Ehiem rates their 300 Watt heater at 159-300 gallons. What would your experience rate it at?
Toasted one on my 180.
I use 2 - 300 watters these days, one frys, I have a back up till the new one shows up. They are titaniums, but I had Ehiem's in there, they have long been my preferred heater.
If you need to raise the temp say 15 F, vs say 4 F, BIG difference, so ambient temps make a large difference in how you rate things, same issue and even more so with chillers and pull down temp requirements.
It's 68F in the winter at our place and the temp is 84F in the 180 due to the cards......open top, wet/dry filter......
This is the Largest electrical cost for most aquarist also, (heating all that water). Sometimes if you have a few tanks, heating one room is cheaper.
So I rate them depending on the pull up temperature. 300 Gallons? Sure, if the temp is only a few degrees different. 100 Gallon in a cold room? Likely okay.