Understanding Solar Panel Systems
Solar photovoltaic (PV) systems convert sunlight directly into electricity using semiconductor cells. A residential solar system typically consists of solar panels mounted on the roof, an inverter to convert DC power to AC power, mounting hardware, wiring, and a metering system. The size of the system you need depends primarily on how much electricity you use and how much sunlight your location receives.
Solar panels are rated in watts (W) under standard test conditions. Common residential panels in 2026 range from 350W to 450W per panel. The total system size is measured in kilowatts (kW)—for example, 20 panels at 400W each equals an 8 kW system. Your actual energy production depends on peak sun hours in your area, which varies from about 3 hours per day in the Pacific Northwest to 6+ hours per day in the Southwest.
This calculator helps you size your system based on your actual electricity usage, estimate the number of panels needed, check if they fit on your available roof space, calculate total cost after incentives, and project your return on investment over the 25-year expected lifespan of the panels.
How to Use This Calculator
Start by entering your average monthly electricity usage in kilowatt-hours (kWh). You can find this on your utility bill. Enter your current electricity rate per kWh and select the panel wattage you are considering. Look up the peak sun hours for your location—the NREL PVWatts calculator or solar irradiance maps can help with this.
Enter your available south-facing roof area in square feet, and adjust the system loss percentage (14% is a good default for most installations). On the cost side, enter the panel cost and installation cost per watt, plus your expected incentive percentage (30% for the federal ITC in the US). The calculator computes everything instantly: system size, panel count, roof fit, total cost, annual savings, payback period, and 25-year net savings with electricity rate escalation.
Solar Cost and ROI Factors
The total cost of a residential solar system includes panels, inverters, mounting hardware, wiring, permits, and labor. In 2026, the average all-in cost is roughly $2.25–$3.00 per watt before incentives. The federal Investment Tax Credit (ITC) reduces the cost by 30%, and many states offer additional rebates or incentives that can lower the net cost further.
Return on investment depends heavily on your local electricity rate. Areas with high electricity costs (like California, Hawaii, or the Northeast) see faster payback periods. The calculator also models electricity rate increases over time—historically, US electricity rates have risen about 2–4% per year, which makes solar increasingly valuable as your alternative cost goes up while your solar production stays essentially free.
Panel Degradation and Long-Term Production
Solar panels degrade slowly over time, typically losing about 0.5% of their output per year. This means that after 25 years, your panels will still produce approximately 87.5% of their original output. This calculator accounts for this degradation in the 25-year savings projection, giving you a more realistic long-term estimate.
Most panel manufacturers warrant at least 80% output at 25 years, and many modern panels exceed this guarantee. Inverters typically last 12–15 years and may need replacement during the panel lifetime. Budget for one inverter replacement when planning long-term costs. String inverters cost $1,000–$2,000, while microinverters last longer and are warranted for 25 years.
Frequently Asked Questions
How many solar panels do I need for my home?
The number depends on your monthly electricity usage, panel wattage, and local sun hours. A typical US home using 900 kWh/month needs about 20–25 panels of 400W each. Enter your specific numbers in this calculator for an accurate estimate.
What is the average payback period for solar panels?
Most residential solar systems pay for themselves in 6–10 years, depending on electricity rates, sun exposure, system cost, and available incentives. After payback, you essentially get free electricity for the remaining 15–20 years of panel life.
What is system loss in solar panels?
System loss accounts for energy lost due to inverter conversion, wiring, shading, dust, temperature effects, and panel degradation. A typical system loss of 12–16% is normal. Higher shading or older equipment increases losses.
What incentives are available for solar installation?
The US federal Investment Tax Credit (ITC) currently offers 30% of system cost. Many states, utilities, and local governments offer additional rebates, credits, or net metering programs. Check DSIRE (dsireusa.org) for your area.
How long do solar panels last?
Most solar panels are warranted for 25 years and can produce electricity for 30–40 years. Panels typically degrade about 0.5% per year, meaning they still produce about 87.5% of their original output at year 25.
Does roof size matter for solar?
Yes. Each standard panel needs about 17.5 square feet of unobstructed roof space. South-facing roofs with minimal shading are ideal. This calculator checks whether your needed panels fit on your available roof area.
Privacy and methodology
This calculator runs entirely in your browser. No data is sent to any server. It uses standard solar sizing formulas based on daily energy usage, peak sun hours, and system losses to determine system size. Cost calculations apply per-watt pricing with incentive percentages. The 25-year projection accounts for annual panel degradation of 0.5% and user-specified electricity rate escalation.