Solar Panel Output Calculator
Solar panel output in kWh per day, month and year from peak sun hours, roof area or a panel count, panel watts, losses and tilt, plus CO₂ offset. How it works
Use the Solar Panel Output Calculator
About the Solar Panel Output Calculator
A 300 ft² roof at 5.8 peak sun hours (the Los Angeles preset), with 400 W panels, 14% system losses and a 30° tilt, fits 13 panels (5.20 kW) and makes about 25.94 kWh a day, 790 kWh a month and 9,474 kWh a year. The solar panel output calculator sizes one panel from its wattage at 20% module efficiency (21.5 ft² for 400 W), counts the whole panels that fit, multiplies system kW by peak sun hours, the share left after losses and a tilt factor, and shows 3,297 kg of CO₂ offset a year.
Yearly output of the same roof in each preset city
| City preset | Peak sun hours | kWh per day | kWh per year |
|---|---|---|---|
| Los Angeles, CA | 5.8 | 25.94 | 9,474 |
| Phoenix, AZ | 6.5 | 29.07 | 10,617 |
| Denver, CO | 5.4 | 24.15 | 8,820 |
| Houston, TX | 5 | 22.36 | 8,167 |
| Miami, FL | 5.2 | 23.25 | 8,494 |
| New York, NY | 4.1 | 18.34 | 6,697 |
| Seattle, WA | 3.5 | 15.65 | 5,717 |
| Chicago, IL | 4 | 17.89 | 6,534 |
| London, UK | 2.9 | 12.97 | 4,737 |
| Berlin, Germany | 3 | 13.42 | 4,900 |
| Madrid, Spain | 5 | 22.36 | 8,167 |
| Toronto, Canada | 3.6 | 16.10 | 5,880 |
| Sydney, Australia | 4.8 | 21.47 | 7,840 |
| Dubai, UAE | 6.2 | 27.73 | 10,127 |
300 ft² of roof, 400 W panels, 14% losses, 30° tilt. The sun-hour presets are the tool’s own rounded planning figures, not values from a solar-resource dataset. For a US address, NREL’s PVWatts calculator gives figures from measured solar data; type its value in place of the preset.
Roof area, panels and yearly output at 5.8 sun hours
| Usable roof area | Whole panels | System size | kWh per year | CO₂ per year |
|---|---|---|---|---|
| 100 ft² | 4 | 1.60 kW | 2,915 | 1,014 kg |
| 150 ft² | 6 | 2.40 kW | 4,372 | 1,522 kg |
| 250 ft² | 11 | 4.40 kW | 8,016 | 2,790 kg |
| 300 ft² | 13 | 5.20 kW | 9,474 | 3,297 kg |
| 400 ft² | 18 | 7.20 kW | 13,117 | 4,565 kg |
| 600 ft² | 27 | 10.80 kW | 19,676 | 6,847 kg |
| 1,000 ft² | 46 | 18.40 kW | 33,522 | 11,666 kg |
Panels = whole panels that fit, rounded down, at 21.5 ft² each (a 400 W panel at the tool’s 20% module efficiency). 400 W panels, 14% losses, 30° tilt.
The tool’s tilt factors on the default roof
| Tilt | Factor | kWh per day | kWh per year |
|---|---|---|---|
| Flat (0°) | 0.87 | 22.57 | 8,242 |
| 15° | 0.95 | 24.64 | 9,000 |
| 30° | 1.00 | 25.94 | 9,474 |
| 45° | 0.97 | 25.16 | 9,189 |
| 60° | 0.89 | 23.08 | 8,432 |
| Vertical (90°) | 0.73 | 18.93 | 6,916 |
The factors are planning assumptions built into the tool, not values from a published table. If the sun hours you type are already for panels at your roof’s tilt, as many solar datasets give them, leave the tilt at 30° (factor 1.00) so the tilt is not counted twice.
How to use this tool
- Pick a city preset to fill in peak sun hours, or type your own value for your location.
- Enter the usable roof area in square feet, or the Number of Panels from a quote (the roof area is then not used), the panel wattage and the system losses in percent, then choose a tilt.
- Read daily, monthly and yearly kWh, the panel count, the system size, the CO₂ offset and the tree equivalent. The Headline figure only changes which of the three energy figures is emphasized.
Frequently asked questions
- How many solar panels fit on a 300 ft² roof?
- 13 panels of 400 W. The tool sizes one panel from its wattage at 20% module efficiency: 400 W ÷ (0.20 × 1,000 W per m²) is 2 m², or 21.5 ft². It divides the usable roof area by that and rounds down to whole panels: 300 ÷ 21.5 = 13.9, so 13, a 5.20 kW system. The 20% is the tool’s assumption, typical of current monocrystalline panels (about 20 to 22%); EnergySage lists 114 ft² for five 400 W panels, about 23 ft² each, which also fits 13 on 300 ft². A less efficient panel is larger, so check your panel’s datasheet, or type the count from a quote in Number of Panels: 15 panels at 400 W make 6.00 kW and 10,931 kWh a year at 5.8 sun hours, whatever the roof area. An area under 21.6 ft², one 400 W panel, gets a message instead of a result.
- How many kWh does a rooftop solar system produce per year?
- A system of 13 panels at 400 W, with 14% losses and a 30° tilt, makes 9,474 kWh a year at 5.8 sun hours, 4,737 kWh at London’s 2.9 and 10,617 kWh at Phoenix’s 6.5. The formula is system kW × peak sun hours × (1 − losses) × tilt factor × 365.25 days.
- What is a peak sun hour?
- One hour of sunlight at 1,000 W per square meter, which is 1 kWh per square meter. A day with 5 peak sun hours brings 5 kWh to each square meter, however the sunlight is spread over the day. A flat surface and a tilted one get different totals, so note which one your figure is for before you pick the tilt.
- What tilt does the tool treat as best?
- 30°, with a factor of 1.00. Flat is 0.87 (8,242 kWh a year on the default roof), 45° is 0.97 and vertical is 0.73 (6,916 kWh). These factors are the tool’s assumptions, not a published table, and roof direction and shading are not modeled.
- How is the CO₂ offset calculated?
- Yearly kWh × 0.348 kg per kWh: 9,474 × 0.348 is 3,297 kg. The 0.348 comes from the U.S. total output emission rate in EPA eGRID2023, 767.2 lb per MWh. EPA’s Greenhouse Gas Equivalencies Calculator uses a different factor, 0.394 kg per kWh of electricity used (eGRID2022, including grid losses), which would give 3,733 kg for the same roof. Emissions differ by region, so the figure is a U.S. average.
- How many trees is that equal to?
- The tool divides the CO₂ by 60 kg, the amount EPA gives as 0.060 metric tons per urban tree per year: 3,297 ÷ 60 is 55 trees. The figure compares emissions, it does not say a tree absorbs that much in your climate.
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