Guided Workflow
Step 5 of 9. Follow the sequence to turn a rough idea into a homeowner-ready solar plan.
Calculate monthly solar loan payments, total interest and amortization. Check down payments and financed costs before comparing a loan with a cash purchase.
Quick answer
Payment = principal x monthly rate / (1 - (1 + monthly rate) raised to negative payment count). At 0% APR, divide principal by payment count.
Notes updated 2026-10-07
Author profile: Marcus Vance . Read the methodology and recheck local tariff, quote, and incentive details before deciding. Attribution is not a record of independent technical review.
Wattcrunch Solar Planning Report
Generated 10/25/2026 · Verify assumptions with your utility and installer.
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Compare cash price, financed principal, interest, term, fees and total payments. A lower monthly payment can arise from a longer term or a larger dealer-fee markup rather than a better purchase.
Subtract the remaining utility bill and recurring solar costs from projected energy savings, then compare that cash flow with the payment. Keep production and export assumptions consistent across offers.
For a new homeowner-owned system installed in 2026, the federal Section 25D credit is unavailable. Do not reduce the loan balance by an assumed 30% federal payment. Historical eligibility and unused-credit carryforwards are different questions. Any lender recast requires checking the actual payment date and contract.
| Fixed APR | Monthly payment | Total interest |
|---|---|---|
| 0% | $133.33 | $0.00 |
| 6% | $202.53 | $12,454.61 |
| 9% | $243.42 | $19,816.32 |
Illustrative assumptions: 15-year fixed-rate loan, 180 regular monthly payments, $0 down, no extra principal payments and no federal credit. Include dealer fees in principal before comparing offers. APRs are examples, not lender offers.
Example and calculation notes updated . Method references are listed below; example rates are not verified utility tariffs.
Payment = principal x monthly rate / (1 - (1 + monthly rate) raised to negative payment count). At 0% APR, divide principal by payment count.
A $12,000 loan at 0% APR over 120 months returns $100.00 per month and $0.00 interest. Fees must be included in principal. This 2026 homeowner scenario assumes no federal credit.
Reproducible example generated from the same calculator functions used by the tool. Inputs are illustrative, not measured customer results.
The calculation assumes fixed interest and regular monthly payments. Dealer fees must be included in financed principal.
Use your bill or written equipment quote, run a conservative scenario, and compare the result with the same inputs in another model.
Calculation notes updated 2026-10-07. Reference links explain the method; they do not establish a current utility tariff or installer price.
Read the full methodology or save and compare your solar plan.
The calculator divides its entered annual rate by 12. Disclosed APR may include finance charges and differ from the contractual note rate.
Worked example: Use the note rate and principal to reconcile the scheduled monthly payment, then compare disclosed APR and total repayment separately.
What to verify: The calculator does not calculate legally disclosed APR or interpret a lending contract.
Method reference (not verification of example prices)Planning notes updated October 12, 2026. Example assumptions are not a local tariff.
Treat the result as a planning range, not a promise. Re-run it with a lower and higher production estimate, your actual utility tariff, and the written installer quote.
Use the method references below to check the formula and its limitations. Input defaults and illustrative scenarios are not current tariffs, guaranteed output or live installer quotes.
Utility export rates, eligibility rules, and installer pricing can change. Confirm the applicable tariff, interconnection requirements, and tax treatment before making a purchase decision.
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Compare the same equipment and cash scope, financed principal including dealer fees, APR, payment term and total interest. A lower advertised APR can accompany a larger financed price. Confirm any paydown or recast requirement in the actual contract.
Start with your actual usage, equipment or quote inputs, keep the displayed units, and change one assumption at a time. The calculation assumes fixed interest and regular monthly payments. Dealer fees must be included in financed principal.
Payment = principal x monthly rate / (1 - (1 + monthly rate) raised to negative payment count). At 0% APR, divide principal by payment count.
A $12,000 loan at 0% APR over 120 months returns $100.00 per month and $0.00 interest. Fees must be included in principal. This 2026 homeowner scenario assumes no federal credit.
The calculation assumes fixed interest and regular monthly payments. Dealer fees must be included in financed principal.
Physical kW and kWh calculations use the same units internationally. Dollar results, regional defaults, emissions factors, incentives and utility guidance are US-oriented. Replace rate and production assumptions with local values; do not apply US tax or export rules abroad.
Use Print / PDF to save this page from your browser without an email address. The solar plan stores supported scenario inputs in your browser; private files should be checked and redacted before uploading for extraction.
Compare cash, loan, lease, and battery scenarios using the same production, rate, and quote assumptions before choosing a financing path.
Or browse all calculators, find rebates in the Incentive Finder, or read our solar guides.