Sizing Solar in Four Numbers

Panel count = daily kWh ÷ (panel watts × sun hours × system efficiency). Monthly usage ÷ 30 gives the daily target; peak sun hours (3.5–6 across the US) say how many full-strength hours the roof sees; and the 20% losses derate accounts for inverters, heat, dust, and wiring. Every input is something you can find on your bill, a solar map, or a panel spec sheet.

The Derate Is Not Optional

New solar owners consistently wonder why production runs 15–25% below the panel ratings. It's physics: inverters lose 3–5%, panels run hot in midday sun losing 5–10%, and soiling plus shading nibble the rest. Sizing without the derate produces a system that under-delivers on paper — the calculator bakes the industry-standard 20% in by default.

Cost Before and After Incentives

The $/W figure ($2.50–3.50 typical) covers panels, inverters, racking, labor, and permits. The federal Investment Tax Credit rebates 30% of that, and many states and utilities add their own. Use the cost line as a sanity check against installer quotes — a bid wildly above $/W × system size deserves a second opinion.