Solarific tells you what angle to mount a solar panel at, anywhere in the world. Give it a city, a US zip code, or a UK postcode and you get the fixed year-round tilt, the direction to face, and a month-by-month table if you are willing to adjust the panels through the year. Every figure is computed from the location's latitude rather than looked up in a table, which is why the coverage runs to 72,000+ cities without gaps.
The reason to trust the numbers is that they were checked against something. The fixed tilt comes from a quadratic in latitude, calibrated against PVGIS v5.2 optimal-slope data for 52 cities spanning 1 to 69 degrees of latitude, and it lands within 3.4° RMSE. The tilt-equals-latitude rule repeated on most solar sites scores 7.3° on the same set, and runs up to 14° too steep once you get as far north as Britain or Canada. Monthly and daily angles come from the sun's declination. The full derivation, including the places where a latitude-only model stops working, is on the methodology page.
The data is free in every sense worth having. The whole dataset is CC0 (public domain): download it as CSV or JSON, republish it, or build a product on it, with or without credit. There is no key, no sign-up, and no rate limit. If you would rather show a city's angle on your own site than copy the figures across, there is a free embeddable widget. Solarific has no accounts and no forms, so it never asks you for anything.
The site is independent, based in the UK, and not affiliated with any panel manufacturer or installer. That independence is the whole point: the angles come out of a published formula, so you can check them yourself instead of taking a vendor's word. Running the site costs money, so some pages carry Amazon affiliate links, always marked as such, and Solarific earns a commission on qualifying purchases at no extra cost to you. That never moves an angle by a degree. If you find an error, or you want to use the data for something, email hello@solarific.co.