The cost nobody adds up
A set of elements at 340 dollars with 120 dollars of fitting, over 120 firings, is 3.83 dollars a firing. A thermocouple at 28 dollars over 60 firings is 47 cents. Six shelves at 55 dollars over 350 firings is 94 cents. Add 60 cents of wash and posts and 1.20 of cones and the wear comes to 7.04 dollars a firing. Against 9.50 of electricity when the elements are new, that is not a rounding error — it is nearly as large.
The reason it feels invisible is that it arrives once. Nobody thinks about elements between the day they go in and the day the kiln stops reaching temperature.
Elements fade before they fail
An old set does not simply die. Its resistance rises, the kiln climbs more slowly, the schedule stretches and the bill goes up with it. At four tenths of a percent per firing of age, a firing at the end of a 120 firing life takes 47.6 percent longer than it did new, and the power cost averaged over the whole set is 11.76 dollars rather than 9.50. The all-in figure with the defaults is 18.80 a firing, of which 62.5 percent is power and 37.5 percent is things wearing out.
Where the two curves cross
Spreading a set of elements over more firings makes the set cheaper per firing. Running elements longer makes the power dearer per firing. One falls, one rises, and there is a point where the sum is at its lowest. It sits at the square root of 200 times the set cost divided by the new-firing power cost times the fade rate — with the defaults, 156 firings.
That is further out than the 120 firing life on the form, which means the elements give out before the power penalty catches up, so there is nothing to gain by replacing early. Change the fade to one percent a firing instead and the crossing point drops to 98 firings, which is inside the life, and running to 120 then costs 18 cents a firing more than changing at 98. Over sixty firings a year that is 11 dollars. Real, findable, and small enough that the honest answer is to fix your fade figure before acting on it.
Two firings, not one
A piece that gets bisqued and glaze fired has been through the kiln twice. At 28 pieces in a load and 18.80 a firing, that is 67 cents of firing per piece per pass and 1.34 for both. Whether that is a lot depends entirely on what the piece sells for, and it is one of the few costs in a pottery that is genuinely fixed per piece rather than per hour.
What the numbers cannot be told from outside
Element life is the input that dominates the answer and there is no honest published figure for it, because it moves with the cone you fire to more than with anything else. Two studios with the same kiln, one firing low and one firing high, are not in the same arithmetic. The same goes for shelf life, which ends when a shelf warps or has been ground back too many times. Both come out of a firing log and nowhere else, which is a good argument for keeping one.
Questions people ask
How much does a kiln firing really cost?
More than the electricity. With the defaults here, 7.04 dollars a firing of elements, thermocouple, shelves, wash and cones, plus an average of 11.76 of power once element fade is allowed for, is 18.80 all in. The wear side is 37.5 percent of it. Your own numbers will differ, and the element life is the one that moves the answer most.
How long do kiln elements last?
There is no figure anyone can give you, and this page asks rather than tells for that reason. It depends most on the cone you fire to, then on how long you hold at temperature and how the kiln is run. The only reliable source is your own firing log — count the firings between the last two element changes and use that.
Do old kiln elements use more electricity?
They make the kiln climb more slowly, so the same schedule takes longer and costs more to complete. How much is a question for your own logs: compare the hours of a firing now with the hours of the same schedule when the elements went in. The page takes that as a percentage per firing of age, because that is a number you can actually measure from two log entries.
Is it worth replacing kiln elements before they fail?
Sometimes, and the page works out where the crossing point sits for your numbers. Spreading the set over more firings makes the elements cheaper per firing; running them longer makes the power dearer. With the defaults the crossing point is at 156 firings and the elements only last 120, so there is nothing to gain. Raise the fade to one percent a firing and it drops to 98, inside the life, and early replacement starts paying — by 18 cents a firing, which is 11 dollars a year at sixty firings.
What does a firing cost per pot?
The all-in cost divided by the pieces in the load, and then doubled for anything that gets both a bisque and a glaze firing. At 18.80 a firing and 28 pieces that is 67 cents a pass and 1.34 for both. Packing the kiln fuller is the only lever that moves it, which is a good reason to know how many the chamber actually takes.