Frequently asked questions
The questions people ask before they trust the numbers. For the full model and every fixed assumption, see How it works.
Is home solar a good investment?
It depends on four things this calculator makes you enter explicitly: how your utility credits exported solar, how much of your production you actually use or store, the price you pay for grid power and how fast it is rising, and the return you could get by investing the same money instead. Under full net metering with rising retail rates, solar often clears a conservative investment bar. Under net billing (California's NEM 3 and similar), the answer hinges on self-consumption and a battery. The tool reports an equivalent return so you can compare it head-to-head with your own opportunity cost.
How does this calculator compare solar to investing the money instead?
Both sides are measured the same way. The cash side grows the system's price at your expected after-tax return, compounded. The solar side is the yearly bill savings — the electricity you stop buying, plus the gasoline a solar-charged EV avoids — reinvested each year at that same after-tax return. Because avoided costs are not taxed but investment gains are, the cash side clears a slightly lower after-tax bar, and the model accounts for that. Whichever pile is larger at your chosen horizon wins.
What is solar “capture” and why does it matter so much?
Solar only earns full value for the kilowatt-hours you use on site or store in a battery. The rest is exported — at retail under full net metering, at a low “avoided cost” rate under net billing, or at nothing under zero-export rules. Capture is the share of your production that earns full value. Under net billing it is the single biggest lever on the result, and it is driven by battery size, roof orientation, and whether large loads (like EV charging) run while the sun is up.
How is charging an EV with solar valued?
Not as avoided electricity — as avoided gasoline. If a solar-charged EV replaces a gas car, each solar kilowatt-hour that charges it displaces gas. The model sizes the EV's yearly need as miles ÷ MPGe × 33.7 kWh (33.7 kWh is the energy in one gallon of gasoline, the EPA gasoline-equivalent figure) and values that energy at the gas it avoids: (miles ÷ MPG) gallons × your price per gallon. Spread across those kilowatt-hours, a solar-charged mile is typically worth about $0.50–$0.70/kWh — well above a $0.30–$0.40 retail rate.
What is “net billing” / NEM 3 and how is it different from net metering?
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 most conservative common choice. Using a high stock-market return here holds solar to a bar it was never trying to clear. The calculator applies tax only to the cash side, since avoided utility and fuel costs 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. 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; entering the number yourself keeps the result honest about what you actually assumed.
What doesn't this calculator account for?
It is a planning model, not an hour-by-hour simulation. It uses a representative day per month rather than 8,760 hourly intervals; it assumes flat monthly household load (no summer-AC spike); it folds partial-peak TOU periods into two buckets; and the added-panels / second-surface feature is a rough scaling estimate, not an interconnection study. It does not model financing, maintenance beyond a warranty-length horizon, roof work, or insurance. Verify anything material against a real quote and your own bill.
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 one person who has installed solar, got it wrong the first time, and did the research since. 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 get in touch — see About.