Solar Loan Calculator

How to use this calculator

Enter four numbers and the calculator returns your monthly payment, total paid over the term, total interest, and year-1 net monthly cash flow (savings minus payment):

1. Loan amount — the financed cash-equivalent cost, including any dealer fee. Do not accept a 2026 payment schedule that assumes a later homeowner Section 25D lump sum. EnergySage’s H2 2025 median implies about $17,430 for a 7 kW cash system at $2.49/W before roof-specific adders; compare that with the total amount financed.
2. APR — the annualised rate from your loan estimate. Q1 2026 ranges: GoodLeap 6.49-8.99%, Sunlight Financial 5.99-9.99%, Mosaic 6.99-9.49%, Dividend 7.49-9.99%, local credit unions 5.49-8.49% (often the best deal for owner-occupants with 720+ FICO).
3. Term — most solar loans are 10, 12, 15, 20, or 25 years. Shorter = less total interest. Longer = lower monthly payment. Use 12 years for the conservative middle of the market.
4. Year-1 monthly bill savings — the average dollars your solar offsets per month in year one. Compute as: annual production × your retail rate ÷ 12. Example: a 7 kW system in Phoenix produces 12,000 kWh × $0.13 = $1,560/yr = $130/mo.

How the math works

Solar loans use standard amortising-loan math (the same formula as a mortgage or auto loan):

monthly_payment = P × r / (1 - (1 + r)^-n)
where:
  P = principal (financed amount, in dollars)
  r = APR ÷ 12 (monthly rate, decimal)
  n = term in months

Worked example for the en-us defaults ($18,000, 7.99% APR, 12 years):

• r = 0.0799 / 12 = 0.006658
• n = 144 months
• monthly = 18,000 × 0.006658 / (1 - 1.006658^-144) = $194.81/month
• Total paid = 194.81 × 144 = $28,053
• Total interest = $28,053 - $18,000 = $10,053

Net cash flow = bill savings - payment. With the default $133/mo savings, net = $133 - $195 = -$62/mo year 1, which improves to roughly $20/mo positive by year 8 once retail rates have escalated 25-30% above today’s level.

What “positive cash flow” actually means in 2026

Most marketers claim solar produces “positive cash flow from day one.” That is true in California (PG&E Tier 4), Hawaii, Massachusetts, and Connecticut at current 2026 rates, where retail electricity is high enough that even a 25-year loan at 7.99% leaves $30-100/mo positive in year one. It is not generally true in Idaho, Washington, Louisiana, or Tennessee where retail rates sit at $0.10-0.13/kWh and a financed system shows -$40 to -$80/mo until year 5-7 when rate escalation crosses the loan payment line.

The honest framing is cash flow, not a promised tax windfall: compare cumulative bill savings with every loan payment, dealer fee, maintenance cost and verified incentive. A financed system may or may not become cumulative-positive within its useful life, especially where export compensation is low.

Loan vs. cash vs. lease — quick comparison

An owned system can beat a 25-year lease because the homeowner retains any verified state or utility incentive, avoids a lease escalator, and keeps the bill savings after the loan ends. Do not count commercial depreciation or a new Section 25D credit in a 2026 homeowner comparison.

What changes the loan economics

Helps (cheaper loan, faster cash-flow positive)

• Credit-union loan or HELOC at 5.5-6.5% rather than dealer-aggregator 7.5-9% products
• No dealer fee — pay attention to APR + dealer fee, not just APR
• Apply only money you actually receive — if the lender permits re-amortisation, a verified state or utility rebate can reduce principal without relying on a nonexistent 2026 homeowner credit
• Battery + TOU rate to capture peak-rate arbitrage in a NEM 3.0 state
• State property-tax exemption (38 states + DC; see DSIRE for your state)

Hurts (more expensive loan, slower payback)

• Dealer-fee buy-downs — a “1.99% APR” loan with a 30% dealer fee on a $20k system effectively costs the same as a 7-8% loan with no fee
• A payment schedule built around a future tax-credit lump sum — Section 25D is unavailable for post-2025 homeowner expenditures, so the payment may rise sharply if the assumed contribution never arrives
• Demand charges or NEM 3.0 export-only credit that cuts year-1 savings by 30-50%
• A roof that needs replacement within 10 years — re-roofing forces panel removal/reinstall, typically $2,000-4,000

Pair this with the payback calculator, system cost calculator, and ROI calculator

The loan calculator answers “what does the monthly payment look like?” Payback answers “when do I break even on the net cost?” ROI answers “what’s my total lifetime return?” Cost gives you the up-front capital to plug in. Run all four — and verify your APR with at least three lenders (one credit union, one bank, one solar-aggregator) before signing.

Sources

• EnergySage Solar Marketplace Report H2 2025 — installed cost, financing mix, APR ranges
• DSIRE database (NC State) — state and local incentives, including loan programs
• Consumer Financial Protection Bureau — Solar Loan Considerations — dealer-fee disclosure rules
• IRS — Residential Clean Energy Credit — Section 25D termination after 31 December 2025 and rules for qualifying earlier expenditures
• SEIA / Wood Mackenzie U.S. Solar Market Insight — financing-product market share