Whatever power immersion you buy, remember that it should be a marine type. If I remember correctly (I'm sure someone will correct me otherwise, I made a mental note at the time as I was thinking of fitting an external immersion), domestic immersions have a secondary over-temperature thermostat which will trip at something like 70 C and need manually resetting. A marine immersion has a secondary thermostat which trips at a higher temperature to cope with engine heating (will it auto-reset?)