Calculator
Formula used
The base payment is calculated using the standard amortization formula:
Each period:
- Interest = current balance × r
- Principal paid = payment − interest, plus any extra payment
- Lump-sum credit is applied at your selected delay month
- New balance = old balance − principal paid − credit applied
How to use this calculator
- Enter the quoted system cost and any sales tax.
- Add closing costs, plus any dealer or origination fees.
- Subtract rebates and set your down payment amount.
- Choose APR, term, and payment frequency.
- Optionally add extra payments and apply a tax credit delay.
- Press Calculate to view results and schedule.
- Download the CSV or PDF to share with stakeholders.
Example data table
| Scenario | System cost | APR | Term | Rebates | Down | Frequency | Extra | Typical outcome |
|---|---|---|---|---|---|---|---|---|
| Starter | 18,000 | 6.50% | 12 yrs | 1,000 | 2,000 | Monthly | 0 | Balanced payment with moderate total interest. |
| Fast payoff | 18,000 | 6.50% | 12 yrs | 1,000 | 2,000 | Biweekly | 25 | Lower interest and earlier payoff date. |
| Low APR offer | 18,000 | 2.99% | 15 yrs | 1,000 | 2,000 | Monthly | 0 | Smaller payment, but watch dealer fees. |
Loan inputs that change payments most
Payment size is controlled by three numbers: financed amount, APR, and term length. Extending a term from 10 to 20 years lowers each payment but increases total interest. Use ranges such as 5–25 years, and test APR changes because compounding grows over periods.
Understanding rolled-in fees and true financed amount
Solar offers may include dealer fees, origination charges, filing costs, and sales tax. When these are rolled into the loan, interest is paid on them too. The calculator separates system cost, tax, closing costs, dealer fee percent, and origination percent, then shows the final loan amount after rebates and down payment. Comparing “loan amount” across quotes reveals which proposal is truly cheaper.
Incentives, rebates, and credit timing
Upfront rebates reduce principal immediately, so they usually create the cleanest savings. Tax credits can be valuable, yet timing matters: if the credit is received after 12 months, interest has accrued for a year. Modeling the credit as a delayed lump-sum helps estimate payoff change and avoids overstating savings. If you will not apply the credit to the loan, choose “do not apply” to keep the schedule consistent.
Payment frequency and extra principal strategy
Monthly schedules are common, but weekly or biweekly payments shorten the interest window by making principal reductions more frequent. Extra payment per period is a simple stress test: add an amount you can sustain and observe reduced total interest and an earlier payoff date. Pairing modest extras with a shorter term can outperform a low-rate, long-term loan carrying high fees.
Reading the amortization schedule for decisions
Each row splits the payment into interest and principal, then lists any extra and credit applied. Early rows are interest-heavy; later rows shift toward principal as the balance falls. Check the credit row to confirm it is applied when expected, and confirm the ending balance reaches zero. Use total paid and total interest to compare options, not the headline payment alone.
FAQs
What does the calculator treat as the financed amount?
Financed amount equals net project cost plus rolled origination fees, minus your down payment. Net cost includes system cost, sales tax, closing costs, and dealer fees, reduced by upfront rebates.
Why can a low APR loan still cost more overall?
Some low-rate offers include dealer fees that increase the financed balance. Even with a smaller payment, interest is paid on that larger balance over time, raising total paid.
How is the tax credit handled in the schedule?
If you choose to apply it, the calculator models the credit as a lump-sum principal payment after your selected delay. It reduces balance immediately at that point and can shorten payoff.
What happens if I add an extra payment each period?
Extra payments are applied directly to principal after interest is calculated. This reduces the remaining balance faster, lowers future interest, and typically moves the payoff date earlier.
Why do weekly or biweekly payments reduce interest?
More frequent payments reduce the time principal sits unpaid. With a lower balance sooner, each later period calculates interest on a smaller amount, which can reduce total interest.
Can I share results with my contractor or lender?
Yes. After calculating, use the CSV for a complete schedule or the PDF for a summary report. Both files are generated from your inputs, making comparison and recordkeeping easier.