Your element isn't the power on the label

Why the label isn't the whole story

A heating element is a resistor. Its rating - say 3,000 W - holds only at its rated voltage. At any other voltage the power follows a simple law: power equals voltage squared divided by resistance. The resistance hardly changes, so the power changes with the square of the voltage.

The same applies whichever voltage your element is designed for. An element rated 3,000 W at 240 V gives about 2,760 W on a 230 V supply; a 1,500 W element on a 120 V supply that sags to 114 V gives about 1,350 W. Manufacturing tolerances add a few percent either way, so two elements with the same label can differ before the supply is even considered.

How much does the mains vary?

More than most people expect. In much of the world the nominal supply is 230 V, and the standards allow it to stray 10% either way for most of the time - anywhere from about 207 to 253 V. In North America, the usual range for a 120 V supply is 5% either way. Within those limits, the voltage at your still depends on:

• Time of day. Voltage tends to sag at evening peaks, when demand is highest.
• Location. Homes at the end of long or rural lines, far from the transformer, often see lower voltages than those close to it.
• Your own wiring. A long or thin extension lead loses voltage under the heavy, continuous load of an element.
• Other supplies. Generators and inverters each have their own voltage, and may hold it less steadily.

Why it matters on a still

Heat-up time

Bringing 25 L of wash from 20 °C to the boil takes about 2 kWh of heat, before any losses. A true 3,000 W does that in about 40 minutes; 2,500 W takes about 48.

Take-off rate and separation

Some heat is always lost through the boiler and column to the room. Only what remains turns wash into vapour, so a drop in power hits the output harder than the power figure suggests. If, say, 500 W is lost, a 3,000 W element leaves 2,500 W for boiling; at 2,500 W, only 2,000 W remains - 17% less power, but 20% less vapour. The run is slower, and on a reflux still the balance between boil-up and cooling shifts with it.

Repeatability

Most power controls work in percent. Sixty per cent of a strong supply and sixty per cent of a weak one are different amounts of heat, so a recipe written in percent drifts with the voltage. A recipe written in watts means the same thing every time - as long as something measures the watts.

Measuring your element's real power

• Measure the resistance. With the element disconnected from the mains, a multimeter across its terminals gives its resistance. Real power is your supply voltage squared, divided by that resistance. The element's resistance rises slightly when hot, so treat the result as a close estimate.
• Use a plug-in energy meter for a plug-in element - but only one rated for the element's full current, which for a 3 kW element is around 13 A.
• Use a clamp meter and a voltmeter on a hard-wired element, or a power-monitoring device made for the load.

What your element really delivers

Enter the rating from the label and your supply voltage. Add a wash volume to estimate the heat-up time.

Steady heat

A heating power regulator sets the heat by switching the element on and off very rapidly, delivering a chosen share of the full power. On its own, that share is a share of whatever the supply provides. Add a measurement of the real power and the control can close the loop: if the voltage sags, the regulator delivers a larger share, and the still receives the heat the recipe asks for - up to what the element can give at that voltage. That is one reason to choose an element with some headroom above the heat you normally use.

Sources and further reading

1. IEC 60038: IEC standard voltages (230 V nominal supply), and EN 50160: Voltage characteristics of electricity supplied by public electricity networks.
2. ANSI C84.1: Electric Power Systems and Equipment - Voltage Ratings (60 Hz).

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