Energy calculator

Kiln firing cost, and where the energy goes

Cost per firing is energy per firing multiplied by your fuel price, and the energy has three parts. First, heating the kiln itself: brick and fibre have mass, and taking that mass from room temperature to cone 6 absorbs a large amount of energy that you never get back. Second, heating the ware and the shelves. Third, the heat leaking out through the walls for the whole length of the firing.

That third part is the only one most calculators model, which is why their numbers come out low. The steady-state loss is what it takes to hold temperature, and a firing spends most of its hours climbing, not holding. This tool integrates the loss across the ramp, adds the thermal mass of the wall you specified and of the load you entered, and divides by the efficiency of your heat source.

Take the calculator's default: a 46 cm (18 in) cube chamber with a 6.5 cm (2.5 in) K-23 brick wall backed by two 5 cm (2 in) layers of ceramic fibre, fired to cone 6 over eight hours. It takes 29.0 kWh. Of that, 19.2 kWh goes into heating the wall, 3.2 kWh into the ware and shelves, and 6.6 kWh leaks out through the walls. A 200 to 280 litre (7 to 10 cubic foot) studio kiln with the same wall takes roughly 45 to 60 kWh. Multiply by your own tariff for the cost, because electricity prices vary more between regions than kilns do between designs.

Kiln wattage: how much power an electric kiln needs

For a studio electric kiln, somewhere around 60 to 70 W per litre of chamber (1.0 to 1.2 W/in³). The calculator's default, a 46 cm (18 in) cube holding 96 litres, comes out at 5.7 kW, about 24 A at 240 V.

There is no single correct answer, so the tool shows you four and lets you compare them. The physics minimum is the steady-state loss plus a margin, and it is always lower than what anyone actually builds, because a kiln sized to its heat loss would take days to climb. Real sizing rules are empirical: Olsen's roughly 1 to 1.2 watts per cubic inch of chamber, the same by surface area at 5 to 7 watts per square inch, and the medians from a survey of 60 commercial electric kilns.

The recommendation the tool gives you sits between the practical minimum (about three times the steady-state loss) and the lowest of the industry rules. It also converts that to amps at 240 V, which is usually the number that decides the project, because the supply you can get to the kiln is a harder constraint than anything else on this page.

How much insulation does a kiln need?

Enough to keep the outside of the kiln below about 70 °C at top temperature. Above that the shell is a burn hazard, which is an insulation problem before it is a fuel problem. Past that point, more brick keeps lowering the loss but starts raising the cost of a firing, because every centimetre of wall is more mass to heat.

The calculator shows it plainly. A 200 litre (7 cubic foot) kiln fired to cone 6 over eight hours takes 48.8 kWh with a 6.5 cm (2.5 in) wall of bare insulating brick and 52.9 kWh with an 11.5 cm (4.5 in) one, although the thicker wall cuts the loss at temperature from 7.1 kW to 4.5 kW. Put two 5 cm (2 in) layers of ceramic fibre behind the thin wall instead and the firing drops to 45.8 kWh, with the shell at 73 °C. Fibre insulates well and weighs very little. The model assumes each layer reaches its full temperature gradient by the end of the firing, which a thick wall on a fast schedule may not, so read these as a comparison between walls rather than a meter reading.

R-value alone will mislead you here, for two reasons the tool handles explicitly. Conductivity is not constant: insulating firebrick conducts roughly twice as much heat at 1260 °C as at 260 °C, so a wall rated on a room-temperature figure looks far better than it performs. And a thick wall on a small chamber does not conduct like a flat slab, because the heat-flow area grows from the inside face to the outside. The solver uses conductivity at each layer's own mean temperature, and counts the thick wall either by mean area or by Langmuir's furnace shape factors, your choice, with the other method's figure shown alongside.

Kiln fuel consumption: electric, gas and wood compared

A gas kiln burns about twice the energy of an electric kiln of the same build, and a wood kiln about five times. The chamber loses the same heat whatever is heating it; what changes is how much of the fuel reaches the chamber. Electric resistance puts nearly all of its input there. A studio gas or propane burner delivers about half, the rest leaving up the flue. Wood is nearer a fifth, which is why wood firings consume quantities that surprise people coming from an electric kiln.

Gas kilns are sized on burner capacity rather than on heat loss. Olsen's rule is 10,000 BTU per cubic foot per hour for insulating brick, and 16,000 to 19,000 for hard brick, which is capacity at full turn-up rather than what you burn while cruising. Wood is scaled from Olsen's fastfire: about a quarter cord per 18 cubic feet to cone 10.