Solar Panel Output Calculator: Daily, Monthly, and Annual kWh by State

To size a solar system for your home: start with your annual electricity consumption in kWh (12 months of electric bills added together, or monthly average x 12). Divide by your state’s annual peak sun hours x 365 x 0.80 system efficiency to get the system size in kW. Divide by your panel wattage to get the number of panels. Example: a home using 12,000 kWh per year in North Carolina (5.0 PSH): 12,000 / (5.0 x 365 x 0.80) = 12,000 / 1,460 = 8.2 kW system. At 400W panels: 8.2 kW / 0.4 kW = 20.5, or 21 panels. Most US homes use 15 to 30 solar panels for a 6 to 12 kW system that covers 80 to 100 percent of their electricity consumption. The exact panel count depends on your roof area, roof orientation, and whether partial or full offset is the goal.

Peak sun hours are a measure of the total solar energy available at a location, expressed in terms of equivalent hours at 1,000 watts per square meter (the standard test condition for solar panel rating). One peak sun hour equals 1 kWh/m2 of solar irradiance. A location with 5.0 peak sun hours per day receives the equivalent of 5 hours of full-intensity sunlight, even though the sun may be above the horizon for 10 to 14 hours. Early morning and late afternoon sun counts for less than the midday sun because the angle is lower and the irradiance is below 1,000 W/m2. The NREL measures peak sun hours through the National Solar Radiation Database (NSRDB) using satellite data and ground measurements accumulated over decades, giving highly accurate annual averages for every US location. The NREL PVWatts tool is the authoritative source for site-specific peak sun hours that accounts for your exact latitude and local cloud patterns.

Solar output is lower in winter for two reasons: shorter days mean fewer hours of sun, and the sun’s lower position in the sky means its rays travel through more atmosphere and hit a tilted roof at a less favorable angle. In June, most US locations receive 30 to 50 percent more solar energy per day than they do in December. This seasonal variation is most pronounced at higher latitudes (Minnesota, Michigan, New England) where the difference between summer and winter sun angles is largest. In the Southwest (Arizona, New Mexico), the seasonal variation is smaller because the sun angle changes less dramatically between summer and winter. The monthly breakdown table in this calculator uses a standard US-average seasonal variation curve. Your actual variation will be slightly different based on your local latitude and weather patterns, but the curve is representative for planning purposes for most continental US locations.

Solar system efficiency (also called the system derate factor or performance ratio) accounts for all the real-world losses between what solar panels are rated to produce under standard test conditions and what actually arrives at your electrical panel. The main factors: inverter efficiency (string inverters typically run 96 to 98% efficient; microinverters similar); wiring and connection losses (typically 1 to 3%); temperature derate (panels lose approximately 0.4 to 0.5% output per degree Celsius above 25C, meaning a panel on a hot Arizona rooftop at 60C loses about 14% output due to heat alone); soiling from dust, pollen, and bird droppings (1 to 5% depending on cleaning frequency and local conditions); partial shading (can dramatically reduce output in string-inverter systems where one shaded panel reduces the output of the entire string); and module mismatch (when panels in a string have slightly different characteristics, the string performs to the lowest-performing panel). The standard derate factor recommended by NREL PVWatts is 0.86 for optimal conditions and 0.80 for typical installations. Systems with significant shading or minimal maintenance may run at 0.70 to 0.75.

A 10kW solar system’s daily output depends on location and system efficiency. At 80% efficiency: Arizona (6.5 PSH): 10 x 6.5 x 0.80 = 52.0 kWh per day, approximately 18,980 kWh per year. Texas (4.9 PSH): 10 x 4.9 x 0.80 = 39.2 kWh per day, approximately 14,308 kWh per year. Florida (5.2 PSH): 10 x 5.2 x 0.80 = 41.6 kWh per day, approximately 15,184 kWh per year. California (5.5 PSH): 10 x 5.5 x 0.80 = 44.0 kWh per day, approximately 16,060 kWh per year. New York (4.0 PSH): 10 x 4.0 x 0.80 = 32.0 kWh per day, approximately 11,680 kWh per year. Washington State (3.5 PSH): 10 x 3.5 x 0.80 = 28.0 kWh per day, approximately 10,220 kWh per year. The US average household uses approximately 10,500 kWh per year, meaning a 10kW system in most US states (outside the Pacific Northwest) would produce more than enough to offset the average American home’s annual electricity consumption.

Yes, particularly for traditional string inverter systems. In a string inverter configuration, solar panels are wired in series (like a chain), and the output of the entire string is limited by the lowest-performing panel. If one panel is shaded by a tree branch, chimney, or rooftop HVAC unit, the output of every other panel in that string is pulled down to match the shaded panel’s reduced output. A single panel shaded to 30% of its normal output can reduce the entire string’s output by 50 to 70%. Microinverters and DC power optimizers solve this by allowing each panel to operate independently at its maximum power point, so one shaded panel only reduces its own output without dragging down the rest of the system. For rooftops with any shading, microinverters or power optimizers are strongly recommended and will pay for their 15 to 20 percent cost premium through higher production. When calculating output for a shaded roof without individual panel optimizers, reduce the system efficiency input in this calculator to 0.70 to 0.75 to account for string-level shading losses.

In the continental United States (which is in the northern hemisphere), solar panels face south for maximum annual output. True south (not magnetic south) is the optimal orientation, producing approximately 5 to 10 percent more annual energy than southeast or southwest-facing panels. East-facing panels produce slightly more output in the morning and west-facing panels produce more in the afternoon; if your utility has time-of-use rates where afternoon or evening power is more valuable, west-facing panels may produce more financial value per kWh even though they generate fewer total kWh annually. For rooftops that cannot accommodate south-facing panels, southeast and southwest orientations lose only 5 to 10 percent of annual production compared to due south. East-facing and west-facing panels lose 15 to 20 percent. North-facing panels are generally not recommended in the US but may be used in some commercial applications where shading concerns, roof space maximization, or building aesthetics require it. The optimal tilt angle for maximum annual output equals approximately your latitude: 33 degrees for Atlanta (Lat 33N), 40 degrees for Denver (Lat 40N), 47 degrees for Seattle (Lat 47N). Flat roofs can use adjustable mounting systems to achieve optimal tilt regardless of roof angle.

Solar panels are rated at 25 degrees Celsius (77 degrees Fahrenheit). In real-world operation, panels mounted on a dark rooftop in summer sun can reach 55 to 70 degrees Celsius (131 to 158 degrees Fahrenheit), which significantly exceeds the test temperature and causes performance degradation. Most crystalline silicon panels have a temperature coefficient of approximately -0.4% per degree Celsius above 25C. A panel operating at 60C (35 degrees above test temperature) loses approximately 0.4% x 35 = 14% of its rated output due to temperature alone. This is the temperature derate factor included in the system efficiency calculation, and it is why summer production in very hot climates (Phoenix in July) can be lower than expected despite the high solar irradiance available. In cooler climates (Pacific Northwest, New England winters), panels actually outperform their rated output on cold clear days, partially compensating for the lower solar resource. The performance impact of temperature explains why some solar performance data shows Arizona producing less than Nevada despite having higher annual solar irradiance: Phoenix’s extreme summer heat suppresses efficiency when irradiance is highest.

A standard 400-watt solar panel measures approximately 65 inches (5.4 feet) by 39 inches (3.25 feet), giving a panel footprint of about 17.6 square feet. In a rooftop installation with appropriate spacing between panels (for maintenance access, fire code setbacks, and mounting hardware), plan for approximately 20 square feet of usable roof area per panel. For a 20-panel, 8kW system: 20 x 20 = 400 square feet. For a 25-panel, 10kW system: 25 x 20 = 500 square feet. These are net usable panel areas; the total roof section used will be somewhat larger due to setback requirements from roof edges, ridges, and valleys. Most residential rooftops in the US have between 1,000 and 2,500 square feet of total roof area, but not all of it is suitable for solar: setbacks from the roof edge typically require a 3-foot clearance on most sides, north-facing sections are excluded in many designs, and roof penetrations (HVAC, vents, skylights) reduce available space. A solar installer will conduct a site assessment to determine exactly how many panels your specific roof can accommodate in a code-compliant layout.

Yes, solar panels produce electricity on cloudy days, but at significantly reduced output compared to clear sunny days. On a heavily overcast day, a solar panel may produce 10 to 25 percent of its clear-sky rated output. On a partly cloudy day, output fluctuates as clouds pass, typically averaging 50 to 70 percent of clear-sky output over the day. This reduced cloudy-day production is already captured in the peak sun hour calculation, which averages irradiance across all days in the year including cloudy days. Seattle’s 3.5 average peak sun hours per day accounts for its high frequency of overcast days; the figure is not the number of sunny days but the total annual solar energy divided by 365 and expressed as equivalent peak irradiance hours. UK, Germany, and the Netherlands all have thriving solar markets with peak sun hours well below 4.0, demonstrating that cloudy climates can still make effective use of solar power when electricity rates are high enough to justify the investment.

The Residential Clean Energy Credit (formerly called the Investment Tax Credit or ITC) allows US homeowners who install solar panels to deduct 30 percent of the total system cost from their federal income taxes. The credit applies to the total installed cost including equipment, labor, wiring, and the inverter. For a $25,000 solar system installation, the 30% credit equals $7,500 off your federal tax bill (not as a deduction from income, but directly from taxes owed). The 30% rate is set through 2032, then steps down to 26% in 2033 and 22% in 2034 under current law. The credit is non-refundable (it reduces your taxes but you cannot receive more back than you owe in taxes for the year), but any unused credit can be carried forward to future tax years. Battery storage systems installed alongside solar also qualify for the 30% credit. Many states offer additional solar tax credits, rebates, or sales tax exemptions on top of the federal credit; the DSIRE database lists all current state and local solar incentives.

Net metering is a billing arrangement where homeowners with solar panels export excess electricity to the grid and receive a credit on their electric bill for that exported energy. The credit rate (what you receive per kWh of export) varies by utility and state. Most US net metering programs provide full retail rate credit (you export at the same rate you would pay to import: $0.16 per kWh exported = $0.16 of credit on your bill). Some utilities offer less than full retail rate; some states have shifted to avoided cost (the wholesale rate at which the utility can purchase power, typically $0.03 to $0.06 per kWh). Net metering does not affect the physical output calculation (your system produces the same kWh regardless of the billing arrangement) but significantly affects the financial value calculation. This calculator uses your entered electricity rate as the value of all production (both consumed on-site and exported). If your utility pays a lower rate for exported power, the actual financial benefit is lower than this calculator shows, because some of your production will be exported at a lower value than the import rate you save on consumption.

Solar panels produce direct current (DC) electricity. Your home runs on alternating current (AC). The inverter converts DC to AC. Solar panel wattage ratings (the 400W on the panel label) are DC watts, measured under standard test conditions (STC). The AC output from the inverter is lower than the DC panel rating by the inverter efficiency (typically 95 to 98% for a modern string or microinverter). When solar installers quote system size, they typically use the DC rating (the sum of all panel nameplate wattages), which is the industry standard in the US. The AC output you will actually see in your monitoring system and on your electric bill is the DC system size x inverter efficiency. This calculator uses DC panel wattage as the input (which is the standard) and accounts for inverter efficiency as part of the overall system efficiency factor (the 4% inverter loss included in the default 80% efficiency). If a solar installer quotes you a “10kW system” in the US, that is almost always the DC nameplate capacity of the panels; the AC output will be approximately 9.5 to 9.8 kW at full sun due to inverter conversion losses.

This calculator provides planning-level accuracy appropriate for initial system sizing and financial analysis. For a 10kW system in a moderate-resource US state, the annual output estimate will typically be within 10 to 15 percent of actual production. Sources of variation between this estimate and real-world production: state-average peak sun hours versus your specific location’s irradiance (use NREL PVWatts for site-specific data); roof orientation and tilt angle (this calculator assumes optimal south-facing tilt; east/west or flat-roof installations produce less); local shading from trees or buildings (not captured in the state-average calculation); specific equipment performance (high-efficiency panels and premium inverters outperform standard assumptions); and weather year variability (actual production varies 5 to 10% from the average year to year). For utility-scale or commercial projects, or any project where precise financial modeling is required, use NREL PVWatts with your exact site coordinates, or commission an energy production assessment from a qualified solar professional. This tool is designed for homeowner planning and comparison purposes, not for engineering specifications.

Modern solar panels from reputable manufacturers carry 25-year product warranties and linear performance warranties guaranteeing at least 80 to 90 percent of rated output at year 25. Solar panel degradation rate is typically 0.5 percent per year for monocrystalline and polycrystalline silicon panels made after 2000. At 0.5% per year, a panel retains approximately 90% of its original output after 20 years and 87.5% after 25 years. First-year degradation is typically higher (1 to 3%) due to a light-induced degradation (LID) phenomenon that occurs in the first few months of operation, then stabilizes at the lower long-term rate. This calculator uses the current nameplate wattage without applying degradation, representing expected output in the first year of operation. For a 25-year financial analysis, use an average of approximately 94 to 95% of the Year 1 output to account for the gradual degradation over the system lifetime. High-quality panels from manufacturers like LG, SunPower, Panasonic, and REC have demonstrated degradation rates of 0.3 to 0.4% per year in independent field studies, providing meaningfully higher long-term output than lower-cost alternatives.

Beyond the 30% federal Residential Clean Energy Credit, most US states offer additional solar incentives. The most comprehensive resource is the DSIRE database (Database of State Incentives for Renewables and Efficiency), maintained by the NC Clean Energy Technology Center, which lists every active federal, state, and local solar incentive and policy by state. Common types of state-level solar incentives include: state income tax credits (California, New York, South Carolina, and others offer state credits in addition to the federal credit); sales tax exemptions on solar equipment (many states exempt solar panel purchases from sales tax); property tax exemptions (preventing the added value of solar panels from increasing your property tax assessment, available in most states); net metering policies (requiring utilities to credit solar owners for exported electricity, strength varies by state); and utility rebates (some utilities and state programs offer upfront rebates per watt or per kW installed). State incentive programs change frequently; checking the DSIRE database for your specific state before commissioning a system is strongly recommended to capture all available incentives and accurately calculate your net system cost and payback period.