If you write about headphones, run a shop, or keep answering the same question in a thread, you can drop the power calculator straight into your own page. It is a single line of HTML, it costs your visitors about 14 kB, and there is nothing to sign up for.
Use it however you like, on any site, commercial or not. The only thing I ask is that you leave the link at the bottom in place.
Most forums strip HTML, so an embed is no use in a reply. For that there is something simpler: every calculator on this site can be linked with its answer already filled in. Set it up the way you want, press Copy link above the result, and paste that link into your reply. Whoever clicks it lands on the working answer rather than an empty form.
The links are readable too, so people can see what they are about before clicking:
thephonepath.com/max-spl/?msSrc=Schiit Magni Heresy&msHp=HD 650
That works on every tool here — power, the matcher, hiss, output impedance, balanced, EQ headroom, all of them.
The widget is a plain iframe pointing at this site, so it updates when the database does and there is nothing for you to maintain. It runs entirely in the visitor's browser: no cookies, no tracking, nothing sent anywhere. It inherits nothing from your page's styling and cannot affect it either.
If you would rather have a different tool as an embed — the matcher, the hiss calculator, something else — say so and I will build it. Same for a light-background version if your site is not dark.