Solar Power System Size Calculator

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Expert Reviewed by: David Chen, Renewable Energy Consultant

Updated: October 2023 | Fact Checked

Planning to go solar? Our solar power system size calculator helps you estimate the precise DC kilowatt (kW) capacity required to cover your home’s electricity needs based on local sunlight and energy consumption.

Solar Power System Size Calculator

Typical range: 3.0 – 6.0 hours
Average is 75-80% (accounting for inverter losses)

Solar Power System Size Calculator Formula

$$System Size (kW) = \frac{Monthly Usage (kWh) \div 30}{Peak Sun Hours \times Efficiency}$$

Formula Source: National Renewable Energy Laboratory (NREL) & U.S. Department of Energy

Variables:

  • Monthly Usage: Your total electricity consumption in kilowatt-hours, found on your utility bill.
  • Peak Sun Hours: The number of hours per day when solar intensity averages 1,000W/m².
  • Efficiency: Accounts for losses due to wiring, inverter conversion, and panel soiling.

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What is a solar power system size calculator?

A solar power system size calculator is a professional tool used to determine the total power output capacity needed for a photovoltaic (PV) installation. It translates your household’s annual energy appetite into a tangible hardware requirement measured in kilowatts (kW).

By inputting variables like geographical sun exposure and historical usage, the calculator ensures you don’t undersize your system (leading to high grid bills) or oversize it (extending your return on investment period).

How to Calculate (Example)

  1. Find your monthly usage (e.g., 900 kWh).
  2. Divide by 30 to get daily usage (30 kWh/day).
  3. Find your sun hours (e.g., 5 hours).
  4. Divide daily usage by sun hours (30 / 5 = 6 kW).
  5. Adjust for 78% efficiency (6 / 0.78 = 7.69 kW).

Frequently Asked Questions (FAQ)

What is a good system size for a standard home? Most American homes require between 6kW and 10kW to cover 100% of their electricity needs.

Do I need a bigger system if I have an EV? Yes, adding an Electric Vehicle typically increases monthly usage by 250-400 kWh, requiring 2-3 extra kW of solar.

Does efficiency change over time? Yes, solar panels degrade about 0.5% per year, but this calculator uses a standard “derate factor” for system losses.

How many panels is a 5kW system? Assuming 400W panels, a 5kW system consists of roughly 13 panels.

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