Frequently asked questions
Questions people ask — or should ask! For the full model and every fixed assumption, see How it works.
Is home solar a good investment?
Let's be clear: solar is almost always a good investment from an environmental standpoint. If you can afford it, do it. Support electrification and the solar transition as much as possible. In my view, the jury is in on this one.
That said, I built this calculator to answer a harder question, which relates to solar as a financial investment. It isn't strictly an investment as much as it is a long term savings tool, but we can compare it to what we would do with that money if we weren't buying solar. Each situation will be different. I would urge you to compare apples to apples in one way though: solar is a stable, long term "investment." So compare it with another similar low-risk investment.
Is this ROI?
No. Simple ROI is (lifetime savings − cost) / cost, which ignores when the savings arrive — and when they arrive is the whole question. A system that returns its cost over forty years and one that returns it over eight look identical on that measure.
What you get instead: payback, the year your savings cover the cost; an equivalent return, the yearly yield your savings pay on the cost within your chosen years, counting the system as worth $0 at the end; and the future value of your savings reinvested, set against simply investing the cash. A negative equivalent return means the system doesn't pay for itself within those years. It isn't a tax loss.
How is charging an EV with solar valued?
As electricity, at whatever you'd have paid the utility for it. A kilowatt-hour that charges the car is worth exactly what one that runs the dishwasher is worth.
This used to work the other way. The model credited solar with the full price of the gasoline avoided, which made a solar-charged mile look like $0.50–0.70/kWh against a $0.39 retail rate. A reader pushed back, and they were right. Gas savings come from owning an electric car, not from owning panels, and you get them whether you charge off your roof at noon or off the grid at midnight. They belong to the car. More on Solar + EV.
The car still moves solar's number, twice over. Its kilowatt-hours are real load, so it changes what size system makes sense. Charge in the middle of the day and it eats production that would otherwise leave at six cents. What switching from gas to electric saves you gets its own line, plus a bundled figure if you're deciding on both at once.
What is “net billing”, and how is it different from net metering and zero export?
Under full net metering, every exported kilowatt-hour is credited at (or near) retail — your meter effectively runs backward. Under net billing (California's NEM 3 and many newer programs), exports are credited at a much lower avoided-cost rate while the power you draw still costs full retail. That gap is why batteries, roof orientation, and daytime usage matter far more under net billing. Zero export means the system never pushes to the grid; anything you can't use or store is thrown away.
What discount / return rate should I use?
Use your genuine opportunity cost for money of similar risk. Solar savings are low-risk and bond-like, so a conservative figure is appropriate — the current 10-year Treasury yield is the conservative choice I use. Using a high stock-market return here holds solar to a bar it was never trying to clear, and don't hold a pre-tax stock assumption against a tax-free savings stream. The calculator applies tax only to the cash side, since avoided utility bills aren't taxed.
How fast will my electricity rates actually rise?
The calculator defaults to a deliberately conservative 5%/yr. For reference, PG&E residential rates have risen roughly 6.75%/yr annualized over the last decade, and PG&E's rate-case filings project further increases of about 8% in 2027 and ~6%/yr from 2028–2030. Your own utility and tariff will differ — enter what you can defend from your bill history.
Does the calculator include the 30% federal tax credit?
Not automatically — and it no longer exists for residential systems. Enter your net system cost if you want the result to reflect an incentive you expect to receive, or your gross cost if you'd rather see the unsubsidized picture. Incentive rules change frequently; entering the number yourself keeps the result honest about what you actually assumed.
What doesn't this calculator account for?
- Your roof's shading. With a quote, the installer's production figure already includes it; without one, you enter a percentage beyond the standard 14% losses built into PVWatts. I expect this will just be a guess.
- The model is built on one typical weather year, so a not so typical year will be different.
- Loans, leases and PPAs: this models a cash purchase, but you can gain some insights from that.
- Demand charges and usage tiers below your top rate.
- Fixed monthly charges, on purpose: solar can't reduce them, and utilities are increasingly turning to them.
- Rules that cap additions to a legacy net-metering system or limit exports. Check the local rules before assuming a system you model here will be allowed.
- Charging a battery from the grid, and what backup power is worth to you.
- Other fees outside the quote, like roof work, insurance and maintenance.
Can I use my own electricity data?
Yes, if your utility offers Green Button downloads (PG&E, SCE and SDG&E do, and many others). Download a year of your electric usage as XML and load it in the calculator, under Household usage. It then runs on your own hours instead of homes like yours, which in my tests on 50 real homes landed within half a percent of an exact calculation. Your browser reads the file: it isn't uploaded or saved. How to get the file.
Can I share a scenario with someone?
Yes. The “Create shareable link” button encodes every input into the URL. Send that link and the recipient sees exactly your scenario, which they can then adjust. Nothing is stored on a server — the state travels in the link itself.
Who built this and can I trust the numbers?
It's built and maintained by David St. Martin, who has installed solar, got it wrong the first time, and did the research since. I believe we're a better community when we share what we know and help each other, each in our own way. Every fixed assumption is listed on the How it works page with a link to its source where one exists; the rest are labeled as modeling estimates. The footer shows when the assumptions were last reviewed. If you find something broken or an edge case that misbehaves, please tell me through the feedback form.