Utility July 13, 2026 · 8 Min Read

Solar Panel Calculator – Guide & Formulas

Estimate the number of solar panels and total system size (kW) needed for your home based on electricity usage, sun hours, and panel efficiency.

Size your residential solar panel array. Input your average monthly electricity bill or kWh consumption, your region’s average peak sun hours, and desired panel wattage to determine the ideal solar system capacity and panel count.

Key Takeaway

Use the free Solar Panel Calculator to estimate the number of solar panels and total system size (kw) needed for your home based on electricity usage, sun hours, and panel efficiency. Get instant results with step-by-step explanations.

How to Use the Solar Panel Calculator

  1. Enter your average monthly electricity consumption in Kilowatt-hours (kWh).
  2. Input the average daily peak sun hours for your location (typically ranges from 3 to 6 hours).
  3. Specify the power rating of the solar panels you plan to buy (standard panels range from 300W to 450W).
  4. Adjust the system efficiency losses factor (default is 20% to cover dirt, inverter losses, and wiring resistance).
  5. Instantly read the required solar array size in kW and the total number of physical panels needed.

The Formula

Daily Energy (kWh) = Monthly kWh / 30 | System Size (kW) = Daily Energy / (Peak Sun Hours * (1 - System Losses)) | Number of Panels = (System Size * 1000) / Panel Wattage

Variable Definitions

  • Peak Sun Hours: The equivalent number of hours per day when solar irradiance averages 1000 Watts per square meter
  • System Losses: Efficiency degradation due to inverter conversion, wire resistance, dirt, and temperature (typically 15% to 25%)
  • System Size (kW): The peak power output capacity of the solar array under standard test conditions

Sizing a Solar Array for a 900 kWh/Month Home

Determine system size and panel count for a home using 900 kWh per month, located in an area with 4.5 peak sun hours, using 400W panels, and assuming 20% system losses.

  1. Step 1: Calculate daily energy requirement: 900 kWh / 30 days = 30 kWh per day.
  2. Step 2: Account for system losses: We need to generate 30 / (1 - 0.20) = 37.5 kWh daily to cover losses.
  3. Step 3: Calculate required system size: 37.5 kWh / 4.5 sun hours = 8.33 kW solar array capacity.
  4. Step 4: Calculate the number of 400-watt panels: (8.33 kW × 1000) / 400W = 20.8 panels. Round up to 21 solar panels.

Frequently Asked Questions

What are "Peak Sun Hours"?

Peak sun hours are not the total hours of daylight. It represents the hours when sunlight intensity is strong enough to generate peak power, equivalent to 1,000 watts of light energy per square meter.

How much roof space do solar panels require?

An average residential 400W solar panel is roughly 17.5 sq ft (approx. 5.4 ft x 3.2 ft). A 20-panel system would require about 350 square feet of unobstructed, sunlit roof space.

Do solar panels work on cloudy days?

Yes, but at reduced efficiency. Solar panels can still produce 10% to 25% of their rated capacity under heavy cloud cover, and will not generate electricity at night.