Worked example
A household using 300 units/month aiming for a 100% offset needs about a 2.5 kW system, needing roughly 250 sq ft of roof space.
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Solar Panel Size Calculator
What system size and roof area you need
Recommended size
2.5 kW
Roof area needed
250 sq ft
≈ 300 units/month generation
Assumes ~4 units/kW/day generation and ~100 sq ft of shadow-free roof per kW — actual space needed varies by panel efficiency, spacing and roof layout.
How Much Roof Space Do You Actually Need?
A quick reference at ~100 sq ft per kW — real space needed varies with panel efficiency, spacing and roof layout.
| System size | Roof area needed |
|---|---|
| 1 kW | 100 sq ft |
| 1.5 kW | 150 sq ft |
| 2 kW | 200 sq ft |
| 3 kW | 300 sq ft |
| 5 kW | 500 sq ft |
| 7.5 kW | 750 sq ft |
| 10 kW | 1000 sq ft |
How Shading and Orientation Affect Your Effective System Size
The roof-area figures above assume unshaded, well-oriented space. A south-facing roof with no obstructions gets close to the standard generation assumption per panel; east- or west-facing roofs generate somewhat less through the day, and any shading — from a water tank, parapet, tree or neighbouring building — cuts output further on the affected panels.
In practice this means a shaded or poorly-oriented roof may need more panels (and more area) than the table above to reach the same effective kW — an installer's site survey, which checks sun-path and obstructions directly, is the only way to confirm your roof's real capacity.
System Size vs. Your AC and Appliance Load
Your "monthly units" input should reflect your real household consumption, not just base lighting and fans — AC in particular can be the single biggest line on your bill, so leaving it out will undersize your system. If you don't know that number yet, check it with the calculators below before sizing your system.
Related calculators
Frequently asked questions
How is recommended system size calculated?
Recommended kW = (monthly units × offset %) ÷ (4 units/kW/day × 30 days) — a standard planning assumption of about 4 units generated per kW installed per day, averaged across the year for Indian conditions.
Why does the calculator round to the nearest 0.5 kW?
Rooftop solar systems are typically sized and quoted in half-kW increments based on standard panel and inverter combinations, so a rounded figure is more useful for getting real quotes.
Should I target 100% offset or more?
It depends on your state's net-metering rules. If your DISCOM credits exported units well, sizing above 100% can make sense; if export credit is low, sizing closer to 100% (or your actual daytime usage) usually gives the best payback. See our net metering calculator.
Does roof shading affect this?
Yes — this calculator assumes shadow-free roof space. Partial shading (from a water tank, trees, or a neighbouring building) reduces effective generation per panel and may mean you need more roof area than shown here for the same output.
How much roof space do I need for a typical system?
Roughly 100 sq ft per kW under standard planning assumptions — so a 1 kW system needs about 100 sq ft, 3 kW needs about 300 sq ft, and 5 kW needs about 500 sq ft. See the reference table above for more sizes.
Does panel orientation affect how much roof space I need?
Yes — a south-facing, unshaded roof gets the most output per panel, so it needs the least area for a given kW. East/west-facing or partially shaded roofs generate less per panel, so you may need extra panels (and area) to hit the same target system size.
What if my roof isn't a simple rectangle?
Installers work around vents, water tanks, staircases and irregular shapes routinely — the sq ft figures here are a planning estimate assuming a clear rectangular area. An installer's site survey will confirm exactly how many panels actually fit your specific roof.
Should I size my system for my whole house or just my AC load?
Size for your total household consumption, not just one appliance — but if AC is your single biggest load, make sure it's reflected in the "monthly units" figure you enter, since undersizing around it is a common mistake. See our AC running cost calculator to check that load specifically.