Solar Loan Calculator: Monthly Payment, Interest and Dealer Fees

A solar loan calculator turns the price, rate and term into a monthly payment, then shows what the loan really costs. With the defaults, $23,100 at 7.99% APR for 20 years costs about $193.07 a month and $46,338 in total, including $23,238 of interest. A dealer fee built into the loan can add thousands more.

How to use the solar loan calculator

1. Cash price of the system. Ask every installer for the cash price as well as the loan price. Default $23,100.
2. Down payment. Money you pay up front. Default $0.
3. Interest rate (APR). From the loan offer. Default 7.99%.
4. Loan term. Solar loans commonly run 10 to 25 years. Default 20.
5. Dealer fee built into the loan. Ask in writing. Default 0%.
6. Advanced fields. Planned lump-sum paydown and Month of the paydown (default month 18), and Extra payment each month.

The results show the monthly payment, amount financed, dealer fee cost, total interest, total you pay and when the loan is paid off. A line chart tracks the balance, and a stacked bar chart splits each year's payments into principal and interest. The amortization schedule has a yearly/monthly toggle and a CSV download. The scenario table compares your loan, a term 10 years shorter, and a loan with no dealer fee but an APR 2 points higher.

How the calculation works

Cash financed    = cash price − down payment
Amount financed  = cash financed ÷ (1 − dealer fee %)
Dealer fee cost  = amount financed − cash financed
Monthly rate i   = APR ÷ 12
Payment          = amount financed × i ÷ (1 − (1 + i)^−months)
Total interest   = payment × months − amount financed
Total you pay    = payment × months + down payment

Assumptions that match the code:

• The dealer fee is grossed up, as lenders usually do: they fund (1 − fee) of the loan face amount, so a 20% fee on $23,100 makes the loan $28,875, not $27,720.
• Fixed rate, monthly payments, no prepayment penalty.
• A lump-sum paydown re-amortizes: the remaining balance is spread over the remaining months, lowering the payment.
• Extra monthly payments go straight to principal and shorten the loan.

Worked example

Default inputs: $23,100 cash price, $0 down, 7.99% APR, 20 years, 0% dealer fee.

• Monthly payment: $193.07. Amount financed: $23,100. Dealer fee cost: $0.
• Total interest: $23,238. Total you pay: $46,338. Paid off in: 20 years.

With no dealer fee in your inputs, the third column only shows a higher rate. Its real use is when your offer has a fee, as below.

Second example, a low-rate loan with a dealer fee: $23,100 cash price, 3.99% APR, 25 years, 20% dealer fee.

• Amount financed: $28,875, of which $5,775 is the dealer fee.
• Monthly payment: $152.25; total interest $16,801; total you pay $45,676.
• The no-fee alternative at 5.99% pays $148.69 a month and $44,608 in total, less than the "cheaper" 3.99% loan.

Paydowns and extra payments on the default loan: a $5,000 lump sum in month 18 lowers the payment to about $149.88 and total interest to $18,650. An extra $50 a month instead pays the loan off in about 12.6 years with $13,592 of interest.

What your result means

Compare the total you pay with the cash price and with expected savings. If the monthly payment is above your current monthly savings for years, the loan costs you money until savings grow. Run the full cash flow in the solar payback calculator using its loan fields, and compare ownership with a lease in the solar lease vs buy calculator. Our guide to how solar loans work explains secured versus unsecured loans and fees.

Rules that change the answer

• Dealer fees. CFPB found hidden fees typically 10% to 30% of the cash price, sometimes over 50%, with loan principal "30 percent or more above the cash price" in some cases (CFPB, 2024 (opens in a new tab)). LBNL's 2025 data show loan-financed systems at a median of about $4.50 per watt, versus $3.00 for cash.
• No tax-credit paydown in 2026. The 25D credit does not apply to installations completed after December 31, 2025 (IRS (opens in a new tab)). Ask whether the loan assumes a month-18 paydown.
• Lender rules vary. Ask how the loan is secured and what happens if you sell or refinance your home before it is paid off.
• Consumer protection. The FTC (opens in a new tab) advises written bids and no pressure signing. See solar sales red flags and lease vs PPA vs loan.

Common mistakes

• Entering the financed price as the cash price, which hides the fee.
• Choosing by APR alone instead of total cost.
• Counting on a federal tax-credit paydown for a 2026 install.
• Ignoring re-amortization terms if you skip the paydown.
• Comparing a 25-year loan's lower payment with a 10-year loan without comparing total interest.

Limits of this calculator

This is an educational estimate, not a loan offer. It assumes a fixed rate and level payments and does not model origination fees outside the dealer fee, autopay discounts, variable rates, payment increases written into specific contracts, or late fees. Your Truth in Lending disclosure shows the official APR, finance charge and total of payments.