Why this site exists
Most energy calculators online are black boxes: you type numbers in, a result comes out, and the assumptions in between — depth of discharge, efficiency, sun hours — are invisible, rounded, or simply wrong. That is a poor fit for decisions that cost real money, like buying a battery bank or sizing an inverter. Watful takes the opposite approach: every calculator shows its formula with your numbers in it, every default assumption is editable, and the assumptions themselves carry named sources and review dates.
How the numbers stay honest
Each calculator's logic lives in a single calculation function that the interface, the example tables and the displayed formula all share — what you see is literally what is computed. That logic is covered by automated tests with worked reference examples, and the planning defaults are sourced from manufacturer documentation and public databases (Victron, Trojan, NREL, PVGIS, Eurostat and others), listed in each calculator's Methodology section. If you find a number that looks wrong, we want to know — that is the point of showing the work.
Who is behind it
Watful is an independent project built and maintained by an engineer from Estonia who works on energy monitoring and control systems — heat pumps, heat-recovery ventilation and off-grid setups — and got tired of re-deriving the same calculations on paper. No company, no content farm; one person and a test suite.
How the site is funded
Right now it is not — there are no ads, no affiliate links and no paid placements, and the "matching gear" suggestions are generic product classes, not sponsored brands. If that changes, affiliate links will be clearly labelled and our privacy policy will be updated first. Recommendations will never be sold: the calculators answer to the formulas, not to advertisers.
Contact
Corrections, questions and suggestions are welcome at the contact address on our privacy page. If you spot an assumption that disagrees with your equipment's datasheet — trust the datasheet, and tell us so we can fix ours.