About the Solar Output Calculator
This calculator estimates how much electricity a solar panel system will generate daily, monthly, and annually based on its rated size and the amount of usable sunlight your location receives. It's built for homeowners or installers sizing a system and wanting a quick output projection before committing to a specific system size.
How It Works
You enter the system's rated size in kW, the average peak sun hours per day for your location, noted to run roughly 3-4 in the Northeast or Pacific Northwest and 5-6-plus in the Southwest, and a system efficiency, or derate, percentage that accounts for real-world losses. The calculator multiplies system size by sun hours and the derate factor to get daily output in kWh, then scales that figure to monthly and annual totals.
Formula & Methodology
The derate factor, defaulted to 80%, is applied directly to the theoretical output of multiplying rated system size by sun hours, representing the gap between a panel's nameplate capacity and its real-world output after inverter conversion losses, wiring resistance, panel heat effects, and dust or debris accumulation. Monthly output uses 30.44 days, the average length of a month across a 365-day year, rather than a fixed 30 or 31.
Examples
Typical Residential Rooftop System
A 6 kW system with 4.5 peak sun hours a day and the default 80% derate produces about 21.6 kWh daily, roughly 658 kWh monthly, and around 7,884 kWh annually.
Larger System in a Sunnier Climate
A 10 kW system in the Southwest with 6 peak sun hours a day and an 82% derate produces about 49.2 kWh daily, near 1,498 kWh monthly, and roughly 17,958 kWh annually.
Advantages
- Turns a system's rated kW size into concrete daily, monthly, and annual production figures for comparison against actual usage.
- Accounts for real-world losses through an adjustable derate factor rather than assuming panels hit their full nameplate rating.
- Lets you compare how sun hours specific to your region change expected output for the same system size.
Common Mistakes
- Confusing total daylight hours with peak sun hours, which are a normalized measure of solar intensity and are almost always a smaller number.
- Skipping the derate factor entirely or leaving it at 100%, which produces an output figure well above what a real installed system delivers.
- Applying a single annual sun-hours average to estimate a specific month's output, when actual monthly production varies with the season.
Edge Cases to Watch For
- Peak sun hours are not the same as daylight hours; entering total daylight hours instead of true peak sun hours will substantially overstate output.
- The 80% derate default is a general industry rule of thumb for a well-installed system and won't reflect unusually shaded panels, poor orientation, or an aging inverter.
- The calculation assumes a constant sun-hours figure year-round, so it represents an annual average rather than accounting for seasonal swings between summer and winter output.
Common Use Cases
- Homeowners evaluating whether a proposed system size will cover their household electricity usage.
- Solar installers providing customers with a quick, ballpark production estimate during initial consultations.
- DIY solar planners comparing how different system sizes or locations affect expected output.