I'm one of the creators of http://unicodepowersymbol.com/ - the campaign to get the IEC 60417-5009 power symbol into unicode.
We've successfully got the following characters into the next version of the Unicode specification
⏻ ⏼ ⏽ ⭘ 🌭
At the moment, the "easiest" way of writing the power symbol in HTML is to use the escape code
I propose that the following entity references be created.
This will make it easier for web developers to include these symbols in online documentation etc.