Solar Panel Output Calculator
See how much energy a solar system makes each month in your area, with your tilt, direction and losses.
Use this when you already know the system size, or when you’re comparing two roofs. It shows energy by month and for the year, then turns the yearly total into dollars at your electricity price.
Change the tilt and the direction to see what each choice costs you. A few degrees of tilt barely matter. Direction matters more.
Loading the calculator...
Planning estimates only. Your roof, equipment, utility rules and weather change the real numbers, so get a written quote before you buy. Data as of October 2026.
How this calculator works
The model starts with monthly sun data, the average of 20 years, and splits it into hourly sun for a typical day of each month. It works out how much of that lands on a panel at your tilt and direction, adjusts for panel heat and the angle sunlight hits the glass, and then takes off system losses and inverter losses.
The default loss is 14%, the same as PVWatts. It covers dirt, shade, wiring, mismatch between panels and downtime. If your roof is shaded, raise it.
The result is a typical year. Weather moves from year to year, so a real year can come in several percent above or below.
What changes the output most
- Direction. In Austin, panels facing east or west make about 12% less than panels facing south. North-facing panels make about 25% less.
- Tilt. In Austin, anything from 15 to 40 degrees on a south roof is within about 2% of the best angle. Seattle likes a steeper roof, but the gap there is small too.
- Shade. Even a little shade on one panel can pull down a whole string, so it’s worth more attention than tilt.
Where the numbers come from
- NREL PVWatts Calculator, used to check the model and for the 14% default loss.
Questions
How many kWh does 1 kW of solar panels make in a year?
In the lower 48, roughly 1,150 to 1,750 kWh, depending on the sun. Seattle sits near the low end and Phoenix near the high end. The tool shows the figure for your ZIP code under “Per kW of panels”.
How much does one solar panel produce per day?
A 400 W panel makes about 1.3 kWh a day on average in Seattle and about 1.9 kWh in Phoenix, with a south-facing tilt of 20 degrees and typical losses.
Why does output drop in winter?
Days are shorter and the sun is lower. In Seattle, December makes about a quarter of what June does. In Austin it’s about two thirds.
Do panels work on cloudy days?
Yes, at lower output. Clouds cut the direct sun, but panels still catch scattered light. The monthly figures already include typical cloud cover.
Does heat reduce output?
Yes. Panels lose roughly a third of a percent of their power for every degree Celsius they run above 25. That’s why a hot, sunny place doesn’t make as much as its sun hours suggest. The model includes it.
Is this the same as PVWatts?
No, it’s a separate model. It uses the same default loss and was checked against PVWatts, but for a formal design use PVWatts or your installer’s software.
More solar tools
Want a real price for your home?
Send one short form and get quotes from solar companies that work in your area.