Solar Panel Size Calculator

Estimate the size of the solar system required to offset your electricity bill.

Varies by location (e.g., 4-5h in US/Europe)

Accounts for inverter loss, wiring, dirt, etc.

How to Size a Solar System

Planning a solar installation starts with understanding your energy needs. This calculator helps you estimate the number of panels required to reach "Net Zero" (producing as much energy as you consume).

Key Factors in Solar Sizing

1. Monthly Energy Usage (kWh)

Check your electricity bill for your average monthly usage in kilowatt-hours (kWh). For a more accurate system, use your highest usage month (usually summer or winter) or an annual average.

2. Peak Sun Hours

This is NOT the length of the day. "Peak Sun Hours" refers to the equivalent number of hours per day where solar irradiance averages 1000 W/m².

  • 3-4 Hours: Northern Europe, Canada, Northern US (Winter).
  • 4-5 Hours: Average for most of the US and Central Europe.
  • 5-6+ Hours: US Southwest, Australia, Middle East.

3. System Efficiency

Solar panels produce DC power, which must be converted to AC by an inverter. Real-world systems have losses due to:

  • Inverter efficiency (90-96%)
  • Wiring losses (2-3%)
  • Dust and dirt on panels
  • Temperature (panels are less efficient when hot)

A standard conservative estimate for total system efficiency is 75% to 80%.

Calculation Steps

The calculator performs the following math:

1. Daily Usage:
Daily kWh = Monthly kWh / 30
2. Required DC System Size:
kW Needed = Daily kWh / (Sun Hours × Efficiency)
3. Number of Panels:
Count = (kW Needed × 1000) / Panel Wattage

Example

If you use 900 kWh per month, live in an area with 5 peak sun hours, and use 400W panels:

  • Daily Usage = 900 / 30 = 30 kWh/day
  • System Size = 30 / (5 × 0.75) = 8 kW
  • Panels = (8 × 1000) / 400 = 20 Panels