About WattCalc

WattCalc is a collection of free calculators for solar power and household energy. It exists because the arithmetic behind a solar system is not difficult, but it is scattered: panel output in one place, cable sizing in another, battery depth of discharge somewhere else again, each with its own assumptions buried in the small print.

What the calculators do

Every tool on the site runs entirely in your browser. There is no account, no upload and no server processing: the numbers you type stay on your machine. That also means the calculators work offline once the page has loaded.

The formulas are the standard ones used in system design, with the conventional safety factors applied and stated on each page. Where a rule of thumb is involved — and in solar design there are several — the page says so rather than presenting an estimate as a precise answer.

What they are not

These are planning tools. They will tell you whether a design is roughly right, what size of component to shop for, and where a system is likely to fall short. They are not a replacement for a site survey, for a shading study, or for the wiring regulations that apply in your country. Anything that will be permanently connected to a building should be signed off by a qualified installer.

Corrections

If a formula looks wrong or an assumption does not match practice where you are, that is worth knowing. The contact page explains how to get in touch.