Calculate daily and annual energy output from solar panels based on wattage, peak sun hours, and system derate factor. Free online solar production estimator.
Understanding how much energy a solar panel produces each day is the first step toward sizing a system and estimating savings. A panel's rated wattage tells you its maximum output under ideal lab conditions, but real-world production depends on how many hours of strong sunlight your location receives and how much is lost to system inefficiencies like wiring, inverter conversion, shading, and temperature.
The standard metric for local sunlight is peak sun hours (PSH) — the number of hours per day your roof receives the equivalent of 1,000 watts per square meter. A location with 5 PSH receives less total energy than one with 6 PSH, directly reducing panel output. The derate factor (typically 0.75–0.85) accounts for all real-world losses between the panel and your meter.
This calculator multiplies your panel's wattage by peak sun hours and the derate factor to give you a realistic daily kWh estimate. Multiply by 365 to project annual output. Use these numbers to decide how many panels you need and what savings to expect on your electricity bill.
Knowing your panel's realistic output prevents over- or under-sizing your solar system. Overestimating production leads to disappointment and unmet savings goals, while underestimating means buying more panels than necessary. This calculator gives you a grounded estimate using your actual location's sunlight data and standard industry derate factors. Having accurate metrics readily available streamlines utility bill analysis, budget forecasting, and investment planning for energy efficiency projects and renewable energy installations.
kWh/day (per panel) = Panel Watts × Peak Sun Hours × Derate Factor / 1,000 kWh/day (system) = kWh/day per panel × Number of Panels kWh/year = kWh/day (system) × 365
Result: 1.60 kWh/day per panel, 11,680 kWh/year total
A 400 W panel in a location with 5 peak sun hours at a 0.80 derate factor produces 400 × 5 × 0.80 / 1,000 = 1.60 kWh per day. With 20 panels, daily system output is 32.0 kWh, and annual output is 32.0 × 365 = 11,680 kWh. This is enough to offset the average U.S. household's electricity consumption of about 10,500 kWh/year.
Solar panels are rated in watts under Standard Test Conditions (STC), meaning 1,000 W/m² irradiance at 25°C. A 400W panel can produce 400 watts at that instant under those exact conditions. In practice, your roof conditions differ, so real output is lower. The rated wattage is still the best starting point for comparisons and sizing calculations.
The southwestern U.S. averages 6–7 peak sun hours, while the Pacific Northwest and Northeast average 3.5–4.5. Hawaii often exceeds 5.5. International locations vary similarly — Australia's outback rivals Arizona, while Northern Europe is comparable to the Pacific Northwest. Always use local data rather than national averages.
Panel orientation and tilt angle significantly affect production. In the Northern Hemisphere, south-facing panels tilted at your latitude angle capture the most annual energy. Microinverters or power optimizers can mitigate partial shading losses that affect string inverter systems. Regular cleaning in dusty areas can recover 2–5% of lost output.
Once you know your daily kWh output, multiply by your electricity rate to estimate daily savings. Factor in net metering policies, time-of-use rates, and seasonal variations for a complete financial picture. Our Solar Savings and Solar Payback calculators can help with the financial analysis.
Peak sun hours represent the equivalent number of hours per day when solar irradiance averages 1,000 W/m². A location receiving 6 peak sun hours gets enough total energy to equal 6 hours of full noon-strength sunlight. This metric simplifies output calculations by normalizing varying sunlight intensity throughout the day.
The derate factor accounts for real-world energy losses in a solar system, including inverter efficiency, wiring losses, soiling, shading, temperature effects, and panel mismatch. A factor of 0.80 means you capture 80% of the panel's theoretical maximum output. The NREL default is 0.86 for well-installed systems.
A 400W panel typically produces 1.2 to 2.4 kWh per day depending on location. In a place with 5 peak sun hours and a 0.80 derate, it produces 1.60 kWh/day. In Phoenix with 6.5 PSH, it might produce over 2 kWh/day.
Yes. Winter brings fewer peak sun hours, lower sun angles, and shorter days. Output can drop 40–60% compared to summer in northern latitudes. However, cooler temperatures slightly improve panel efficiency, partially offsetting the lower irradiance.
This calculator provides a solid estimate using the same method professional installers use for initial sizing. For detailed accuracy including roof angle, shading analysis, and local weather patterns, use NREL's PVWatts or get a professional site assessment.
Yes, but production drops significantly. On overcast days, panels may produce only 10–25% of their rated output. Peak sun hours already account for average cloud cover, so using annual PSH values gives you a realistic year-round estimate.
Panel wattage (like 400W) is the maximum output under Standard Test Conditions (STC): 1,000 W/m² irradiance, 25°C cell temperature, and AM 1.5 spectrum. Real-world output is always lower due to temperature, angle, shading, and system losses, which the derate factor captures.
Divide your annual electricity usage (from your utility bill) by the annual output per panel. For example, if you use 10,000 kWh/year and each panel produces 550 kWh/year, you need about 18–19 panels. Use our Solar Panel Count calculator for a quick estimate.