Watt density (WPSI) is a term used in the heating industry that describes the amount of permissible wattage over a surface area of a heater. Water has a typical watt density of 65 WPSI where as Oil would have much less because of its viscosity. When ordering a heater, it is always important to indicate to our sales group the medium for which you are heating.
Typically when you are heating unclean filmy water it is important to maintain the cleanliness of your tank and consider watt density as a way to help prolong your heaters life. If deposits are in contact with the tubular element, it will stick to the sheath and will eventually lead to failure because the heat from the inner tubular coil cannot dissipate into the liquid medium easily.