Formulas used

• Monthly payment: M = P[r(1+r)^n] / [(1+r)^n − 1], where r = annualRate/12, n = months
• Total payments: Monthly payment × months
• Total interest: Total payments − principal
• Break-even: Upfront costs ÷ monthly savings
• APR (approx): Effective rate including points and fees solved via IRR on payment stream