Getting your system size wrong costs you money in two distinct and opposite ways, and most first-time buyers only worry about one of them — usually the more obvious one.
Undersizing is the mistake everyone anticipates. If your system generates meaningfully less than your household consumes, you're still buying a significant amount of grid electricity every month, your monthly savings are smaller than they could be, and your payback period on the investment stretches out longer than necessary. This is the mistake that gets caught easily, because the bill savings simply look disappointing from month one.
Oversizing is the less obvious mistake, and in this specific region, it matters more than buyers typically assume. As covered in the net-metering topic, export electricity is credited at a DISCOM-set buyback rate that is meaningfully lower than the rate you'd otherwise pay to import the same unit of power. This means that generating far more electricity than your household actually consumes doesn't return full value — you're effectively selling your surplus at a discount rather than saving the full retail-equivalent amount you would have saved through direct self-consumption. An oversized system also means a larger upfront cost and larger EMI, without a proportional increase in real monthly savings once you're generating well beyond your consumption needs.
So how should the right size actually be determined? Three inputs matter, in this order of importance. First, your trailing 6 to 12 month average electricity bill — not a single month, which can be skewed by an unusually hot or unusually mild period — gives a realistic picture of your baseline consumption across seasons. Second, your household's actual usage pattern matters, not just the total: a household that runs its washing machine, water pump, or home office equipment during daylight hours benefits more from direct self-consumption per unit of solar generated than a household whose usage is concentrated in evening hours after panels have stopped producing. Third, your available shadow-free roof space places a physical ceiling on what's achievable regardless of what the ideal number on paper might be.
This is precisely why a credible vendor's recommendation should follow a site visit and a review of your actual consumption history, rather than a blanket "everyone in this area should get 3kW" answer given over a phone call. The right number for your specific household could reasonably be smaller or larger than a neighbor's system, even if your homes look similar from the outside.
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