Solar Loan Calculator
Use this solar loan calculator to estimate monthly payments, total interest and the full cost of financing a solar installation. Keep the cash price, financed fees and down payment separate. You can also compare the loan’s first-year payments with entered energy savings and see how an optional extra payment changes the schedule.
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Versioned calculationFormula v1.0.1
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Report a calculation issue Solar Loan Calculator: purpose and method
Use this solar loan calculator to estimate monthly payments, total interest and the full cost of financing a solar installation. Keep the cash price, financed fees and down payment separate. You can also compare the loan’s first-year payments with entered energy savings and see how an optional extra payment changes the schedule.
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Method and test recordFormula v1.0.1
- Recorded scope
- Not recorded
- Publisher
- CalculatorGeek
- Reviewed by
- CalculatorGeek Editorial Team
- Recorded review date
- Not recorded
- Next source review
- Not scheduled
- Definition fixtures
- 3 configured scenarios
- Published examples
- 3 shown below
Recorded method
No tool-specific review method has been recorded.
These records describe the published model and reference tests. A test-case count is not a certification of every possible input or an independent specialist review. Editorial policy
Known limitations
Method sources
Latest model update
2026-10-28 - Created from finished package copy and deterministic contract.
Reference inputs and expected results
Up to 12 examples from the configured definition fixtures are shown. Expected values use the stated output units; invalid inputs are intended to be rejected.
| Case | Inputs | Expected result |
|---|---|---|
| Financed fees included once | Cash Price: 24000; Financed Fees: 2000; Down: 4000; Upfront Fee: 100; Rate Pct: 6; Months: 120; Annual Energy Savings: 2400; Annual Maintenance: 100 | Principal: 22000; Monthly: 244.24510427163; Total Interest: 7309.4125125959; Total Outlay: 33409.412512596; Year1 Net: -630.94125125959 (allowed numeric tolerance: 1.0E-6) |
| Zero-rate level payment | Principal: 12000; Rate Pct: 0; Months: 60 | Monthly: 200; Total Interest: 0; Total Payments: 12000; Payoff Month: 60; Final Payment: 200 (allowed numeric tolerance: 1.0E-6) |
| Zero-rate extra principal keeps payment | Principal: 12000; Rate Pct: 0; Months: 60; Extra Month: 12; Extra Amount: 600; Recast: false | Monthly: 200; Payoff Month: 57; Total Interest: 0; Total Payments Including Extra: 12000 (allowed numeric tolerance: 1.0E-6) |
Report an issue with this tool. Include the page URL, units, expected answer, and steps to reproduce. Do not include sensitive personal information.
On this page
Begin with the cash price and loan disclosure
Request a cash quote for the same equipment and installation scope as the financed offer. The Consumer Financial Protection Bureau’s solar financing report identifies cash-price markups and payment-reset structures as issues borrowers should examine. Its historical tax-credit discussion does not establish eligibility for a new project.
Enter either an itemized cash price plus financed fees, or the lender’s disclosed principal. If the disclosed principal already contains a dealer or origination charge, do not add it again. Enter charges paid separately at closing as upfront fees.
Use the annual note interest rate for the monthly amortization calculation. A disclosed APR may include additional finance charges and should be recorded for comparison, but it is not automatically the note rate. Enter the repayment term in months and the down payment you actually plan to make.
What the monthly payment includes
For a fixed-rate, fully amortizing loan, the monthly payment covers interest and repayment of principal. With principal P, monthly rate r and n payments:
Monthly payment = P × r ÷ [1 − (1 + r)⁻ⁿ].
The monthly rate is the annual nominal note rate divided by 12. At a zero interest rate, payment is simply principal divided by the number of months. The schedule calculates interest on the opening balance, applies the scheduled payment and then applies any entered extra principal payment.
The final payment is reduced to the remaining amount due. Total interest is the sum of the schedule’s interest charges. Total financing outlay includes scheduled payments, extra payments, the down payment and fees paid outside the loan.
Example: compare a $24,000 cash price with financing
Suppose the installation cash price is $24,000, financed fees are $2,000, the down payment is $4,000 and an additional $100 fee is paid at closing. At a 6% note rate over 120 months:
| Result | Illustrative amount |
|---|---|
| Amount financed | $22,000 |
| Monthly scheduled payment | $244.25 |
| Total scheduled payments | $29,309.41 |
| Total interest | $7,309.41 |
| Total outlay including down payment and upfront fee | $33,409.41 |
This costs $9,409.41 more than the stated cash price, consisting of financed fees, interest and the separately paid fee. Displayed payments are rounded; schedule totals use unrounded calculations with a final payment adjustment.
If annual energy savings are entered as $2,400 and maintenance as $100, first-year savings after loan payments are approximately negative $630.94. The system may still have a longer-term benefit, but the financing does not produce a positive first-year cash result in this example.
Extra payments and payment changes
An extra principal payment normally reduces the outstanding balance. In the keep-payment scenario, the regular payment stays the same and the modeled payoff occurs sooner. If your lender allows a recast, the alternative scenario recalculates future payments over the remaining original term after the extra payment.
Do not assume either treatment matches your contract without checking. A loan with an introductory payment, an expected large prepayment or a contractual payment increase needs those specific terms. This calculator’s standard schedule does not silently treat such a loan as a level-payment agreement.
Keep incentives and electricity savings realistic
For a new installation completed in 2026, do not deduct the former 30% federal residential solar credit. The IRS Form 5695 instructions state the relevant credit ended after 2025. A confirmed local reimbursement can be entered as an extra payment when it arrives; it should not reduce the original principal unless actually applied at closing.
Energy savings are separate from loan repayment obligations. Use a production and tariff estimate that accounts for grid purchases, export compensation and continuing charges. A lower payment alone does not prove that borrowing is cheaper: extending the term can increase total interest.
For a broader borrowing comparison, use the personal loan cost calculator or inspect the draw and repayment phases with the HELOC payment calculator.
Frequently asked questions
Is the loan paid off when solar reaches payback?
Not necessarily. Project payback and loan payoff measure different things. The loan balance depends on its repayment schedule, while project payback depends on cumulative benefits and costs.
Can the calculator guarantee a tax refund for an extra payment?
No. Enter only money you reasonably expect to receive under a verified program, and compare a scenario without it.
Is the calculator’s cost comparison a legal APR disclosure?
No. The schedule and cash-price comparison are planning calculations. Use the lender’s required disclosure for its stated APR and applicable loan terms.
What if my monthly bill savings are seasonal?
The first-year comparison uses an annual total. Actual monthly cash flow can vary with production, consumption and utility settlement timing.
Enter the complete financed offer, compare it with the cash quote, and review the payment schedule before relying on a monthly savings claim.