The Electrical Safety First socket overload calculator that is embedded into many websites which Electrical Safety First want you to do and actively promote it, is actually hosted on another one of their domains (http://www.twothirtyvolts.org.uk/)
Unfortunately the domain is not using a secure certificate which means that secure browsers like Google chrome will not show the calculator. Not even the Electrical Safety First Website itself! (See screenshot and link https://www.electricalsafetyfirst.org.uk/guidance/safety-around-the-home/overloading-sockets)
If you have this embedded into your site then you will most likely find that it is not working through certain browsers. It may even make your whole site or page show up as not secure in the browser.
I have contacted Electrical Safety First and offered my help to fix this. In the mean time, if you want this to work properly on your website, I have built it on my own server which you can use here: https://www.my-local-trades.co.uk/socket-overload-calculator/
You can also embed it into your website by using the follow code:
<iframe src="https://www.my-local-trades.co.uk/socket-overload-calculator/" title="Socket Overload Calculator"></iframe>