How It Is Calculated
This calculator estimates the energy output and cost savings of a solar panel system. The calculations follow these steps:
- Total system size (kW): Multiply the wattage of one panel by the number of panels, then convert to kilowatts.
W = panel wattage (W) × N = number of panels
S = total system size (kW) = (W × N) / 1000
- Daily energy production (kWh): Multiply system size by peak sun hours and system efficiency.
H = peak sun hours per day (h/day)
E = system efficiency (decimal, e.g., 80% = 0.80)
D = daily energy (kWh) = S × H × E
- Monthly energy (kWh): Multiply daily energy by 30.
M = monthly energy (kWh) = D × 30
- Annual energy (kWh): Multiply daily energy by 365.
A = annual energy (kWh) = D × 365
- Annual cost savings ($): Multiply annual energy by the electricity rate.
R = electricity rate ($/kWh)
C = annual cost savings ($) = A × R
All results are rounded to two decimal places.
Assumptions
- One month is assumed to have 30 days, and one year 365 days.
- System efficiency accounts for all losses (inverter, wiring, temperature, shading, soiling, etc.). Typical values range from 70% to 90%.
- Peak sun hours represent the average daily equivalent of full sunlight for your location. In the US, this is often between 4 and 6 hours.
- The electricity rate is constant and does not include taxes, fees, or tiered pricing.
- Panel wattage is the rated power under Standard Test Conditions (STC).
Worked Example
Inputs:
- Panel wattage = 300 W
- Number of panels = 10
- Peak sun hours per day = 5 hours/day
- System efficiency = 80%
- Electricity rate = 0.12 $/kWh
Step 1: Total system size = (300 × 10) / 1000 = 3.00 kW
Step 2: Daily energy = 3.00 × 5 × 0.80 = 12.00 kWh
Step 3: Monthly energy = 12.00 × 30 = 360.00 kWh
Step 4: Annual energy = 12.00 × 365 = 4380.00 kWh
Step 5: Annual cost savings = 4380.00 × 0.12 = 525.60 $
Results: Total system size = 3 kW, Daily energy = 12 kWh, Monthly energy = 360 kWh, Annual energy = 4380 kWh, Annual cost savings = 525.6 $.
Practical Tips
- Use realistic peak sun hours for your location. You can find maps or databases from weather services.
- System efficiency depends on equipment quality and installation. Inverters typically have 95-98% efficiency; wiring and temperature losses add up.
- Consider using the Electricity cost calculator to compare your current bills with solar savings.
- For grid-tied systems, check with your utility about net metering policies, which can affect actual savings.
- If you plan to add batteries, the Power factor correction calculator may help optimize your system.
- Always consult a professional installer for a site-specific assessment.