cover a quarter, so check the billing period before you do anything else. If you can, add up the last four bills to get a full year. That one number does most of the work.
Why a year and not one bill? Because consumption in Bengal swings a lot. A family that runs two air conditioners from April to September can easily use twice as much in June as in January. Size to June and you'll have too much solar for half the year. Size to January and the summer bills stay high.
Step 2: divide by what one kW makes in a year
On the plains of Bengal and the Bihar border, a well-placed rooftop plant makes roughly 3.4 to 4 units per kW per day, averaged over the year. For a first estimate we use the middle of that, about 1,350 units per kW a year.
So the sum is simple: yearly units ÷ 1,350 = plant size in kW. A family using 4,000 units a year needs about 3 kW. One using 6,750 units needs about 5 kW.
A quick table
| Average units a month | Suggested size |
|---|---|
| 150 | 1.5 kW |
| 250 | 2 kW |
| 350 | 3 kW |
| 500 | 4.5 kW |
| 750 | 6.5 kW |
| 1,000 | 9 kW |
Treat these as starting points. The site survey moves them, sometimes a lot, once we see the shadow of a water tank or the building next door.
Step 3: check the two limits
Your sanctioned load. In West Bengal a rooftop plant can't be larger than the load sanctioned on your connection. If the bill says 3 kW and the sums say 5 kW, either we size down or we apply to WBSEDCL to raise the sanctioned load first. It's a routine application, but it takes time, so it's worth knowing early.
Your roof. Allow about 90 to 100 square feet of shadow-free flat roof for each kW. A 3 kW plant wants 270 to 300 sq ft; 5 kW wants 450 to 500. If space is tight, an elevated structure lifts the panels above tanks and stair rooms and frees up area you'd otherwise lose. There's more on this in how much roof space solar needs.
Why bigger isn't better under net metering
This one catches a lot of people out. Under West Bengal's net metering rules, units your plant sends to the grid are credited against units you take from it, and any credit left over carries forward to the next bill. But on 1 April every year the bank is wiped clean. Whatever you haven't used is gone, and nobody pays you for it.
So a plant that makes more than you use in a year is giving the extra away. Sizing to yearly consumption gets the best return out of every panel. We explain the rules properly in net metering in West Bengal.
Where the subsidy fits
Under PM Surya Ghar, the central assistance for homes grows up to 3 kW and then stops. You can still install more, and many families should, but the assistance is capped at the 3 kW level. To claim it the panels must be DCR (made with Indian cells), and that changes the maths for bigger homes. See DCR or Non-DCR panels.
Mistakes we see often
- Sizing to the summer bill, then exporting units all winter that lapse in April.
- Forgetting a second meter. Net metering works per consumer number, so a separate meter for the ground floor needs its own system.
- Ignoring shade. One tall tree to the south can cost more generation than you'd think, especially in winter when the sun sits low.
- Planning for an air conditioner you haven't bought yet. It's fine to allow for it, but tell us, so we can check the sanctioned load and the roof together.
Try it with your own numbers
Our size estimator does the arithmetic above from either your units or your bill amount. For a firm answer, send photos of your last three bills and your roof on WhatsApp. We'll come back with a size and a written note on how we got there.
Quick answers
How many kW of solar do I need for a 3BHK in Siliguri?
It depends on consumption, not rooms. A 3BHK using about 350 units a month typically needs around 3 kW. With two or three air conditioners in regular use it can be 5 kW or more.
Can I install more solar than my sanctioned load?
No. In West Bengal the plant cannot exceed your sanctioned load or contract demand. You can apply to WBSEDCL to raise the load first.
Is 3 kW of solar enough for a home?
For a home using about 300 units a month, yes. A 3 kW plant makes roughly 306 to 360 units a month on average in Bengal.