Hello Rainy.
A 250 watt heat lamp is going to cost you about £25 to brood 24/7 for 6 weeks, if you keep them under a lamp till then.
You would benefit from running more than 20 chicks under it to bring the costs down,(which works out just over £1 each till off heat).
This would depend on where you keep them and how well insulated the chick room is.
Using a red heat emitter gives the chicks 24 hour light to feed..they will sleep ok with it on...A dull emitter will give heat but no light so they are subject to daylight feeding..So short daylight hours restrict the amount of feeding they can do...I use these if the daylight is at least 12-14 hours, even then I keep my eye on them.
I have never used an electric hen but have in the past used haybox brooders...
Have a look at my web page there are a few notes on the blog about breeding.www.castlefarmeggs.co.uk