GridCalcs exists because most free engineering calculators give you a number and nothing else. If you are signing off a design — a cable run, a string layout, a battery bank — an unaudited number is worse than no number. So every calculator here publishes the formula, the constants, the assumptions, and the cases where the model breaks down.
The tools are built and maintained by working electrical engineers in the solar and energy-storage field. They are used daily for preliminary design and cross-checking, and published so you can use them too. There is no account, no paywall, no tracking, and no server: every calculation runs in your browser, and your inputs never leave your machine.
Good. That means you checked the work, which is the entire point. Write to hello@gridcalcs.com with the inputs, the result and what you believe is wrong. Errors with sources get fixed fast.