The Solar Panel Payback Calculator Pack — Seven linked calculators, every formula checked by recalculation

Seven calculators for the solar decision: what the system costs after incentives, what it produces and saves, how long it takes to pay back with and without rising power prices, whether a solar loan leaves you ahead each month, and what a change in the export credit does to all of it. Start with sheet 4: in the worked example payback is 14.7 years once prices rise and panels age, against 17.5 on a flat-price estimate. The federal tax credit is set to 0% because the IRS states it is not available for property placed in service after December 31, 2025 -- check irs.gov and a tax pro. Every price, rate and production figure is an example to overwrite with your own quote and bill. These are estimates, not financial or tax advice. Every formula was recalculated and checked against an independent implementation before this shipped.

What is in it

7 calculators, one per sheet. Overwrite the blue cells with your own numbers and everything else follows — nothing is locked, there are no macros, and it opens in Excel, Numbers, LibreOffice and Google Sheets.

• Cost after incentives — What does the system cost you once incentives come off?
• Production and savings — How much electricity will it make, and what is that worth?
• Simple payback — How many years until the savings repay the cost?
• Payback with rising rates — When does it pay back if power prices rise and panels age?
• Solar on a loan — With a solar loan, are you ahead or behind each month?
• Lifetime savings and ROI — Over the life of the panels, what does solar return?
• Export rate sensitivity — What happens to payback if the export credit changes?

Every formula was checked

This pack contains 47 formulas. Before it was published, every one of them was recalculated by a spreadsheet engine and compared against an independent implementation of the same arithmetic written in Python. All 47 agreed to within a cent, and the workbook carries a Verification sheet saying so.

That is an unusual thing to be able to claim about a spreadsheet, and it is the main reason to buy this one rather than download a free template. A formula that looks right and is wrong is worse than no formula at all, because you will act on it.

What each calculator assumes

There is a sheet listing this, because a calculator that hides its assumptions is just a number generator:

• Tax credits are applied to the cost after cash rebates. Whether a credit exists, and whether you can use it, are for irs.gov, your state and a tax professional. Not tax advice.
• Output, self-use share and both rates are your estimates for year one. Fixed monthly connection charges are not reduced by solar and are left out.
• Electricity prices never change and the panels never lose output. Both are wrong; this is the baseline sheet 4 corrects.
• Price rise and output loss are constant every year, and upkeep grows with the savings. Payback is in fractional years with savings arriving evenly.
• Fixed-rate, fully amortizing loan with level monthly payments. A loan that expects a lump-sum paydown (for example from a tax credit) re-amortizes if you miss it; that is not modeled.
• Savings arrive at the end of each year. Inverter replacement, roof work under the panels and resale value are not counted unless you add them to the cost. The discount formula needs the savings growth to differ from the discount rate.
• Simple payback, flat prices -- this sheet isolates the export rate. A negative 'credit needed' means the target is met without any export credit at all.

What this is not

Not financial, tax or legal advice, and not a recommendation to buy, sell, borrow or sign anything. These are calculators: they show what a choice costs, not which choice is right for you. Where a published rate or threshold is used it is the one current when the pack was built — those change, the arithmetic does not.

Honest about the format

One .xlsx workbook, 7 calculator sheets plus Start Here, Assumptions and Verification. Not a PDF, not a physical product, not a Notion template. Available immediately after purchase; nothing ships.

Refunds

If this isn't what you expected, email me within 30 days and I'll refund you in full — no need to explain. Because it's an instant download I can't take the file back, so I'd rather you just had your money back.