Solar Loan Payoff Calculator: Time and Interest to Clear It
See how long a solar panel loan takes to clear at a fixed monthly payment, and how much of that money is pure interest rather than principal.
Adjust the inputs and select Calculate for a full breakdown.
Year-by-year payoff schedule
Compare Common Scenarios
How the numbers shift across typical situations for this calculator:
| Scenario | Time to pay off | Total interest | Total paid |
|---|---|---|---|
| $25k · 7% · $400/mo | 6y 6m | $6,186.03 | $31,186.03 |
| $18k (post-credit) · 6% · $300/mo | 6 years | $3,454.14 | $21,454.14 |
| $40k · 8% · $500/mo | 9y 7m | $17,351.18 | $57,351.18 |
| $12k · 9% · $250/mo | 5 years | $2,932.14 | $14,932.14 |
How This Calculator Works
Enter the current loan balance, the loan APR, and the fixed monthly payment. The calculator simulates interest and payments month by month and counts the months until the balance reaches zero. If you've received the 30% federal solar tax credit, apply it as a lump-sum prepayment to the balance first.
The Formula
Debt Payoff Time
B = balance, P = fixed monthly payment, r = monthly rate (APR ÷ 12), n = months to clear
Worked Example
A $25,000 solar loan at 7% APR paid down at $400 a month clears in 78 months — about 6.5 years — with roughly $6,186 of interest along the way. Most solar loans are structured to assume the borrower applies the 30% federal tax credit (~$7,500 on this loan) as a prepayment within 12 to 18 months; doing so re-amortizes to a lower payment or shorter term.
Key Insight
Solar loans hide their true cost in 'dealer fees'. Low-advertised-rate solar loans (1.99%, 2.99%) typically bake a 10% to 30% markup into the principal to compensate the lender for the below-market rate — meaning a $20,000 system becomes a $26,000 loan. A higher-rate loan (or HELOC at 8%) on the true $20,000 cost often beats the low-rate-high-principal solar loan on total cost. Always compare the all-in amount financed, not just the headline rate, and consider cash or HELOC if available.
Frequently Asked Questions
How is solar loan payoff calculated?
Interest charged monthly on the remaining balance, monthly payment applied, balance reduced — counted until it reaches zero. Apply the 30% federal tax credit as a lump-sum prepayment if received, then recalculate.
What's the deal with solar loan dealer fees?
Low-rate solar loans (1.99% to 3.99%) typically bake a 10% to 30% markup into the financed principal to compensate the lender for the below-market rate. A $20,000 system becomes a $26,000 loan. Compare the all-in amount financed, not just the rate — a higher-rate loan on the true cost often wins.
How does the federal tax credit work?
The federal Residential Clean Energy Credit covers 30% of solar installation cost through 2032 as a tax credit (reduces tax owed dollar-for-dollar). Most solar loans assume you apply this ~30% as a principal prepayment within 12 to 18 months, which re-amortizes the loan to a lower payment.
Cash, loan, or HELOC for solar?
Cash: cheapest (no financing cost or dealer markup). HELOC: usually beats solar loans on total cost (lower rate, no dealer markup, possibly tax-deductible interest). Solar loan: convenient but watch the dealer-fee markup. Lease/PPA: no upfront cost but you forfeit the tax credit and most of the savings.
Should I prepay a solar loan?
Apply the tax credit as a prepayment as planned — that's built into most solar loan structures. Beyond that, prepayment depends on the rate: solar loans above 7% to 8% often beat expected investment returns after tax; below that, investing the difference may win. Check for prepayment penalties first.
Related Calculators
Methodology & Review
The payoff is simulated month by month: interest is charged on the balance, the fixed payment is deducted, and the months are counted until the balance reaches zero. Solar loans often assume a large principal prepayment when the 30% federal tax credit arrives — that re-amortization is not modeled here; the calculator assumes a fixed payment on the full balance.
Written by Ugo Candido · Last updated May 17, 2026.