Solar Panel Size Calculator
FREE · NO SIGNUPWork out your system size from your roof area or your appliances, and see how many panels you need and how much roof they actually take. For what a system would save you, use the Chennai savings calculator instead.
I have this much roof
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What you run, and for how long
Wattages are indicative. The refrigerator is modelled on its compressor duty cycle rather than 24 hours at rated draw, because doing it the other way inflates every household result.
I want this system size
Panel count changes with wattage, which is why it is a poor basis for comparing quotes. The 600W and 700W series cut the count further without changing capacity.
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Roof area needed
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Of which panel glass
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Per day
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Per month
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Per year
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This tool tells you what fits. It does not price anything or estimate savings — that is the savings calculator's job, and keeping them separate is what keeps both honest.
See what this would save you →Panel count
How many solar panels do I need?
The number of solar panels you need depends on your system size and the wattage of the panels used. With the 545 to 700 watt modules common in 2026, one kW takes roughly two panels, so a 3 kW system uses about five to six panels and a 5 kW system about eight to ten. On the 700W series a 3 kW system is under five panels. Your system size follows from your annual electricity consumption, or from the usable roof area you have available.
A 3 kW system, by series
Same capacity. Fewer, larger modules.
550W
6 panels
600W
5 panels
700W
5 panels
Roof area barely moves — it follows kW, not panel count. Only the module count and the number of fixings change.
Panel count is not a quality measure. A system using fewer, higher-wattage panels is not better or worse than one using more, lower-wattage panels of the same total capacity. What matters is total kW, the technology, and the warranty. Do not compare quotes on panel count alone.
The gap everyone notices
What the 50 sq ft per kW figure actually covers
The panels themselves occupy considerably less area than the installed system requires. We plan on 50 square feet of usable roof per kW, which is the figure the sizing above uses. At 550 watts per module that is a little under two panels of glass, and it assumes the array is laid out sensibly — tilt, the spacing needed between rows so panels do not shade each other, and access for cleaning all have to fit inside it. On a congested terrace the real figure runs higher, which is what the survey establishes.
One kW, in numbers
Glass area against the planning figure.
Panel surface
~50 sq ft
Planning figure per kW
50 sq ft
Tilt, row spacing, walking access and edge setbacks all have to fit inside that figure — the four things below.
Tilt
Panels are angled rather than laid flat, so each row occupies more horizontal space than its own dimensions.
Inter-row spacing
A tilted row casts a shadow behind it in low sun. Rows must be spaced so they do not shade the row behind, particularly in winter when the sun is lower.
Access
Somebody has to walk between rows to clean panels and inspect connections, which matters more in Chennai than most places given how quickly panels soil.
Edge setbacks
Arrays are kept back from parapet edges for safety and wind loading reasons.
And separately, usable area is less than total area
Water tanks, staircase headrooms, lift machine rooms, parapet walls and neighbouring buildings all reduce what is genuinely available and shade-free through the middle of the day.
A common working assumption is that around 70% of a terrace is usable, which is what the tool defaults to — though it varies widely and can only be established properly by a survey. If you have measured your shade-free area, slide it to 100%.
What eats the other 30%
Water tanks and sumps
Staircase headroom
Lift machine room
Parapet setback
Neighbouring buildings
Which method
Sizing by appliances versus sizing by bill
Sizing from your electricity bill is considerably more accurate than sizing from a list of appliances. A bill records what you actually consumed. An appliance list records what you think you use, and people consistently underestimate running hours and forget appliances entirely. Use the appliance method when there is no bill to work from, such as a new building or an off-grid site.
Which input to trust
Your bill · what you did consume
A measured record. Start here whenever one exists.
An appliance list · what you think you use
Hours get underestimated and appliances get forgotten. Use it only when there is no bill.
When the appliance method is the right one
A new property with no consumption history
An off-grid site with no connection and therefore no bill
A specific circuit you want to back up, such as sizing a hybrid battery
A rough sanity check before you have your bill to hand
When to use the bill instead: everywhere else. It is the only input that reflects what you actually paid for.
Chennai savings calculator →Honest limits
Why a survey still matters
A sizing calculator works from area and averages. It cannot see shading across the day, the structural condition of your roof, its orientation, your sanctioned load, or obstructions that reduce usable area. Those are established at a site survey, and any of them can change the answer significantly.
| What the calculator cannot see | Why it changes the answer |
|---|---|
| Shading through the day | Which a roof plan cannot show. A terrace that looks clear at 10am can be substantially shaded by 2pm |
| Orientation and roof pitch | Which affect generation on sloped roofs |
| Structural capacity | Particularly on older buildings and sheet roofs |
| Sanctioned load | Which caps your system size regardless of roof area |
| Access | Which affects both installation and long-term maintenance |
This tool tells you what fits
See what a system this size would save you
The savings calculator tells you what it is worth, using your electricity bill, the PM Surya Ghar subsidy and Tamil Nadu's tariff structure. Both tools share the same generation and area constants, so the sizes agree.
Use the Chennai solar savings calculator →Common questions
Panel count and sizing, common questions
It depends on system size and panel wattage. With the 545 to 700 watt modules common in 2026, one kW takes roughly two panels, so a 3 kW system uses about five to six panels and a 5 kW system about eight to ten. On the 700W series a 3 kW system is under five panels. System size itself follows from your annual consumption or from your usable roof area.
About six panels at 550 watts each. At 450 watts it would be seven. Panel count varies with wattage, so it is not a useful basis for comparing quotes. Total system capacity in kW is what matters. What a 3 kW system costs in Chennai.
Roughly 80 to 100 square feet of usable, shade-free area per kW. The panels themselves occupy only about 50 square feet per kW; the rest is tilt, inter-row spacing so panels do not shade each other, and access for cleaning and maintenance.
Because total roof area is rarely all usable. Water tanks, staircase headrooms, lift machine rooms, parapet setbacks and shading from neighbouring structures all reduce what is genuinely available through the middle of the day. If you have measured your shade-free area, you can slide the usable share up to 100%.
Neither is inherently better at the same total capacity. Higher-wattage modules mean fewer panels and slightly less roof area, which helps on constrained roofs. What matters is total kW, the panel technology and the warranty, not the panel count.
You can, and the tool supports it, but it is less accurate. People consistently underestimate running hours and forget appliances. Use the appliance method for a new building, an off-grid site with no bill, or when sizing a battery for specific backup circuits. Otherwise use your bill.
Divide your usable shade-free roof area by roughly 90 square feet to get the approximate maximum in kW. A 450 square foot usable terrace supports around 5 kW. Bear in mind that your sanctioned load may cap the system below what the roof could physically carry.
The panel count and roof area calculations apply anywhere. The generation figures are based on Chennai's solar resource of approximately 5.3 peak sun hours a day, so output will differ in other regions, generally lower in the north and comparable across much of southern India.