Solar Panel Calculator
Wondering how many solar panels your home needs? Enter your monthly electricity bill, local rate, and average sun hours to get a quick estimate of system size and potential savings.
How Solar Panel Sizing Works
Solar panel sizing starts with your electricity consumption. Your monthly electric bill divided by your utility rate gives you monthly kWh usage. Divide that by 30 to get daily needs.
Next, divide daily kWh by your area's peak sun hours. This gives you the system size in kilowatts needed to offset your usage. The Southwest US gets about 6 peak sun hours daily, while the Northeast averages closer to 4.
Finally, divide the system size in watts by individual panel wattage to get the number of panels. Modern residential panels typically range from 350W to 450W. Higher-wattage panels mean fewer panels for the same output, which matters when roof space is limited.
Understanding Solar Savings
Solar savings depend on three main factors: how much electricity you use, what you pay per kWh, and how much sun your location gets. Areas with high electricity rates and abundant sunshine see the fastest payback periods.
The federal Investment Tax Credit (ITC) currently covers 30% of system costs through 2032. Many states offer additional rebates, performance incentives, or favorable net metering policies that credit you for excess power sent to the grid.
Most homeowners see payback periods of 6-10 years, after which the electricity is essentially free for the remaining 15-20 years of the system's life. In high-rate areas like California or Hawaii, payback can be as fast as 4-5 years.
Factors That Affect Real-World Performance
This calculator provides estimates based on ideal conditions. Several factors cause real-world production to differ. Panel orientation matters: south-facing roofs in the Northern Hemisphere perform best. East or west-facing panels produce about 15-20% less.
Roof pitch affects how directly panels face the sun. The ideal angle roughly equals your latitude. A flat roof or steep pitch reduces output somewhat compared to the optimal angle.
Temperature also plays a role. Solar panels actually perform slightly worse in extreme heat. High desert areas get lots of sun but lose some efficiency on the hottest days. Snow coverage temporarily blocks production in winter but can increase it through ground reflection on clear days after snowfall.
Frequently Asked Questions
How many solar panels does a typical home need?
The average US home uses about 900 kWh per month and needs a 6-8 kW system, which translates to roughly 15-20 panels of 400W each. Your actual number depends on your electricity usage and local sun hours.
What are peak sun hours?
Peak sun hours measure how many hours per day the sun delivers full-strength irradiance (1,000 W/m²). The US averages 4-6 peak sun hours, with the Southwest getting more and the Northeast getting less.
How much do solar panels cost?
As of 2024, residential solar costs average $2.50-$3.50 per watt before incentives. A 7 kW system costs roughly $17,500-$24,500 before the 30% federal tax credit.
How long do solar panels last?
Modern solar panels are warrantied for 25-30 years and can produce electricity well beyond that. They typically degrade about 0.5% per year, meaning they still produce around 87% of original capacity at year 25.
Does this account for weather and shading?
This is a simplified estimate based on average peak sun hours. Real-world production varies with weather, panel orientation, roof pitch, shading, and temperature. A professional site assessment gives more precise numbers.