small heater

I'm looking for a reliable heater for my 20H. I'd ideally like to get 2 smaller heaters as a safety measure. Is the heater wattage equation 3 watts per gallon? If so, is there really a 30 watt heater out there? Suggestions?

 

For small tanks, I like to use submersible heaters so that they're not as much of an eyesore as non-submersible ones (The chart on this website might help you figure out the wattage you'll need too):

Pro-Heat Titanium IC Heaters - Marine Depot - Marine and Reef Aquarium Super Store

 

Thanks. Looks like I'd need 80 watts of heater to bring the temp up 10 degrees in my 20 gallon tank. Or two 40 watt heaters. But I'm finding that 50 watts are the smallest size heaters out there. Would I be asking for trouble if I put in two 50 watt heaters? I'm definitely going with submersible since they'll be in my sump. Any recommended brands? I've heard good things about Finnex...