How to size a heater??

[Fishinwall]

Is it best to oversize an aquarium heater or undersized and have two heaters?
I planned on putting two in as a backup anyway.
I have a 65 gal display with a 20 gal sump.
Was planning on buying a Eheim jäger but wasn't sure if I should buy 150 or 200 watt.
Thanks.

 

[preston469]

I would over size, I have the same setup but my sump is a little larger, I use the 200 Watt and it's been great never hurts to have a backup.

 

[Texas Reefer]

I have 2 undersized heaters, one in the sump and one in the DT. I've heard stories of heaters getting stuck in the "on" position and cooking a tank. The way I'm doing it, any heater getting stuck "on" will have less of an affect, and my temp is a very stable 78.7.

 

[Fishinwall]

Thanks I think I'm going to go with two 150w...they should fit my sump better too.