How to Choose the Right Solar System Size

"What size solar system do I need?" is usually the first real question after "should I go solar at all." There's no single fixed package that fits every roof — the right size depends on how much roof space you have, how much electricity you actually use, and what kind of building it is. Here's how we actually work it out, which is also exactly the logic behind our solar calculator.

Method 1: Available roof area

As a rule of thumb, a rooftop solar installation needs roughly 100-110 square feet of shadow-free roof per kW of system capacity (we use ~107 sq.ft./kW in our estimates), accounting for panel spacing, walkways and mounting structure. So a roof with 500 sq.ft. of usable space could comfortably support a system in the 4-5kW range. This sets the practical ceiling on system size for a given roof, regardless of what your bill might justify.

Method 2: Your electricity consumption

A well-sized system should generate roughly what you consume over a month — oversizing wastes money on panels producing power you can't use or export efficiently, while undersizing means you're still paying for a meaningful chunk of grid electricity. As a working estimate, 1kW of rooftop solar in this region generates around 120 units (kWh) per month. If your monthly bill shows you're using, say, 480 units, a 4kW system would be a reasonable match — before accounting for roof constraints or subsidy considerations.

Method 3: Venue type and load pattern

The type of building changes both the tariff you pay and when you actually use electricity, which matters for sizing:

• Homes typically pay a lower domestic tariff and have load spread across the day and evening — ACs, refrigeration, lighting. Residential systems are also the only ones eligible for the PM Surya Ghar and UPNEDA subsidy, which meaningfully changes the economics (see our subsidy guide).
• Shops often run refrigeration, lighting and displays through business hours — daytime-heavy load, which lines up well with solar generation.
• Offices have concentrated daytime HVAC and equipment load during business hours, again a good match for solar's generation window.
• Factories usually need a proper load assessment — machinery, shift patterns and peak demand vary too much for generic per-kW rules of thumb to be reliable, which is exactly why we do an on-site assessment before quoting any commercial or industrial system.

A worked example

Take a home with a ₹4,000 average monthly bill, 400 sq.ft. of usable roof, and one AC. Using the bill (₹4,000 ÷ ~₹7/unit tariff ≈ 570 units/month ÷ 120 units per kW ≈ 4.75kW), the roof area (400 ÷ 107 ≈ 3.7kW), and a small bump for the AC load, these methods triangulate to a system somewhere in the 3.5-4.5kW range — which is also comfortably within the subsidy-eligible bracket that gets the full ₹78,000 central subsidy. This is exactly the kind of calculation our solar calculator runs automatically when you enter your own numbers.

Why oversizing and undersizing both hurt

An oversized system costs more upfront and, under standard net metering, surplus exported beyond what your annual consumption can offset doesn't necessarily translate into proportional financial benefit — you're spending on capacity you don't need. An undersized system is cheaper today but leaves you still paying a real electricity bill every month, stretching out how long it takes before solar starts saving you money net of what you could have generated. The right size sits where your roof, your consumption and your budget genuinely intersect — which is exactly what a proper site visit is for, rather than a one-size package.