Solar Size Calculator

by
🛡️ Reviewed by David Chen, CFA — Energy Infrastructure Specialist

Switching to renewable energy starts with precision. Use our solar size calculator to determine the exact solar panel system capacity required to offset your monthly electricity bills based on your local sun exposure and system efficiency.

Solar Size Calculator

Your average monthly electricity cost.
Price per kilowatt-hour from your utility.
Average peak sunlight hours in your area.
Standard is 75% – 85% (accounting for losses).
Recommended System Size
—
—

Solar Size Calculator Formula

$$System Size (kW) = \frac{(Monthly Bill / Rate) \times 12}{365 \times Sun Hours \times Efficiency}$$

Formula Source: National Renewable Energy Laboratory (NREL)

Variables Explained

  • Monthly Bill: The total amount paid to your utility provider each month.
  • Electricity Rate: The cost per unit of energy (kWh). This allows us to calculate your total energy consumption.
  • Peak Sun Hours: Not just daylight, but the hours when solar intensity is at its peak (averaging 1,000 watts per square meter).
  • Efficiency: Accounts for energy lost during conversion (DC to AC), wiring resistance, and panel degradation.

Related Calculators

Solar Payback Period Calculator Battery Storage Capacity Tool Carbon Footprint Offset Estimator Monthly Electricity Bill Forecaster

What is a Solar Size Calculator?

A solar size calculator is an essential tool for homeowners and businesses looking to transition to green energy. It translates your financial energy costs into physical hardware requirements. By analyzing your consumption patterns and geographic solar potential, it estimates the total kilowatt (kW) capacity needed for your roof.

Choosing the right size is critical; an undersized system won’t eliminate your utility bill, while an oversized system may lead to unnecessary upfront costs that take longer to recoup through net metering.

How to Calculate Solar System Size (Example)

  1. Determine Monthly Usage: Divide your bill ($150) by your rate ($0.15) = 1,000 kWh per month.
  2. Calculate Annual Usage: 1,000 kWh x 12 months = 12,000 kWh per year.
  3. Daily Target: 12,000 kWh / 365 days ≈ 32.88 kWh per day.
  4. Apply Solar Factors: Divide daily usage by peak sun hours (e.g., 5) and efficiency (0.75).
  5. Final Result: 32.88 / (5 * 0.75) = 8.77 kW System.

Frequently Asked Questions (FAQ)

What is the average solar system size for a house? Most American homes require a system between 6 kW and 10 kW to cover 100% of their electricity needs.

Does a solar size calculator account for roof angle? Basic calculators assume ideal orientation. For precise engineering, tilt and shading factors are added to the efficiency variable.

Is 100% offset always better? Not necessarily. Depending on your utility’s net metering policy, sizing for 80-90% offset might provide a faster Return on Investment (ROI).

How many panels are in a 5kW system? Using modern 400W panels, a 5kW system would consist of approximately 12 to 13 solar panels.