About

A payback number is somebody's spreadsheet

A payback period is not a fact about a product. It is the output of a spreadsheet, and whoever built the spreadsheet chose every number that went into it.

That is easy to say and hard to act on, because the inputs are rarely shown. A proposal arrives saying the system pays for itself in eight years. Somewhere behind that sentence sits an annual production estimate, a value per kilowatt-hour, an assumption about how fast electricity prices rise, an assumption about how fast the panels lose output, and a decision about how to treat the money if it is borrowed. Move the utility rate escalator from 2% to 4% and the eight years become six, with nothing about the roof having changed. The customer is shown the six.

Three questions that sent me to the tariff sheet

Three questions sent me looking for the arithmetic, and they turned out to be badly served:

What is a kilowatt-hour I export actually worth? Not the retail rate on the bill — the rate the tariff pays for exports, in the hour it leaves the house. Under a net billing tariff that number can be a fraction of what the same kilowatt-hour costs to buy, and it varies by hour and by month. Nearly every payback explanation on the web still quietly values exports at retail, which is the assumption that makes solar look best.
What did the end of the federal credit do to the number? The Residential Clean Energy Credit under 26 U.S.C. § 25D covered 30% of a qualifying system and ran out for expenditures made after 31 December 2025; the Energy Efficient Home Improvement Credit under § 25C ended the same day. Removing 30% from the cost side does not lengthen a payback by 30% — it lengthens it by however much depends on where the other assumptions sit. While researching this, I found pages ranking at the top of ordinary searches that still described these systems as eligible for a 30% credit months after both had expired. Anyone doing this sum from what the web says is doing it wrong.
Why do three quotes for one roof disagree? Same address, same south-facing plane, paybacks of seven, nine, and fourteen years. The gap was never in the hardware. It was in production estimates, degradation rates, escalation rates, and a dealer fee buried in a financed price that the cash price did not carry.

So each page here takes one input, shows where the official number for it is published, and works the arithmetic in the open. Nothing on this site tells you the answer for your house. It tells you how to get it, and how to see which assumption a stranger's answer is resting on.

No licence, no commission, only arithmetic

I'm Priya Raghunathan. This started as a spreadsheet for a decision I was making about my own house, kept growing footnotes, and eventually became easier to publish than to keep explaining.

A word about my standing, because in this subject a claim of expertise is usually the opening of a sales pitch. I do not work in the energy industry. I am not an installer, an electrician, an engineer, a certified energy auditor, a tax adviser, or a financial adviser. I hold no licence and no certification. I have never sold, financed, or installed a system, and I take no commission, referral fee, or advertising money from anyone who does — the ads on this site are placed automatically by Google and I have no idea which company will appear in them.

What I offer instead is arithmetic you can check. Every calculation is written with its inputs listed and its formula shown, so you can substitute your own utility's numbers and get your own answer. Where a page uses a rate, a credit rate, or a code requirement, it names the document — the tariff schedule, the commission decision, the code section — and links to it. A figure I cannot point you at the source for has no business being inside a payback sum in the first place.

No figure without the document behind it

The rule is that a figure has to come from the body that sets it, not from an article about it. In practice that means, in this order:

Your utility's own tariff sheets. Rate schedules, net metering and net billing tariffs, interconnection tariffs, and time-of-use period definitions are published documents. They are the only authority on what your kilowatt-hour costs and what your exported kilowatt-hour earns, and they differ enough between utilities that a national average is close to useless for one house.
State regulator decisions. Where export compensation or a rate design has been changed, the change lives in a public utility commission proceeding — California's net billing tariff, for example, comes out of a CPUC decision with the reasoning and the schedules attached. Pages here cite the decision rather than the news coverage of it.
Federal technical sources. NREL's PVWatts documentation for production modelling, including what its default loss and inverter assumptions actually are, and Department of Energy and EIA material for fuel prices and equipment performance.
DSIRE and the programme administrators themselves for state and utility incentives, with the programme's own page checked for remaining budget, because a rebate that has exhausted its funding is worth nothing in a payback calculation no matter how many sites still list it.
Standards and code bodies — UL and NFPA for battery listing and installation rules, AHRI for how heat pump performance is rated — where the question is what a piece of equipment is permitted or certified to do.
The IRS's own text for anything about a federal credit. Pages quote the code section and the agency page, note the date read, and stop there. Tax strategy is somebody else's subject.

Each rate stamped with the day I read it

Every page shows when it went up and when it was last re-read, and each rate is stamped with the day I took it off the schedule. Tariffs and incentive budgets run on calendars that have nothing to do with mine, and here an out-of-date page is worse than no page at all: it hands you a confident number that is wrong.

What stays outside the spreadsheet

It is not tax, financial, or investment advice, and it does not tell you whether to buy anything. It is arithmetic and sourcing.
It does not review your proposal, price your roof, or tell you whether a quote is a good one.
It does not recommend or rank installers, lenders, or equipment brands, keep a directory, or take a fee from any of them.
It is not about broken equipment. Repair quotes, contractor selection, permits, and what a failed system should cost to fix are a different problem with different sources, and this site stays out of it.
It does not assume the answer is yes. Plenty of these calculations come out saying that air sealing pays back faster than panels, or that a battery is a comfort purchase rather than an investment. The point is to be able to tell.

Found an error in the arithmetic?

The figures here rest on documents that get revised without warning: a rate schedule is replaced, a rebate exhausts its budget, a newer commission decision supersedes the one a page cites. If a rate, a formula, or a code reference does not hold up when you check it against the source, send it my way with the figure named. A corrected page shows the old value beside the new one and the date it moved, because a number that changes while nobody says so is the exact habit this site was built to argue with.