I found this on the internet at lakesidepottery.com and don't want to loose the information.
Estimating electric kiln firing costs we, at lakeside pottery ceramic school and studio, have been operating electric kilns for years several times each month. Many potters and clay facilities have asked us or used our consulting services to determine what is the actual cost per firing operating an electric kiln. The electric cost is perceived to be their highest concern, yet through our collective experiences, it is the least of our concerns as demonstrated below.
Step 1: kiln power consumption
look at your kiln manual or at label on the kiln’s controller and find the watts value.
It may be indicated in kilowatts or kw. If all you see is amps and voltage, multiply the number of amps by the voltage to get the watts value. Then, divide the watts by 1,000 to get the kilo watts value (e.g., 48 amps x 240 volts = 11,520 watts / 1,000 = 11.52kw). * for the example below, let’s say that the kiln power consumption is 11.52 kw.
Step 2: firing duration
clock the duration of firing. With computerized kilns, measure the candling or preheating time prior to bisquing (remove moisture) separately. * for the example, let’s say that the kiln was preheating at 180 degrees f for 7 hours and bisqued fired for 9 hours.
Step 3: duty cycle
the kiln's elements are not engaged for the full duration of firing and are powered only sometimes as needed to maintain the proper firing curves. The terminology for this is duty cycle. For example, if the elements are powered constantly, duty cycle is 100%. If they are powered half of the time, duty cycle is therefore 50%. 1. Duty cycle for low firing or bisque firing (cone 04 or 06) is about 50%
2. Duty cycle for glaze firing (cone 6) is about 65%
3. Duty cycle preheating (180 degree f) is about 15% note that duty cycle is also effected by:
1. Kiln’s load mass (number of pots, number of shelves, shelves thickness).
2. Firing target temperature and hold up time.
3. Kiln’s brick thickness (3” inches is more efficient than 2” brick).
4. Kiln element’s age (the older they are, the longer they have to be on to reach temperature).
5. Kiln room temperature.
6. Whether or not envirovent is used (when used, some efficiency is lost). * for the example, the duty cycle is 15% for 7 hours (preheating), 50% for 9 hours (bisquing) and 65% for 10 hours (glaze firing).
Step 4: your electric cost per kwh
electric bills can be confusing with all the different charges (generation, supplier and delivery charges). The easiest way to estimate the final cost per kwatt / hour (kwh) that includes all charges, take your total monthly charge and divide by the total kwh used to get the cost per kwh. * for the example, the total monthly bill cost due is $275 and the total kwh used is 1,527 kwh. Therefore, the cost per kwh is 275/1527 = .18 (18 cents per kwh). Note that in the summer, cost per kwh can almost double.
Step 5: calculation of firing electricity cost
kiln kwatt x firing hours x duty cycle x cost per kwh
- preheating: 11.52 x 7 x 0.15 x 0.18 = $2.18
- bisquing: 11.52 x 9 x 0.5 x 0.18 = $12.35
- glazing: 11.52 x 10 x 0.65 x 0.18 = $13.48
- total cost for candling, bisque (cone 06) and glaze firing (cone 6): 2.18+12.35+13.48 = $28.00
step 6: other firing costs
- fans: use the same calculation as above to figure out operating fans costs (about $1.00 for 24 for two fans)
- kiln cost: let’s assume that your kiln purchase cost is $2500 and lasted 400 firings. Cost per firing will therefore be $6.25
- kiln maintenance: kiln shelves, elements, relays and thermocouple require replacement approximately every 100 firings (150 firings if used for low fire only). If you are able to use your own labor to replace parts, the cost for the above parts is about $375 (add $250 for labor). Cost per firing is therefore $3.75 or $5.75 (w/labor) per firing.
- space used: if you rent space, monthly rental cost for the kiln room will add to “per firing cost”. Let’s say your kiln room is 9x6’ = 54 sq ft. For example, if your rent is $2 a sq ft per month, the kiln room cost is $108 per month. If you fire 5 times a month, cost per firing is $21.60
- insurance: insurance cost for a house or a facility can double in some cases due to having a kiln. In our case (lakeside pottery), insurance cost increased by $1,200 a year. At a 5 times a month firing rate, the cost per firing will be $20.00
- misc other costs: kiln shelves maintenance (labor, parts and material), kiln wash, witness cones. For this example, let’s assume $2.00 per firing
if all variables are added up using the information above with a real situation example, cost per firing is way more than just the electric cost. For example, electric cost per glaze firing is $13.48. But, the total cost taking in all variables, adds up to $70 per firing ($13.48 + 1 + 6.25 + 5.75 + 21.6 + 20 + 2