Solar System Size Calculator
Find the exact kW system size, panel count, and roof space you'll need — sized to your real usage and local sunlight.
Why Live-Data System Sizing Is the First Decision, Not the Last
Most people start shopping for solar by asking what a system costs, but cost only makes sense once you know the size. A 4kW system and a 10kW system are different products entirely, and quoting cost before sizing is like asking "how much is a car" without saying how many seats you need. This calculator flips the order: usage and sunlight first, size second, cost later.
The math is straightforward once you break it down. Take your monthly kWh usage, convert it to a daily number, then divide by your area's peak sun hours — the average number of hours per day the sun delivers roughly 1,000 watts per square meter. A home using 30 kWh a day in a city with 5 peak sun hours needs roughly a 6kW system before accounting for real-world losses, which is why we apply a derate factor rather than the raw number.
Choosing Your Offset Percentage
100% offset means the system is sized to fully cancel your average bill over a year — not every single month, since production varies by season. Some homeowners intentionally undersize to 70-80% offset to lower upfront cost, especially if net metering isn't available or roof space is limited. Others oversize past 100% to account for future electric vehicle charging or planned appliance upgrades. There's no universally "correct" number, only the one that matches your budget and roof.
Once you have a kW target, the panel count and roof space fall out automatically from panel wattage. Higher-wattage panels mean fewer panels for the same output, which matters most on roofs with limited usable area. If your roof space is the binding constraint rather than budget, try increasing the panel wattage input to see how it changes your footprint.
Why Your System Might Produce Less in December Than in July
The kW number this calculator gives you is sized against an annual average of peak sun hours, not a single day. In practice, production swings by season — a system in a place with real winters can produce 40-50% less power in December than in June, purely from shorter days and a lower sun angle. Sizing against the annual average means you'll produce a surplus in summer and a deficit in winter, which nets out close to your target over a full year, but won't offset your bill evenly month to month. If your utility only credits net-metering surplus within the same billing cycle rather than rolling it over, ask your installer about that before assuming this calculator's output maps directly onto every monthly bill.
Frequently Asked Questions
How do I know what size solar system I need?
Your system size depends on your monthly electricity usage and how much sun your area gets. This calculator divides your daily energy need by your local peak sun hours to size a system that can realistically cover your usage.
What does offset percentage mean?
Offset percentage is how much of your electricity usage the solar system is designed to cover. A 100% offset aims to eliminate your grid bill entirely, while a lower offset covers only part of it, usually to reduce upfront cost.
Why does panel wattage matter for the panel count?
Higher wattage panels produce more power per panel, so you need fewer of them to hit the same system size. Standard residential panels today range from 350W to 450W, and this calculator defaults to 400W but lets you adjust it.
How much roof space does a typical solar system need?
A standard residential panel is roughly 17.5 square feet. A 6kW system with 400W panels needs about 15 panels, which works out to roughly 260 square feet of usable, unshaded roof space.
Is this calculator using real, live data?
Yes. Every calculation fetches current peak sun-hour data live from NASA's POWER satellite database for your exact coordinates, rather than pulling from a static, pre-set table — so sizing reflects real solar conditions at your specific location.