I agree - use a non breakable design like the titanium models. As for wattage - that largely depends on how stable the temp is in the area where the tank is kept. If the room gets very cold 500W may not be adequate, on the other hand if the room has minimal temp swings 250W should be fine. Thermodynamically, water holds a high calorie (heat) content liter for liter and is not prone to rapid temp changes compared to other liquids.