Solar estimates have a credibility problem. The average online solar calculator is a lead form wearing a tool costume — it asks for your address, refuses to show math, and emails your number to three installers. SolarCalc was built as the opposite: every assumption printed, every formula in the open, no address required and nothing to sign up for.

The estimate is the product

Type a panel count or a roof area and the page answers the questions that actually matter: how big, how much it makes, what it saves, how long until it’s paid for itself. Behind the single result sits a model we state in plain view — NREL-style peak-sun-hour ranges you can override, a loss stack that compounds the way physics does (each loss on what’s left, not on the nameplate), and a payback line that escalates your electricity rate while your panels quietly degrade.

Facts with dates on them

Solar math drifts. The biggest example is the federal tax credit: the 30% residential credit ended for installations completed after December 31, 2025 — a change most of the solar web still hasn’t caught up with, because “30% off” converts better than “no credit under current law.” This site defaults to the law that exists, labels the legacy option for systems that genuinely qualify, and dates the article that explains it (current status). Same discipline everywhere: presets are cited as approximations, ranges are printed as ranges, and anything that expires gets an expiry date.

What we deliberately don’t do

No quotes, no lead-gen, no installer marketplace, no “enter your address to continue.” The calculator runs entirely in your browser and can’t send your numbers anywhere — there is nowhere to send them. Roof surveys, structural checks, interconnection paperwork and bids all belong to professionals with boots and licenses; a calculator’s job is to make you numerate enough to argue with a quote.

The family it lives with

SolarCalc sits on nxtolerance.com beside sibling tools like PowerBill (what your appliances cost to run — the natural input to solar sizing) and EVCharge (charging time and cost with the taper modeled). One question per site, honest arithmetic, nothing phoned home.

Corrections welcome

Irradiance data updates, incentives change, tariffs get renamed and repriced. If a preset, a range or a dated status contradicts a current primary source, the contact page is the fix path — corrections citing a source are the best mail this site gets.