I was going to provide a link to the best definition of watts density available on the web, but they are in very short supply, so here goes.
Watts density is the amount of heating energy emanating from any given amount of surface area of the hot part of an element.
In Imperial Britain and the States, this is usually described in terms of watts per square inch, but could be expressed (to keep the Eurocrats happy) as watts per square cm, mm, or indeed, meter. But why bother.
For 8mm diameter elements, simply divide the heating power output (in watts) of the given element by the length of hot section, ie. subtracting the cold ends at either end of the element, which can be 2″, 4″, 6″ or any custom length.
So, for the benefit of a simple example, if we have a 1kW, 54″ element with 2″ cold sections, we first take 4″ (2 ends of 2″) of cold off the total length of the element to give our hot section, 50″. Then we divide the power output of 1000w by 50″ to give us a watts density of 20w/in².
Now imagine we have a 1kW, 104″ element with 2″ cold ends (c/e), giving a hot length of 100″, do the maths as above, and you should get a watts density of 10w/in². Now, picture an inch square in your mind, or draw it if you prefer. Now, fill the box with 10 or 20 little boxes, happy faces, “watt monsters”, whatever crumbles your cookie. The size of the square doesn’t change between the 2 examples, but the size of the contents do, more watts per square inch means more squashed, little watts trying to get out of the same space. Now draw a box, mentally or on paper, and squeeze 40 of your little watt icons in that box. While your at it, have think about how long a 1kW element with 2″ c/e, would need to be to give a watts density of 40w/in²?
Different watts densities are requires for different applications. Some are set in stone, for obvious reasons, such as oil heaters needing to be no more than 12w/in², others are a little more flexible and open to “interpretation”. The gaffer always told me 5w/in² for elements in still air, but as can be seen below, grill elements can work at up to 42w/in². Fortunately, we don’t supply many of them, cos there wouldn’t be much of a guarantee at that top end. We are mainly suppliers of immersion heaters for water, with soft water being fine between 5075w/in², but hard water being better suited to 40w/in² or below.
Our PEA Range is designed for forced air heating units, where the airflow needs to be 2m³/s. The simple way this is achieved is that every meter of element equates to 1kW, so if you want 3kW you have to bend/coil a 3m element into your available space.
Below is a detailed table of watts density in inches and equivalent cm, plus details of colour change and suitable applications.
W/in2 
W/cm2 

94 
14.5 
Immersed only 
83 
12.9 
Immersed only 
73 
11.3 
Immersed only 
63 
9.7 
Immersed / High Arflow / Machined Fit 
52 
8.1 
Immersed / High Arflow / Machined Fit 
42 
6.5 
Equivalent to Grill / Radiant / Orange 
31 
4.8 
Glow Red 
21 
3.2 
Colour Change 
10 
1.6 
Little or no Colour Change Still Air 
9 
1.5 
Little or no Colour Change Still Air 
8 
13.0 
Little or no Colour Change Still Air 
7 
1.1 
Black Heat 
6 
1.0 
Black Heat 
5 
0.8 
Black Heat 
4 
0.6 
Black Heat 
3 
0.5 
Black Heat 
2 
0.3 
Black Heat 
To convert to other element diameters, please divide your solution as above by the following factors. If you think of the 1kW, 54″ element with 2″ cold sections that we started with, only this one is 12mm in diameter, share our 20w/in² initial answer by the factor of 1.484, gives a watts density of 13.48w/in², which makes sense as a wider element will have more surface area for the 1000w to “escape” from.
mm
diameter 
FACTOR 


8 
0.99 
8.5 
1.05 
9.5 
1.175 
10 
1.24 
11 
1.36 
11.5 
1.42 
12 
1.484 
12.7 
1.57 
13.4 
1.657 
16 
1.98 
20 
2.474 
50.8 
6.28 