Inputs
Example data table
A typical mid-sized construction deployment can use the following values to sanity-check outputs.
| Scenario | System Cost | Down Payment | APR | Term | Payment Frequency | Compounding | Expected Outcome |
|---|---|---|---|---|---|---|---|
| Rooftop PV on site office | $18,000 | $2,000 | 8.5% | 7 years | Monthly | Monthly | Monthly payment near $250–$275 |
| Warehouse auxiliary load | $65,000 | $5,000 | 7.2% | 12 years | Monthly | Daily (365) | Payment increases slightly vs monthly compounding |
| Remote pump station | $24,500 | $0 | 0.0% | 5 years | Monthly | Annual | Equal payments; interest remains $0 |
Formula used
- Loan amount (principal): P = (Cost − Down Payment − Upfront Rebate) + Financed Fees + Financed Rebate
-
Payment-period rate from nominal APR:
i = (1 + APR/m)^(m/p) − 1
m = compounding periods per year, p = payments per year.
-
Payment per period (amortizing):
PMT = P · i / (1 − (1 + i)^−n)
n = term years × payments per year. If i = 0, PMT = P/n.
- Effective annual rate (EAR): EAR = (1 + APR/m)^m − 1
How to use this calculator
- Enter the total installed solar system cost from your estimate.
- Add down payment and any lender fees, then choose if fees are financed.
- If incentives exist, enter a rebate and select how it affects the loan.
- Provide the nominal APR, term length, payment frequency, and compounding method.
- Optional: add an extra payment to model faster payoff.
- Press Submit to view payments, totals, and the schedule, then export CSV or PDF.
Financing inputs that drive the loan amount
In construction estimates, the financed principal typically starts as System Cost − Down Payment. This calculator also models lender fees and incentives. Financing a $350 fee adds $350 to principal and increases every future payment. If a $2,000 credit is applied upfront, the loan drops immediately; if it is financed, payments stay higher because interest is charged on that amount.
APR, compounding, and the real yearly cost
Lenders quote a nominal APR, but compounding changes the effective annual rate (EAR). With an APR of 7.2%, daily (365) compounding produces an EAR near 7.465%, slightly higher than the nominal rate. EAR helps compare offers that use different compounding rules.
Term length trade-offs for jobsite budgets
Longer terms reduce the periodic payment but increase total interest. A 12-year term spreads principal over 144 monthly periods, easing cashflow pressure on site operations. For higher APRs, moving from 7 years to 12 years can add thousands in interest, even when the payment looks “manageable.”
Extra payments and accelerated payoff planning
Extra payments reduce principal earlier, shrinking later interest charges and shortening the schedule. Even $25–$50 per period can remove several payments near the end of the loan. Use the amortization preview to see when the balance reaches zero and export CSV for internal review.
Example calculation snapshot for common scopes
| Scope | Inputs | Key outputs |
|---|---|---|
| Site office PV | $18,000 cost, $2,000 down, $350 financed fee, 8.5% APR, 7 years | Loan $16,350 · Payment $258.93/month · EAR 8.839% |
| Warehouse auxiliary | $65,000 cost, $5,000 down, $600 financed fee, 7.2% APR, 12 years, daily(365) | Loan $60,600 · Payment $630.36/month · EAR 7.465% |
| Remote pump station | $24,500 cost, 0% APR, 5 years | Loan $24,500 · Payment $408.33/month · Total interest $0 |
FAQs
1. Does the calculator use APR or interest rate per period?
Enter the nominal APR. The tool converts it into a payment‑period rate using your chosen compounding and payment frequency, then computes the amortized payment and schedule.
2. What is the difference between APR and EAR?
APR is the quoted nominal annual rate. EAR includes compounding effects. If interest compounds more often than annually, EAR is higher and is better for comparing two financing offers.
3. Should fees be financed or paid upfront?
Financing fees increases the loan amount and interest paid over time. Paying fees upfront raises initial cash needs but lowers principal, reducing total interest and the scheduled payment.
4. How should I handle rebates or tax credits?
If you expect the credit to reduce what you borrow, choose an upfront reduction. If the loan covers the full cost and you receive the credit later, choose financed to keep principal higher.
5. Why does payment change when I switch compounding?
Different compounding rules change the effective rate per payment period. Even with the same APR, daily compounding can produce slightly higher payments than monthly compounding.
6. How is extra payment applied?
Extra payment is added to each scheduled payment and goes to principal after interest is covered. This shortens the payoff time and usually reduces total interest in the schedule.
7. Can I use the exports for bid documentation?
Yes. Use CSV for spreadsheet review and PDF for client or internal approvals. The exports include inputs, totals, and an amortization schedule preview to support sign‑off.