Inputs
For advanced realism, use panel sizing plus loss factors. Orientation factors near 1.00 indicate a well-aimed array.
Formula used
Monthly energy output is estimated using a derate approach:
Energy(kWh) = DC_kW × SunHours/day × Days × OverallFactor
OverallFactor combines performance ratio, inverter efficiency, orientation factors, and losses from temperature, soiling, wiring, availability, shading, and age degradation.
OverallFactor = PR × InvEff × (1-Temp) × (1-Soil) × (1-Wire) × (1-Avail) × (1-Shade) × Tilt × Azimuth × Age
Monthly value splits energy into self-consumed savings and exported credits using your rate and credit percentage.
How to use this calculator
- Select the month and year, then confirm days.
- Enter panel count and wattage, or system size.
- Set average sun hours for your location and season.
- Adjust losses and orientation factors for realism.
- Add your retail rate, export share, and credit percentage.
- Press Calculate to see output and monthly value.
- Use the download buttons to export CSV or PDF.
Tip: If you lack detailed loss data, keep defaults and only adjust sun hours and rates.
Example data table
Sample scenarios to illustrate typical ranges. Replace with local data for accuracy.
| Scenario | DC kW | Sun hours/day | Days | Overall factor | Monthly kWh | Rate | Monthly value |
|---|---|---|---|---|---|---|---|
| Urban rooftop | 5.50 | 4.8 | 30 | 0.79 | 626 | 0.18 | ~113 |
| Sunny region | 7.15 | 6.2 | 31 | 0.82 | 1,123 | 0.20 | ~220 |
| Partial shading | 4.40 | 5.0 | 30 | 0.66 | 436 | 0.17 | ~74 |
Values are approximate and depend on system design and tariffs.
Monthly energy output drivers
Monthly production depends on DC capacity, peak sun hours, and days in the period. The calculator multiplies these three inputs and then applies an overall factor to represent real performance. In many residential systems, performance ratio and inverter efficiency account for most predictable reductions, while soiling and shading create location specific variance.
Loss factors and realistic ranges
Typical combined losses fall between 15% and 30% across a year. Temperature losses rise in hot months, while soiling losses rise in dry, dusty seasons. Wiring and availability losses are usually smaller, but they matter when comparing scenarios. Orientation factors let you test tilt and direction differences; values above 1.00 can represent an optimized seasonal angle.
Self consumption versus export value
Financial impact is shaped by how much energy you use on site versus send to the grid. Self consumed kWh is valued at the full retail rate, while exported kWh is valued using the credit percentage you enter. If the credit rate is lower than retail, improving daytime usage can increase monthly value without changing production.
Interpreting the results section
Use monthly kWh to compare against bills, and use average daily output to sanity check against system size. The effective value per kWh summarizes blended savings and credits into one number. The overall factor helps you audit assumptions; a low factor usually signals heavy shading, high losses, or aggressive degradation settings.
Planning scenarios and sensitivity checks
Run three cases: conservative, expected, and optimistic. Change only one parameter at a time, such as sun hours or shading, to see sensitivity. For budget planning, pair the monthly value with your tariff and estimate seasonal averages. For environmental reporting, the CO2 avoided figure converts kWh into kilograms using your chosen grid intensity. When modeling upgrades, increase panel count and keep loss assumptions constant to isolate capacity effects. For storage, reduce export percentage to reflect higher self use. Document each run, then compare totals using the chart for quick decisions later securely.
FAQs
What are peak sun hours?
Peak sun hours represent the equivalent number of hours per day at 1,000 W/m². They compress daily sunlight into a single value, so regions with clearer skies or longer days have higher totals.
Should I use system size or panel sizing?
If you know panel count and wattage, use those because they define DC capacity directly. If you only know the installer’s kW rating, enter system size and keep panel inputs at reasonable defaults.
How do I estimate exported energy percentage?
Export percentage is the share of production not used instantly at home. Start with 20%–40% for many homes, then adjust based on daytime usage patterns. Adding daytime loads generally lowers export and increases retail savings.
What does performance ratio represent?
Performance ratio captures real world system behavior beyond sunlight and capacity. It reflects losses from temperature, inverter conversion, wiring, mismatch, and other effects. Well designed systems often land around 0.75–0.85 across a year.
Why does the overall factor change so much?
The overall factor multiplies several assumptions, so small changes compound. Shading and temperature losses can dominate, while tilt and azimuth factors can raise or reduce output versus an ideal orientation. Review each input to keep assumptions realistic.
Does this model battery storage?
It estimates storage effects indirectly through export percentage. If you add a battery and expect higher self use, reduce exported energy and rerun the calculation. For detailed storage dispatch modeling, use hourly data and battery specifications.
Notes and disclaimers
- This tool provides estimates, not engineering guarantees.
- Actual production varies with weather, maintenance, and equipment.
- Tariffs and export credits differ by utility and plan.
- Use a professional site survey for investment decisions.