';steps+='1. Monthly Energy Usage: $'+bill+' / $'+rate+' = '+(bill/rate).toFixed(0)+' kWh
';steps+='2. Daily Generation Needed: '+(bill/rate/30.42).toFixed(2)+' kWh/day
';steps+='3. System Size (adj. for efficiency): '+(dailyKWh/sun).toFixed(2)+' / 0.77 = '+sysSize.toFixed(2)+' kW
';steps+='4. Gross System Cost: '+sysSize.toFixed(2)+' kW * 1000 * $'+cpw+' = $'+grossCost.toLocaleString()+'
';steps+='5. Incentives Applied: -$'+taxValue.toLocaleString()+'
';steps+='6. Net Investment: $'+netCost.toLocaleString()+'
';steps+='7. Yearly Avoided Costs: $'+bill+' * 12 = $'+annualSavings.toLocaleString()+'
';steps+='8. Payback: $'+netCost.toLocaleString()+' / $'+annualSavings.toLocaleString()+' = '+payback.toFixed(2)+' years';document.getElementById('stepsOutput').innerHTML=steps;document.getElementById('stepsOutput').style.display='block';}else{document.getElementById('stepsOutput').style.display='none';}}
Using the Solar Calculator
The solar calculator is a professional-grade tool designed to help homeowners and business owners estimate the size and financial viability of a solar photovoltaic (PV) system. By inputting basic utility data, this tool provides a realistic look at how much you can save and how long it takes for the system to pay for itself through avoided electricity costs.
- Avg Monthly Bill
- The amount you pay your utility company on average each month. This determines the total energy offset needed.
- Electricity Rate ($/kWh)
- The price you pay per kilowatt-hour. You can find this on your latest utility statement.
- Daily Sun Hours
- Also known as "Peak Sun Hours." This varies by geography (e.g., Arizona averages 6 hours, while Seattle may average 3.5).
- Incentives & Tax Credits
- The Federal Investment Tax Credit (ITC) currently allows for a 30% deduction on installation costs.
How the Solar Calculation Works
To determine the system size, the solar calculator accounts for panel efficiency and real-world losses (like wiring, inverter heat, and dirt). Most systems operate at roughly 75% to 80% efficiency compared to their laboratory rating. The primary formula used is:
System Size (kW) = (Monthly kWh / 30.42) / (Peak Sun Hours × 0.77)
- Monthly kWh: Calculated as Total Bill divided by Rate.
- 30.42: The average number of days in a month.
- 0.77: The standard derate factor for system losses.
- Peak Sun Hours: The intensity of solar radiation in your specific location.
Solar ROI Example
Example: A homeowner in Florida pays $200 per month for electricity at a rate of $0.15/kWh. They receive an average of 5.5 sun hours per day and the local installer quotes $3.00 per watt.
Step-by-step solution:
- Calculate usage: $200 / $0.15 = 1,333 kWh per month.
- Daily requirement: 1,333 / 30.42 = 43.8 kWh per day.
- Required System: 43.8 / (5.5 * 0.77) = 10.34 kW.
- Gross Cost: 10,340 Watts * $3.00 = $31,020.
- Apply 30% Tax Credit: $31,020 * 0.70 = $21,714 Net Cost.
- Annual Savings: $200 * 12 = $2,400.
- Payback Period: $21,714 / $2,400 = 9.05 Years.
Common Questions
What are "Peak Sun Hours"?
Peak sun hours do not represent the total time between sunrise and sunset. Instead, it is a measurement of the intensity of sunlight. One peak sun hour is defined as 1,000 watts of solar energy per square meter. Most of the US falls between 3 and 6 peak sun hours per day.
Does the solar calculator include maintenance?
Generally, solar panels require very little maintenance other than occasional cleaning. Most high-quality panels are warrantied for 25 years. However, you should factor in a possible inverter replacement after 12-15 years, which usually costs about $1,500 to $2,500 depending on system size.
Why is my payback period longer than expected?
The solar calculator payback period is highly sensitive to the cost of electricity. If your utility rates are low (under $0.10/kWh), your savings accrue more slowly. Conversely, in high-rate areas like California or Massachusetts, payback periods can be as short as 5 to 6 years.