Calculating solar panel output takes one formula and three numbers: your panel’s rated wattage, your local peak sun hours, and a loss factor. The wattage printed on a panel is a lab result, not a promise, so your roof, shade, climate, and utility rate all rewrite that number before it reaches your bill. This guide shows you how to compute solar panel output for a single panel, a full system, and a full year, then turns that output into solar savings using current California sun hours and utility pricing.
Quick Answer: The Solar Panel Output Formula
- Formula: Rated wattage × peak sun hours × derate factor = daily watt-hours
- Example: 400 W × 5 hours × 0.80 = 1,600 Wh, or 1.6 kWh per day
- Derate factor: use 0.75 to 0.85 to cover heat, wiring, inverter, and dust losses
- Whole system: use the total array wattage in the same formula
- Monthly and yearly: multiply the daily result by 30 or 365
The Solar Panel Output Formula, Explained
The Three Inputs
Every solar panel output calculation uses the same three inputs. Get each one right, and the result lands close to what your monitoring app will show.
| Input | What it means | Typical value |
|---|---|---|
| Rated wattage | The panel’s maximum power under lab test conditions (STC) | 350 to 450 W for current residential panels |
| Peak sun hours | Hours per day of full-strength sunlight (1,000 W per square meter). This is not the same as daylight hours | About 4 to 6 hours across the U.S. |
| Derate factor | The share of rated power you keep after real-world losses | 0.75 to 0.85 |
Watts vs. Kilowatt-Hours
Mixing up power and energy is the most common mistake in any solar panel calculation. Your utility bills you in kilowatt-hours, so kWh is the number that matters for money.
| Unit | What it measures | Example |
|---|---|---|
| Watt (W) | Power at one moment | A 400 W panel |
| Kilowatt (kW) | 1,000 watts, used for system size | An 8 kW system (20 panels × 400 W) |
| Watt-hour (Wh) | Energy produced over time | 1,600 Wh per panel per day |
| Kilowatt-hour (kWh) | 1,000 Wh, the unit on your utility bill | 1.6 kWh per panel per day |
STC vs. PTC: Which Rating Is Your Quote Using?
Manufacturers publish Standard Test Conditions (STC) wattage, which is the number on the panel label. Some installers also cite PVUSA Test Conditions (PTC), which simulates a more realistic operating temperature. If a quote projects your production from STC alone, ask which figure was used, because the gap can shift your savings estimate.
| Rating | Conditions | What to expect |
|---|---|---|
| STC | Lab test at 77°F (25°C) cell temperature, full overhead sun, no shade | The highest number, printed on the panel |
| PTC | Closer to real operating temperature | Typically 10% to 15% lower than STC |
How to Compute Solar Panel Output in 4 Steps
- Find your panel’s rated wattage. Check the label on the back of the panel near the junction box, or look up the model number on the manufacturer’s datasheet. If you only have voltage and current, multiply them: a panel with a maximum power voltage of 37.0 V and a maximum power current of 10.8 A rates at about 400 W.
- Find your peak sun hours. Enter your address in PVWatts, the free calculator from the National Laboratory of the Rockies (NLR, formerly NREL). Its solar radiation figure in kWh per square meter per day is the same number as your peak sun hours.
- Choose a derate factor. Use 0.80 as a planning default. Drop to 0.75 for a hot, shaded, or older roof. Move up to 0.85 for a clean, cool, unshaded, well-oriented one.
- Multiply, then scale. Multiply the three numbers for one panel per day, then multiply by panel count, days, or months to get system, monthly, and annual output.
Worked Example: A 400 W Qcells Panel in Los Angeles County
SCE territory in Los Angeles County averages roughly 5.3 peak sun hours per day. Using a 0.80 derate factor:
Output = 400 W × 5.3 hours × 0.80 = 1,696 Wh, or about 1.7 kWh per day
| Scale | Calculation | Result |
|---|---|---|
| One panel, per day | 400 × 5.3 × 0.80 | 1,696 Wh (1.7 kWh) |
| One panel, per month | 1.696 kWh × 30 | About 51 kWh |
| One panel, per year | 1.696 kWh × 365 | About 619 kWh |
| 20-panel, 8 kW system, per day | 1.696 kWh × 20 | About 34 kWh |
| 20-panel, 8 kW system, per month | 33.9 kWh × 30 | About 1,020 kWh |
| 20-panel, 8 kW system, per year | 33.9 kWh × 365 | About 12,380 kWh |
These are annual averages. Winter months produce noticeably less than summer, and our month-by-month solar savings chart shows what that swing looks like on a real bill.
Solar Panel Output by Wattage (Quick Reference)
Use this table to skip the math for common panel sizes. It applies a 0.80 derate factor to three peak sun hour levels, in kWh per panel per day.
| Panel rating | 4 peak sun hours | 5 peak sun hours | 6 peak sun hours |
|---|---|---|---|
| 300 W | 0.96 kWh | 1.20 kWh | 1.44 kWh |
| 350 W | 1.12 kWh | 1.40 kWh | 1.68 kWh |
| 400 W | 1.28 kWh | 1.60 kWh | 1.92 kWh |
| 430 W | 1.38 kWh | 1.72 kWh | 2.06 kWh |
| 450 W | 1.44 kWh | 1.80 kWh | 2.16 kWh |
Want to know whether your own system is producing a normal amount? Compare it against our guide to average solar panel output benchmarks by system size and location.
How Your Location Changes the Result
The same panel produces different output in different cities because peak sun hours change. A quote built on a national average, instead of your ZIP code, can leave you with a system that is undersized or oversized for your home.
| Location | Planning value for peak sun hours | 400 W panel, per day (0.80 derate) |
|---|---|---|
| Los Angeles County, CA (SCE) | About 5.3 | About 1.7 kWh |
| Houston, TX | About 5.0 to 6.0, depending on season | About 1.6 to 1.9 kWh |
| Chicago, IL | About 3.8 to 4.2 | About 1.2 to 1.3 kWh |
Treat these as planning values, then confirm yours in PVWatts. Texas usually has lower electricity rates per kWh than California, so more output does not always mean more dollars saved.
What Reduces Real-World Output (and Sets Your Derate Factor)
The derate factor is a bundle of small losses. Knowing what is inside it helps you decide whether to use 0.75, 0.80, or 0.85 for your own roof.
| Loss | Typical impact | What causes it |
|---|---|---|
| Inverter conversion | About 3% to 5% | Converting DC panel power to AC household power |
| Temperature | Up to about 12% on the hottest afternoons | Panels lose output as they heat above 77°F |
| Soiling | About 2%, more in dusty or pollen-heavy areas | Dust, leaves, and debris on the glass |
| Wiring and connections | About 2% | Resistance in cables |
| Panel mismatch | About 2% | Small output differences between panels in a string |
| Orientation and tilt | 10% to 20% for east or west roofs | Compared with a south-facing roof at a moderate tilt |
| Shading | Site-specific, can be large | Trees, chimneys, vents, and neighboring roofs |
PVWatts starts with roughly 14% total system losses plus an inverter efficiency near 96%. Multiply those together, and you land near 0.83, which is why 0.75 to 0.85 is the accepted range, and 0.80 is a safe planning number.
Temperature Loss: A Quick Calculation
Every panel has a temperature coefficient that shows how much power it loses per degree Celsius above 25°C (77°F). Qcells panels carry a coefficient of -0.34% per degree Celsius.
- Cell temperature on a hot Los Angeles roof: about 60°C (140°F)
- Degrees above the test condition: 60 – 25 = 35°C
- Power loss at that moment: 35 × 0.34% = about 11.9%
That loss only applies during the hottest hours, so it averages out over a full year, but it is one reason a lower temperature coefficient matters in hot climates.
Degradation: Output in Year 10 and Year 25
Most panels lose roughly 0.5% of their output per year as materials age. A calculation that only shows first-year production is telling you part of the story.
| Panel age | Output vs. year 1 | 400 W panel in Los Angeles County, per day |
|---|---|---|
| Year 1 | 100% | 1.70 kWh |
| Year 10 | About 95.5% | About 1.62 kWh |
| Year 25 | About 88% | About 1.49 kWh |
You can compare specific warranty terms and degradation caps in our Qcells panel specifications review.
From Solar Panel Output to Solar Savings
Output is measured in kWh, but savings depend on what each kWh is worth. Multiply your output by the rate you would otherwise pay. At SCE’s current average residential rate of 34.5 cents per kWh, the 8 kW example above (about 1,020 kWh per month) carries roughly $350 of monthly electricity value.
That figure is the ceiling, not the guarantee. Only the kWh you use or store are worth your full retail rate.
| Where the kWh goes | What it is worth |
|---|---|
| Used in your home as it is produced | Your full retail rate |
| Stored in a battery and used in the evening | Your full retail rate, often at a peak-hour price |
| Exported to the grid under NEM 3.0 | A much lower export credit |
That difference is why your output number and your savings number can tell different stories. See how NEM 3.0 changes solar savings for the export side, and use the solar panel payback period guide to turn yearly savings into a break-even timeline.
Tools That Do the Math for You
You do not have to run this formula by hand every time. Here is when each option makes sense.
| Tool | Best for | Limits |
|---|---|---|
| Manual formula | Fast sanity check on a quote, panel by panel | Ignores site-specific shading and hourly weather |
| PVWatts | Free, address-level monthly and annual production | Estimates kWh only, not dollars. See how accurate PVWatts really is |
| Axia Estimate solar savings calculator | Turning output into dollars using your address, roof orientation, and utility rate | An estimate, not a firm quote |
| Installer proposal | A design built around your exact roof | Check which rating (STC or PTC) and derate factor it assumes |
If you want a projection built around your actual home instead of a rough national average, get a free solar estimate in a few minutes.
Common Mistakes When Calculating Solar Panel Output
- Using daylight hours instead of peak sun hours. Ten hours of daylight is not ten hours of full-strength sun.
- Skipping the derate factor. Watts × hours alone overstates output by 15% to 25%.
- Confusing kW and kWh. One measures power, the other measures energy.
- Using STC wattage in a hot climate without adjusting. Heat pulls real output below the label.
- Judging a 25-year system by year-one output. Build degradation into the projection.
- Using a national sun-hour average. Use your own ZIP code or address.
What Axia Estimate Does Differently on Every Install
Once you know how to estimate output yourself, it is easier to spot which installers give honest projections and which round up.
Qcells Panels Built for California Conditions
As an exclusive Qcells partner, Axia Estimate uses American-made panels backed by a 25-year comprehensive warranty covering the panels, workmanship, and performance. Their degradation rate and temperature coefficient both feed directly into the output math above.
Installers Who Explain the Math, Not Just the Sale
Axia Estimate’s CSLB-licensed consultants walk through real output projections during a free consultation, and every quote includes a written projection tied to your specific roof, not a generic per-panel estimate from a brochure. Read Axia by Qcells customer reviews from homeowners who compared projected output to what actually showed up on their app. With 180+ five-star Google reviews, the pattern homeowners cite most is that the production numbers they were quoted matched reality.
Beyond the Panel: Batteries, Shade, and ADUs
Panel output is only part of the picture once you factor in how you will use the electricity.
- Batteries: Under NEM 3.0, using your own solar in the evening is worth far more than exporting it at midday. Sizing storage around your calculated daily output often does more for your bill than adding panels. Try the battery storage and EV charging estimator.
- Roof and shade: A roof near the end of its life can force a costly panel removal later. Check whether your roof is ready for solar before you finalize a design.
- Small roofs and ADUs: Output per square foot matters more than total wattage. Higher-efficiency panels produce more energy in less space, which is especially relevant for ADU solar requirements in Los Angeles.
Know Your Numbers Before You Sign Anything
Calculating solar panel output is not complicated once you know the formula, but it is the detail most quotes gloss over. With utility rates continuing to climb across California, Texas, Florida, and Illinois, the gap between a rough estimate and a site-specific projection can mean thousands of dollars over the life of your system. If you want that projection built around your actual roof and usage, you can go solar now and get a consultation based on real numbers.
Frequently Asked Questions About Calculating Solar Panel Output
How do I calculate my solar panel’s output?
Multiply the panel’s rated wattage by your location’s peak sun hours, then multiply by a derate factor between 0.75 and 0.85. A 400-watt panel with 5 peak sun hours and a 0.80 derate factor produces about 1,600 watt-hours, or 1.6 kWh, per day.
How do I compute the output of a whole solar system?
Add up the wattage of every panel to get total array wattage. Twenty 400-watt panels equal 8,000 watts, or 8 kW. Use that total in the same formula, then multiply the daily result by 30 for a monthly estimate or 365 for a yearly one. PVWatts can also show how production changes month by month.
What derate factor should I use to calculate solar panel output?
Use 0.80 as a planning default. Use 0.75 for a hot, shaded, or older roof and 0.85 for a clean, cool, well-oriented one. The range exists because inverter, wiring, heat, dust, and mismatch losses combine to remove roughly 15% to 25% of the rated output.
What are peak sun hours and how do I find mine?
Peak sun hours measure the daily equivalent of full-strength sunlight at your location, not total daylight hours. Enter your address in PVWatts and read the solar radiation value in kWh per square meter per day. Most of the U.S. falls between about 4 and 6 hours.
Why is my actual output lower than my panel’s rated wattage?
Rated wattage is measured under lab conditions that rarely occur on a rooftop. Heat, shading, dust, panel age, and inverter losses combine to reduce real-world output by roughly 10% to 25% compared with the STC label.
Does panel wattage matter more than efficiency when calculating output?
Not necessarily. A higher-wattage panel is not always more efficient, since wattage can reflect a larger panel. Efficiency determines how much output you get per square foot, which matters most when roof space is limited.
Does a battery change my solar output calculation?
A battery does not change how much your panels generate, but it changes how much of that output saves you money. Under current net billing rules, stored solar used during peak evening hours is worth significantly more than solar exported to the grid.