The wattage should be marked on the old element. It is a good idea to take it to the parts store when you get the replacement. There are both 'high density' and 'low density' hot water heater elements. Low density elements are considered better, they last longer and are less prone to build up. The low density elements have more surface area and therefore do not need to get as hot.
Types of Hot Water Heater Elements
There are three types of hot water heater elements, 'Low Density' and 'High Density' and 'Extra Low Density'. Density has to do with the surface are of the heating element. Low density elements have more surface area. They are also coated and/or made from different materials to resist corrosion.
Low Density Elements
This type of element is made from copper with a magnesium oxide and nickel coating. This type of element provides more heating surface and is less prone to corrosion. Low density elements are a little more expensive than high density types. This style will have a screw type fitting to install it.
High Density Elements
This type is the least expensive and will likely be he type installed originally in your water heater. This type will come in either a screw type or flange type connection. High density elements are the most suceptible to corrosion and failure due to the high heat generated. This type is made from copper and may have a zinc coating.
Extra Low Density Elements
This type is the best element that you can get. Some of them come with a lifetime gaurantee. They are made from high grade stainless steel. They are less prone to build up and will not burn out. These are also the most expensive elements.
You may have trouble finding a low density element with a flange assembly. You can purchase an adapter kit to use a screw in type on a flanged set up. It's up to you. For a screw type element you should not have a problem finding either density. Of course the low density elements cost more. Did you think you would get something for nothing? It's not a lot, a few dollars more for the better one. Make your choice on the density thing, use the salesman in you need to. After that you are ready to get things back together.
The wattage on hot water heater elements can range from 1500 to 5500 watts. You want to get elements that closely match the wattage you have on your hot water heater.