Solar Panel Calculator Formula
Understand the math behind the solar panel calculator. Each variable explained with a worked example.
Formulas Used
Panels Needed
num_panels = panels_neededSystem Size
system_size = panels_needed * panel_watts / 1000Annual Production
annual_production = panels_needed * panel_watts / 1000 * sun_hours * 365 * efficiency_loss / 100CO2 Avoided per Year
co2_avoided = panels_needed * panel_watts / 1000 * sun_hours * 365 * efficiency_loss / 100 * 0.92 / 2204.6Variables
| Variable | Description | Default |
|---|---|---|
monthly_kwh | Monthly Electricity Usage(kWh) | 900 |
sun_hours | Peak Sun Hours per Day(hours) | 5 |
panel_watts | Panel Wattage(watts) | 400 |
efficiency_loss | System Efficiency(%) | 80 |
daily_kwh | Derived value= monthly_kwh / 30 | calculated |
system_kw | Derived value= daily_kwh / (sun_hours * efficiency_loss / 100) | calculated |
panels_needed | Derived value= ceil(system_kw * 1000 / panel_watts) | calculated |
How It Works
How to Calculate Solar Panel Needs
Sizing a solar system depends on your energy consumption, local sunlight, and panel efficiency.
Formula
System Size (kW) = Daily kWh / (Sun Hours * System Efficiency)
Panels Needed = System Size * 1,000 / Panel Wattage
Peak sun hours vary by location: 4-5 hours in the northern US, 5-7 in the south and west. System efficiency accounts for inverter losses, shading, and panel degradation.
Worked Example
A home using 900 kWh/month in a location with 5 peak sun hours, using 400W panels at 80% efficiency.
monthly_kwh = 900sun_hours = 5panel_watts = 400efficiency_loss = 80
- 01Daily usage: 900 / 30 = 30 kWh/day
- 02System size: 30 / (5 * 0.80) = 7.5 kW
- 03Panels needed: ceil(7,500 / 400) = 19 panels
- 04Annual production: 19 * 400 / 1000 * 5 * 365 * 0.80 = 11,096 kWh
Ready to run the numbers?
Open Solar Panel Calculator