Start with your electricity use
Everything begins with how much electricity you actually use, measured in kilowatt-hours per year. Pull it from your utility bills — most show twelve months of history. A typical home uses somewhere around 10,000 to 11,000 kWh a year, but real usage varies enormously with climate, home size and habits, so use your own figure rather than an average.
Factor in your sun
The same panel produces far more in Arizona than in Seattle. The shorthand is peak sun hours — the equivalent hours of full-strength sun your location averages per day, usually between 3 and 6. After accounting for real-world losses of about 20%, the system size you need is roughly your annual kWh divided by (peak sun hours × 365 × 0.8). The system-size calculator does this for you from your usage and region.
Convert kilowatts to a panel count
Once you have a system size in kilowatts, the panel count is simple arithmetic. Modern residential panels are about 400 to 450 watts each, so panels are roughly system kW × 1,000 ÷ panel watts. A 7.5 kW system at 425-watt panels is about 18 panels. Higher-wattage panels reach the same system size with fewer modules — useful when roof space is limited.
Roof space and orientation
Each panel occupies roughly 18 square feet, so panel count and available roof area have to agree. Orientation matters too: south-facing roofs produce the most, while east- or west-facing surfaces give up roughly 10 to 20%. Shading from trees or chimneys can disproportionately cut output, so a smaller unshaded array sometimes beats a larger shaded one.
Should you size to cover 100%?
Not always. Your local net-metering or export rules, your roof space and your budget all influence the target. Some owners deliberately offset 80 to 90% to avoid over-producing under unfavourable export rates. And if you’re about to add an EV or a heat pump, size up — both raise your annual usage substantially, so plan for the loads you’ll add, not just today’s bill.
A quick worked example
Suppose your bills show 9,600 kWh a year and you live somewhere averaging 4.5 peak sun hours. Dividing 9,600 by (4.5 × 365 × 0.8) gives about 7.3 kW. At 430-watt panels that is roughly 17 panels needing about 305 square feet of well-oriented roof. Now add an EV that uses 3,000 kWh a year and your target climbs to about 12,600 kWh, pushing the system to roughly 9.6 kW, or about 23 panels. The same exercise for a smaller, mild-climate home using 5,000 kWh at 5 peak sun hours lands near 3.4 kW — about 8 panels. The method never changes; only your three inputs do.
Common sizing mistakes to avoid
The most frequent error is sizing to square footage instead of actual usage — two homes of identical size can differ twofold in consumption. The second is ignoring future loads: fitting exactly enough for today, then adding an EV or heat pump a year later and wishing you had gone bigger. The third is underestimating shading, which quietly erodes the production a paper calculation promises. The fourth is forgetting that export rules vary — in some areas over-producing earns very little, so the largest possible array is not always the smartest one. Running your own numbers, then sanity-checking them against your roof and your utility’s rules, avoids all four and leaves you with a figure you can trust.
The bottom line
There is no universal panel count — only your three numbers run through one formula. Pull your annual kWh, find your peak sun hours, pick a panel wattage, and the answer falls out. Do that before you talk to an installer and you’ll know whether a quote is sized to your home or to their inventory, which puts you in a far stronger position to compare proposals.