Solar Panel Sizing Calculator

A solar panel calculator (also called a solar panel energy estimator or solar array size calculator) determines how many panels and how much roof space you need based on your electricity consumption and local sunlight. Whether you start from your monthly bill or average daily kWh, the tool applies your target offset and peak sun hours to output a recommended system size in kilowatts.

Our solar system size calculator supports 50% to 150% bill offset scenarios, modern panel wattages from 350W to 500W, and adjustable peak sun hours for any U.S. region — helping you calculate solar panel output before talking to installers.

Every solar panel sizing calculator follows the same core physics: your panels must produce enough daily kilowatt-hours to match (or exceed) your consumption, adjusted for how many hours of strong sunlight your roof receives.

1. 1
   Determine Daily Energy Usage

   From your bill: Daily kWh = (Monthly Bill ÷ $0.16/kWh) ÷ 30 days. Or enter average daily kWh directly from your utility statement.

2. 2
   Apply Target Offset

   Target Daily Production = Daily Usage × (Offset% ÷ 100). A 100% offset matches your bill; 150% supports EV charging or future load growth.

3. 3
   Divide by Peak Sun Hours

   Required kW = Target Daily kWh ÷ Peak Sun Hours. Southwest U.S. averages 5–6 hrs; Northeast averages 3.5–4.5 hrs.

4. 4
   Convert kW to Panel Count

   Panel Count = ceil(System kW × 1,000 ÷ Panel Wattage). A 6.7 kW system with 400W panels needs 17 panels.

Raw panel count from a solar panel calculator assumes unobstructed roof space. In practice, usable area is always less than total roof square footage. Before finalizing hardware purchases, account for:

• Fire setbacks — building codes require 3-foot clearance along ridges and eaves on many roofs
• Roof obstructions — vents, skylights, chimneys, and HVAC units reduce available mounting area
• Shading — trees, neighboring structures, and dormers cut daily production and may disqualify sections entirely
• Panel spacing — each module needs ~17.5–19.5 sq ft including gaps for mounting rails and maintenance access

Add a 10–15% buffer above the calculator's roof area estimate. If your available south-facing roof is insufficient, consider higher-wattage panels (450W–500W) or a ground-mount array.