Every solar panel has a wattage on its label, usually around 400 watts. That number comes from a lab test, not from your roof. To calculate solar panel output you can trust, you need three inputs: the panel’s size in kilowatts, the peak sun hours at your address, and a loss factor for real-world conditions.
Quick answer
Daily output (kWh) = panel kW × peak sun hours × 0.80
Example in Los Angeles: 0.4 kW × 5.5 hours × 0.80 = 1.76 kWh per day, or about 640 kWh per year for one 400-watt panel.
What you will learn in this guide
- The solar panel output formula and a fully worked example
- Where the typical 15 to 20 percent of real-world losses comes from
- California peak sun hours by season and by region
- Expected output for common home system sizes in Los Angeles
- How roof direction, heat, shade, and NEM 3.0 change what you actually save
The Solar Panel Output Formula
The Formula in Plain Language
Solar panel output is the energy a panel delivers to your home, measured in kilowatt-hours (kWh). The formula uses three inputs.
| Input | What it means | Typical value in Los Angeles |
|---|---|---|
| Panel size (kW) | Panel wattage divided by 1,000 | 0.40 to 0.45 kW |
| Peak sun hours | Hours per day of full-strength sunlight, as a daily total | About 5.5 on an annual average |
| Loss factor | Share of power left after heat, dust, wiring, and inverter losses | 0.80 to 0.85 |
Worked Example: One 400 Watt Panel in Los Angeles
| Step | Calculation | Result |
|---|---|---|
| Convert watts to kilowatts | 400 W ÷ 1,000 | 0.4 kW |
| Multiply by peak sun hours | 0.4 kW × 5.5 hours | 2.20 kWh per day (before losses) |
| Apply the 0.80 loss factor | 2.20 × 0.80 | 1.76 kWh per day |
| Monthly output | 1.76 × 30 days | About 53 kWh |
| Yearly output | 1.76 × 365 days | About 640 kWh |
With a more efficient setup and a 0.85 loss factor, the same panel produces about 1.87 kWh per day, or roughly 680 kWh per year.
Why Real Solar Panel Output Is Lower Than the Label
Rated Wattage vs Real-World Output
Panel wattage is measured at 77 degrees Fahrenheit with sunlight hitting the panel at a perfect angle. Almost no roof matches those conditions for long. A 400-watt panel often delivers closer to 320 to 360 watts at a typical moment, which is why the label alone never predicts your real production.
Where the 15 to 20 Percent Loss Comes From
| Loss source | Typical range | What causes it |
|---|---|---|
| Heat | 5 to 10 percent | Panels lose output as cells get hotter than 77°F |
| Dust and soiling | 2 to 5 percent | Dirt, pollen, and ash block sunlight |
| Inverter conversion | 2 to 4 percent | Converting DC power to AC power for your home |
| Wiring | 1 to 2 percent | Electrical resistance in cables and connections |
| Panel mismatch | 1 to 2 percent | Small differences between panels on the same string |
| Shading | 0 to 5 percent or more | Trees, chimneys, vents, and nearby rooflines |
These losses do not all hit at their maximum at once. Combined, most well-designed systems land around 15 to 20 percent, which is why 0.80 to 0.85 is the standard loss factor.
How to Calculate Solar Panel Output for Your Home: 5 Steps
Step 1: Find Your Panel Wattage
Check the panel datasheet or your quote. Most modern residential panels are rated between 400 and 450 watts. Divide the wattage by 1,000 to get kilowatts.
Step 2: Look Up Peak Sun Hours for Your Address
Peak sun hours are not the same as daylight hours. They count the hours of full-strength sun your roof receives, added up across the day. Use the California table below, or enter your address in the free NREL PVWatts calculator for a location-specific figure.
Step 3: Apply Your Loss Factor
Use 0.80 for a conservative estimate and 0.85 for a clean, unshaded, well-designed system. Do not skip this step. Leaving out losses is the most common reason quoted output and real bills do not match.
Step 4: Multiply by the Number of Panels
Take the daily kWh for one panel and multiply it by the total number of panels on your roof. Twenty 400-watt panels at 1.76 kWh each produce about 35 kWh per day.
Step 5: Convert to Monthly and Yearly Output
Multiply daily output by 30 for a monthly estimate and by 365 for a yearly estimate. Remember that summer months run well above the average and winter months run below it.
California Peak Sun Hours and Seasonal Output
Peak Sun Hours in Los Angeles by Season
One annual average hides large seasonal swings. The table below shows approximate planning ranges and what one 400-watt panel produces at a 0.80 loss factor.
| Season | Peak sun hours per day | 400 W panel output per day |
|---|---|---|
| Winter (December to February) | 3.5 to 4.5 | 1.1 to 1.4 kWh |
| Spring (March to May) | 5.5 to 6.5 | 1.8 to 2.1 kWh |
| Summer (June to August) | 6.5 to 7 | 2.1 to 2.2 kWh |
| Fall (September to November) | 4.5 to 5.5 | 1.4 to 1.8 kWh |
| Annual average | About 5.5 | About 1.8 kWh |
These are planning ranges. Confirm the figures for your exact address with PVWatts before relying on them.
Coastal vs Inland Southern California
Where your home sits changes the numbers. Coastal areas get marine layer mornings, often called June gloom, while inland valleys and desert areas get more direct sun and more heat.
| Area | Examples | Typical yearly output per installed kW |
|---|---|---|
| Coastal | Santa Monica, Malibu, Long Beach | 1,450 to 1,600 kWh |
| Central LA and valleys | Encino, Pasadena, Burbank | 1,550 to 1,700 kWh |
| Inland and high desert | Palmdale, Lancaster, Inland Empire | 1,700 to 1,900 kWh |
A quick shortcut: multiply your system size in kW by the yearly figure for your area to estimate annual kWh.
Solar Panel Output by System Size in Los Angeles
Using 5.5 peak sun hours and a 0.80 loss factor, every installed kilowatt produces about 4.4 kWh per day. Here is what common system sizes produce with 400-watt panels.
| System size | Panels (400 W) | Daily output | Monthly output | Yearly output |
|---|---|---|---|---|
| 4 kW | 10 | 17.6 kWh | About 535 kWh | About 6,400 kWh |
| 6 kW | 15 | 26.4 kWh | About 800 kWh | About 9,600 kWh |
| 8 kW | 20 | 35.2 kWh | About 1,070 kWh | About 12,800 kWh |
| 10 kW | 25 | 44.0 kWh | About 1,340 kWh | About 16,100 kWh |
This table shows what a system produces. If you want to work backward from your electric bill to the right system size, read our guide on what size solar system you need.
What Changes Your Solar Panel Output
Roof Direction and Tilt
| Roof direction | Output compared with south-facing |
|---|---|
| South | Baseline (best annual production) |
| Southeast or southwest | About 95 to 99 percent |
| East or west | About 80 to 90 percent |
| North | Generally not recommended |
A tilt close to your latitude captures the most sunlight across the year. Not sure how your roof stacks up? Start with how to tell whether your home is good for solar.
Shade and Inverter Type
Even a small shadow from a tree or chimney can cut output far more than the shaded area suggests, especially on systems with string inverters. Microinverters and power optimizers limit the damage by letting each panel perform on its own.
Heat and Temperature Coefficient
Most panel datasheets list a temperature coefficient, often around negative 0.3 percent per degree Celsius. A cell running 35 degrees Celsius above the 25-degree test temperature loses about 10 percent of its output at that moment. This is why a hot August afternoon in the San Fernando Valley can produce less than a cool, clear April day. For a deeper look at how panel design affects performance, see our guide to solar panel efficiency.
Dust, Soiling, and Degradation
- Soiling: dust and pollen can cost 2 to 5 percent between rain events. See solar panel maintenance basics for what homeowners should do.
- Degradation: panels typically lose a fraction of a percent of output per year, so year 10 output is slightly lower than year 1.
Solar Panel Output vs Real Savings Under NEM 3.0
Production Is Not the Same as Bill Savings
Output tells you how much electricity your panels make. Savings depend on when you use it. Under California’s NEM 3.0 rules, power you use at home while the sun is up is worth full retail rates, while power you export to the grid earns a much lower credit. Read NEM 3.0 explained and how to read your solar bill to see how production and credits appear on your statement.
How a Battery Fits In
A battery does not increase production. It stores midday surplus so you can use it in the evening instead of exporting it at a low rate. Once you know your daily output, you can judge how much surplus there is to store. Our guide on whether you need a battery with solar walks through that decision.
Skip the Math: Use the Free Solar Output Calculator
Manual math gets you close, but your roof pitch, shading, and panel model all shift the final number. Our free solar panel output calculator pulls these factors together and compares the result with your current electric bill in about a minute.
- Uses peak sun hours for your location
- Accounts for roof orientation and typical system losses
- Shows a realistic production estimate instead of a label number
How to Check Your Real Output After Installation
Once your system is running, compare real production with your estimate.
- Open your monitoring app and note total kWh for a full month.
- Compare it with your estimate from the formula or the calculator.
- Account for weather, since cloudy weeks and marine layer reduce output.
- If production runs more than 10 to 15 percent below the estimate over a clear month, ask your installer to check for shading, inverter alerts, or heavy soiling.
Before you sign anything, review these questions to ask before signing a solar contract, since some proposals still quote lab wattage instead of realistic output.
Frequently Asked Questions About Solar Panel Output
How do I calculate solar panel output for my house?
Multiply each panel’s size in kilowatts by your local peak sun hours, then multiply by 0.80 to 0.85 to account for real-world losses. That gives daily kWh per panel. Multiply by your number of panels, then by 365 for yearly output.
How much power does a 400 watt solar panel produce per day in California?
A 400-watt panel typically produces about 1.8 kWh per day on an annual average in Los Angeles, using 5.5 peak sun hours and a 0.80 loss factor. Expect roughly 1.1 to 1.4 kWh in winter and 2.1 to 2.2 kWh in summer.
How many peak sun hours does California get?
Most of Southern California averages about 5.5 peak sun hours per day over a full year. Winter days run lower, around 3.5 to 4.5 hours, and summer days run higher, around 6.5 to 7 hours.
Why is my solar output lower than the rated wattage?
Rated wattage is measured in lab conditions at 77 degrees Fahrenheit with ideal sunlight. On a real roof, heat, dust, inverter and wiring losses, shading, and panel angle typically reduce output by 15 to 20 percent.
How much does heat reduce solar panel output?
Most panels lose about 0.3 percent of output for every degree Celsius above the 25-degree test temperature. On a hot afternoon, panel cells can run far hotter than the air, which can cost roughly 10 percent of output at that moment.
Does a battery increase solar panel output?
No. A battery does not make your panels produce more electricity. It stores surplus power so you can use it later instead of exporting it at a lower credit rate, which can increase how much of your output saves you money.