Evaporation is also a factor in the heater capacity if we where to figure up the exact heating capacity needed.
Obviously something could be developed that would give the user an approxomation as to how many watts of heater would be needed.
The biggest factors in sizing a heater in my view are the following
- Total water in the system
Temp you want to keep the tank
Temp the tank is above room temp
Does the tank get any sun
How much evaporation does the tank have
I dont think more water current would affect the heater's performance much since the heater is still trying to heat the same volume of water.