How to use this calculator
This tool calculates the standard amortizing-loan payment formula M = P · r · (1+r)n / ((1+r)n − 1), where M is the monthly payment, P is the principal, r is the monthly interest rate (APR ÷ 12), and n is the total number of monthly payments.
Three numbers to compare
- Monthly payment is the number salespeople lead with. Lower isn’t always better — a 25-year term lowers the monthly but raises lifetime interest dramatically.
- Total interest tells you what the loan actually costs. A 10-year term can cut this in half compared to a 25-year term on the same principal and APR.
- Total paid is principal plus interest — the full lifetime cost of the system after financing.
What this calculator doesn’t show
- Dealer fees folded into the principal. If your installer is using a $5/W dealer fee, your “loan amount” here should be the financed price — typically thousands more than the cash price.
- Re-amortization. If your loan re-amortizes after an expected tax-credit payment, the monthly shown here applies only if you make that lump-sum payment. Otherwise the loan recalculates.
- The federal tax credit. The credit reduces your federal tax bill in the year of installation. Subtract roughly 30% of the cash system price from your true lifetime cost if you can use the credit.
Comparing term lengths
For a $30,000 loan at 4.99% APR, here’s how term length affects lifetime cost:
| Term | Monthly | Total interest | Total paid |
|---|---|---|---|
| 10 years | $318 | $8,160 | $38,160 |
| 15 years | $237 | $12,660 | $42,660 |
| 20 years | $198 | $17,496 | $47,496 |
| 25 years | $175 | $22,560 | $52,560 |
| 30 years | $161 | $27,925 | $57,925 |
Choosing 25 years over 15 years saves you $62/month but costs an extra $9,900 over the life of the loan.