How solar array sizing actually works
The whole calculation hangs on one idea: a solar panel only produces its nameplate wattage under laboratory test conditions, and a real roof only sees the equivalent of a few of those perfect hours per day. That equivalent figure is called peak sun hours — the number of hours of 1,000 W/m² sunlight that would deliver the same total energy as your actual day. So a 1 kW array in a location with 4.5 peak sun hours yields roughly 4.5 kWh a day before losses. Working backwards, the array size you need is daily kWh ÷ (peak sun hours × derate), and the panel count is that wattage divided by the wattage of one panel, rounded up.
The derate factor is where most rough estimates go wrong. Between DC-to-AC inverter conversion, voltage drop in the wiring, panels running hot in summer, dust and pollen on the glass, and small mismatches between modules in a string, a well-built system typically delivers meaningfully less to the meter than the panels' combined nameplate rating. Using something in the region of 80% keeps the estimate honest. Shading is separate and much more brutal: a single chimney, vent stack or tree branch crossing one panel can pull down a whole string unless the system uses microinverters or DC optimisers, so it is worth having an installer run a shade analysis on the actual roof rather than assuming a clean sky.
Roof area here is estimated at about 18 ft² per panel, which reflects a common residential module footprint plus a little spacing. Real layouts are constrained by roof planes, ridges, valleys, obstructions and the setback distances fire codes require around the array edges, so the usable area on a given roof is almost always smaller than its total surface. Also remember that offsetting 100% of your usage is a choice, not a requirement — local rules on net metering, export limits and interconnection capacity may cap how big a system you are allowed to connect.
Planning estimate only. Actual solar output varies substantially with location, roof pitch and orientation, shading, weather and equipment, and the figures above do not account for incentives, taxes or interconnection requirements. Confirm sizing, structural capacity and permitting with a licensed solar installer and your utility before buying anything.