Solar Panel Output Calculator

About the Solar Panel Output Calculator

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.

When This Page Helps

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.

How to Use the Inputs

1. Enter the wattage of a single solar panel (check the panel's spec sheet, e.g. 400 W).
2. Enter your location's peak sun hours per day (check NREL's PVWatts or your installer's data).
3. Enter the system derate factor (default 0.80 covers typical losses).
4. Enter the number of panels if you want total system output.
5. Review daily kWh output per panel and for the full system.
6. Check the annual output to compare against your electricity usage.

Example Calculation

Tips & Best Practices

• Peak sun hours vary by season — use an annual average for planning, but expect higher output in summer and lower in winter.
• The derate factor typically ranges from 0.75 to 0.85; use 0.80 as a conservative default.
• South-facing roofs in the Northern Hemisphere receive the most sunlight; east/west facing roofs lose 10–20%.
• Shading from trees or chimneys can dramatically reduce output — even partial shade on one panel affects the whole string.
• Panel output degrades about 0.5% per year, so year-20 output is roughly 90% of year-1.
• Higher-efficiency panels (21%+) produce more watts per square foot, not more watts per watt rating.
• Check NREL's PVWatts tool for location-specific peak sun hours and production estimates.

Understanding Solar Panel Ratings

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.

Location Matters: Peak Sun Hours by Region

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.

Maximizing Your Panel Output

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.

From Output to Savings

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.

Frequently Asked Questions

• What are peak sun hours?

  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.

• What is the derate factor?

  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.

• How much electricity does a 400W solar panel produce per day?

  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.

• Does panel output decrease in winter?

  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.

• How accurate is this calculator?

  It gives 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.

• Do solar panels produce electricity on cloudy days?

  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.

• What's the difference between panel wattage and actual output?

  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.

• How many solar panels do I need for my home?

  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.