System Sizing Inputs
Check your electricity bill or use the Appliance Builder tab to estimate.
Auto-filled from state selection. US average is 4.5 hr/day.
MPPT is more efficient and recommended for larger systems.
Accounts for inverter, wiring, soiling, and temperature losses. Typical: 75–85%.
Enter Your Details
Fill in your daily energy usage, peak sun hours, and panel wattage to see how many solar panels you need, your system size, and 25-year savings projection.
How to Use the Solar Panel Calculator
Choose Your Calculation Mode
Select one of four tabs: System Sizing to find how many panels you need, Battery Storage to size your backup battery bank, Cost and Payback for a 25-year financial projection, or Appliance Builder to estimate your daily energy use from individual devices.
Enter Your Energy Usage and Location
In System Sizing, enter your daily kWh usage (from your electricity bill or the Appliance Builder), select your US state to auto-fill peak sun hours, then adjust panel wattage and type. The performance ratio (80% default) accounts for real-world losses.
Review Results and Environmental Impact
See the number of panels, system size in kW, daily and annual output, minimum inverter size, and your solar coverage percentage. The environmental section shows annual CO2 avoided in kilograms and the equivalent number of trees planted each year.
Run the Payback Analysis and Export
Switch to the Cost and Payback tab to enter your system cost, electricity rate, and 30% federal tax credit. Review the 25-year savings chart showing when cumulative savings cross your net system cost (break-even year). Export your results to CSV or print a summary report.
Frequently Asked Questions
How many solar panels does the average US home need?
The average US home consumes about 30 kWh per day (10,500 kWh/year). With 400W panels, 4.5 peak sun hours, and an 80% performance ratio, you need approximately 21 panels (about 8.3 kW system). Homes in sunnier states like Arizona or Texas need fewer panels because they receive more peak sun hours per day. Homes in the Pacific Northwest or Northeast require more panels to meet the same energy demand. Your actual number depends on your specific consumption, local sun hours, and whether you intend to fully offset your bill or only a portion of it.
What is the difference between MPPT and PWM charge controllers?
MPPT (Maximum Power Point Tracking) charge controllers are more efficient — typically 92–98% — because they actively optimize the voltage-current relationship from the solar array to maximize power extraction. PWM (Pulse Width Modulation) controllers are simpler and cheaper but operate at about 75–80% efficiency because they connect panels directly to the battery, wasting power when panel voltage exceeds battery voltage. For larger systems (above 200W) and lithium batteries, MPPT is strongly recommended. For small systems under 100W or with closely matched panel and battery voltages, PWM can be a cost-effective choice.
How is the battery bank size calculated?
Battery bank size equals your daily energy usage multiplied by the number of backup days you want, divided by the battery's depth of discharge (DoD). For example: 30 kWh/day × 2 days = 60 kWh raw energy needed. With LiFePO4 at 90% DoD: 60 ÷ 0.90 = 66.7 kWh battery capacity. With lead-acid at 50% DoD: 60 ÷ 0.50 = 120 kWh — almost double the physical size. To convert kWh to amp-hours, divide by system voltage: 66.7 kWh × 1,000 ÷ 48V = 1,389 Ah for a 48V system. Battery capacity is also affected by temperature — lead-acid loses significant capacity below 0°C while LiFePO4 maintains better cold-weather performance.
What is the US federal solar tax credit and how does it work?
The Investment Tax Credit (ITC) allows US homeowners and businesses to deduct 30% of the total installed solar system cost from their federal income taxes. It applies to equipment and installation costs. For example, a $15,000 system qualifies for a $4,500 credit, reducing your net cost to $10,500. The 30% rate is guaranteed through 2032, then steps down to 26% in 2033 and 22% in 2034. This is a dollar-for-dollar credit against your tax liability, not a deduction, making it highly valuable. Many states also offer additional incentives on top of the federal ITC. Always consult a tax professional for guidance on your specific situation.
What are peak sun hours and how do they affect my system size?
Peak sun hours are not simply daylight hours — they represent the cumulative daily solar energy available at a location expressed as equivalent hours of full-strength irradiance (1,000 W/m²). A location with 5 peak sun hours receives the same energy as 5 hours of peak noon sun regardless of the actual hours of daylight. Arizona averages 6.5 peak sun hours while Washington state averages 3.5. Doubling your peak sun hours roughly halves the number of panels needed for the same daily energy production. The built-in state lookup table uses NREL/PVGIS-derived averages for the continental US. Actual values vary by season, roof orientation, and local shading conditions.
How accurate is the 25-year payback projection?
The projection is a useful planning estimate but carries several simplifying assumptions: annual electricity rate increases uniformly at the entered percentage (historically ~3% in the US), solar output remains constant each year (in reality panels degrade ~0.5% per year), and the net system cost is fixed after the federal tax credit. Real-world factors that can improve the projection include state tax credits and rebates, net metering income from excess solar exported to the grid, and rising electricity rates faster than assumed. Factors that can worsen it include shading, soiling, inverter replacement costs (~$1,500–$3,000 after 10–15 years), and lower electricity rates than projected. Despite these simplifications, the calculator provides a solid baseline for comparing solar to other investments.