Heaters work on a "X" degrees above room temp.
Sooo, if the temp is lower then usual room temp(68-72) you would need even more wattage then normally recommended.
The size of the storage vessel also comes into play.
your 100 watt heater isn't even close to enough.
I use a 300w titanium in one 32g brute and aqueon 250 in another.
The titanium really works much faster.
Finnex Titanum Heaters With Controllers
For the couple bucks I would go with the 500 watt!
I can warm 32 g of tap(54 degrees???) in 3-4 hours easy in my 70 degree basement .
You could also fill 5 g buckets and bring them in to warm to room temp a day early??