How does the solar loan calculator work?
The calculator applies the standard loan amortization formula to your system cost and financing terms, then compares the result against your estimated electricity bill savings.
Enter cost & down payment
Set your total system cost and any down payment you plan to make.
Enter your loan terms
Use the APR and term length your lender quoted, or compare a few options.
View your payment breakdown
See your monthly payment, total interest, and total cost of the loan.
Why total interest matters as much as the monthly payment
Two loans with very different total costs can have similar-looking monthly payments if their terms differ enough. A longer term spreads the same loan amount over more months of interest charges, lowering the monthly payment but often substantially raising the total amount you pay over the life of the loan. This calculator shows both figures together so a lower payment doesn't hide a much higher total cost.
P is your loan amount, r is your monthly interest rate (APR ÷ 12), and n is your total number of monthly payments (years × 12).
Worked example: financing a $25,000 system
A $25,000 system, no down payment, 6.5% APR over 15 years
Suppose your solar installer quotes a $25,000 system cost with no down payment, financed at 6.5% APR over a 15-year term, and you estimate your electricity bill savings at $150 per month once the system is running.
- Loan amount: $25,000 − $0 = $25,000.
- Monthly rate: 6.5% ÷ 12 ≈ 0.5417%.
- Number of payments: 15 years × 12 = 180.
- Monthly payment: ≈ $217.78.
- Net monthly cost: $217.78 − $150 ≈ $67.78.
Over the full 15-year term, the total amount paid comes to about $39,200 — the original $25,000 loan amount plus roughly $14,200 in interest.
Result: about $217.78 per month, $14,200 in total interest, and a net monthly cost of about $67.78 after estimated savings.
Comparing loan terms
Run the calculator a second time with a different term length — say, 10 years instead of 15 — to see the tradeoff directly. Shorter terms raise the monthly payment but can cut total interest substantially, since the balance is paid down faster and accrues interest for less time overall. There's no universally correct choice; it depends on how much monthly payment you're comfortable with versus how much total interest you'd rather avoid.
Common ways to use this calculator
Run each lender's quoted rate and term to compare monthly payments and total interest side by side.
See how much a down payment reduces your monthly payment and total interest.
Add your estimated bill savings to see your true net monthly cost, not just the loan payment.
Weigh total interest paid against the opportunity cost of paying cash upfront.
Privacy and appropriate use
Your inputs are processed directly in your browser and are not sent to a database. This is a planning estimate; your actual loan terms, fees, and payment schedule depend on your specific lender's agreement — review your loan documents for the exact figures.