Calculate kiln electricity costs: what a firing really costs you

Most studios hang their per-kilo price on a number that once felt about right. What is missing is the other side: what a firing actually costs you. If you want to calculate kiln electricity costs, you do not need last winter’s hunch. You need a formula and a few assumptions you can swap for your own.

This piece stays on the cost side. It does not tell you what to charge. It tells you how to get to the amount below which you lose money. Every figure is a worked example with named assumptions, not a market study and not a tariff comparison.

The formula: calculate kiln electricity costs

The honest version is not simply power times hours. A kiln only draws full power part of the time: close to its rated load while climbing, much less while holding. That is why a load factor belongs in the sum.

Energy used (kWh) ≈
  rated power (kW)
  × firing time (h)
  × load factor

Electricity cost (€) =
  energy used (kWh)
  × electricity price (€/kWh)

The load factor is a rule of thumb, not a measurement. In practice it sits roughly between 0.5 and 0.7: the kiln is not at full load the whole time. Use 0.6 if you have no better number, and replace it as soon as you do.

The only reliable figure comes from a plug-in energy meter on your own kiln, averaged over several firings. Everything else is an estimate that shows the order of magnitude until that measurement exists.

Rated power is on the type plate, in kW. Firing time is from start to programme end, not just the hold. Take the electricity price from your last bill, the working price including taxes and levies, not the wholesale rate.

A worked example

We will run a glaze firing in a mid-size electric kiln. Every input is an assumption. Swap in your numbers; the steps stay the same.

Assumptions for this example:

• Kiln: electric, about 60 litres usable
• Rated power: 6 kW (type plate)
• Glaze firing duration: 10 hours
• Load factor: 0.6 (rule of thumb, as above)
• Electricity price: 0.30 €/kWh (an example, not a market price)

Energy used:

6 kW × 10 h × 0.6 = 36 kWh

Electricity cost:

36 kWh × 0.30 €/kWh = 10.80 €

A bisque firing in the same kiln, shorter and a little lower in load, for comparison. Assumptions: 8 hours, factor 0.55, same electricity price.

Bisque energy: 6 × 8 × 0.55 = 26.4 kWh

Bisque electricity: 26.4 × 0.30 = 7.92 €

Both firings back to back, electricity only: 10.80 € + 7.92 € = 18.72 €. That is what the meter sees for a piece that needs bisque and glaze. Everything that follows does not show up on the electricity bill and still belongs in the cost price.

If your kiln is 9 kW instead of 6, energy scales linearly: 9/6 = 1.5, so 54 kWh instead of 36 kWh in the glaze firing, 16.20 € instead of 10.80 € at the same price and factor. That is why the type plate comes before you touch the per-kilo rate.

What belongs besides the electricity

Electricity is the visible block. Next to it sit four chunks studios often forget in the spreadsheet row labelled “firing costs”.

Kiln depreciation. The kiln is paid for, or it will be. Its purchase price has to be spread across the firings it will do in its life. Example: 4,800 € purchase, 800 firings over its useful life. 4,800 ÷ 800 = 6.00 € per firing. Whether 800 firings fit your kiln is your number. What matters is the calculation, not the amount.

Shelves and props. Shelves crack, props go blunt, a new shelf costs more than it looks. Example: 400 € of replacements over 400 firings, so 1.00 € per firing. Ignore it and you subsidise every firing by exactly that euro.

Labour. Loading, choosing the programme, waiting, unloading, sorting. Take the minutes seriously even if you do not pay yourself a wage. Example: 45 minutes per firing, so 0.75 hours, at 24 €/h as a stated hourly rate (opportunity cost, not a tariff). 0.75 × 24 = 18.00 €. That is often the largest block, larger than the electricity. Set it to zero and you pretend kiln time is free.

Rent for the kiln’s space. The kiln does not stand in nowhere. A share of the studio rent belongs on the firing, or only the open studio carries it. Example: 2.00 € per firing as a rough share. Here too: put in your own figure, the calculation stays the same.

Glaze and materials we leave out here if you pass them on separately. If firing and material sit in one per-kilo price, they belong in the same cost price. Keeping them separate is clearer once you know both figures anyway.

Together for the glaze firing from the example:

• Electricity: 10.80 €
• Kiln: 6.00 €
• Furniture: 1.00 €
• Labour: 18.00 €
• Rent (share): 2.00 €
• Total: 37.80 € per firing

The 37.80 € is not a price to members. It is what this one glaze firing costs you under the assumptions above, before any margin.

From cost price to selling price

Now the conversion that makes the per-kilo rate honest. You need a realistic fill weight, not the type-plate maximum. A half-full kiln costs almost as much as a full one. Use the weight you actually pack in a normal, well-loaded firing.

Assumption: 12 kg of ware when the kiln is reasonably full.

Cost per kilo, glaze firing:

37.80 € ÷ 12 kg = 3.15 €/kg

Check: 12 × 3.15 = 37.80. The division lands.

If the kiln is only half full, 6 kg, the total stays 37.80 €. Per kilo that is 37.80 ÷ 6 = 6.30 €/kg. Same firing, double the per-kilo cost, only because you fired air. That is why studios bill by weight and still watch utilisation.

On top of the 3.15 € comes the margin, plus what you price in for risk: breakage, a refire, the firing that does not fill. How you pour that into a pricing model, per kilo, per piece or flat, is in the guide How pottery studios really calculate firing costs. This piece stops at the cost price. The selling price is cost plus margin, and the margin is a decision, not a formula.

For context: in that guide the stated ranges for a high glaze firing sit around 6 to 18 € per kilo. A cost price of 3.15 €/kg under the assumptions above leaves room underneath. Whether 6 € is enough for you depends on whether your real numbers look like 3.15 €, not on whether the range exists somewhere on the web.

If you do not want to redo the sum every time

You do the sum above once, thoroughly, then only again when the electricity price, the kiln or the fill weight moves in a way you feel. What hurts after that is not the formula. It is sticking it to every piece: which member, which weight, which temperature, which rate.

That is where firing-cost billing starts. You work the cost price as here, set the model up once, and log pieces next to the kiln. The monthly total follows from the firings you logged, not from a spreadsheet nobody opens any more.