Price of electric kettle Vs hob - Straight answer possible?

The reason it's difficult is because it is hard to measure the small quantity of gas involved.

Bottled gas is much more expensive than mains gas.

The energy that is "wasted" from your gas hob actually goes to heat your house, so apart from in Summer, it is not really wasted.

The first step is to establish how much energy (in kWh) is required to boil the required amount of water. There are formulas to do it based on the specific heat capacity of water - it works out at about 0.091kWh per litre (assuming the initial water temp is 20degC)

It is relatively easy to work out the kettle cost based on the unit rate you are paying. If a unit is costing €0.35, the kettle will use about 3.2c worth of electricity per litre boiled. That assumes a kettle which is 100% efficient. While electric kettles would be pretty efficient, I would say a 90% efficiency figure would be more like it - so let’s say the kettle is costing approx 3.5c per litre of water to go from 20degC to boiling.

To look at gas,

You still need 0.091kWh of energy per litre of water you wish to boil. LPG bottled gas delivers about 7.1kWh per litre. The price of a litre of LPG varies greatly, but let’s say it’s €1.00 per litre on average at present. So each kWh costs €0.141 (vs €0.35 per unit of electricity in the example above). So gas appears, and probably is significantly cheaper. The big variable with gas is the efficiency - I.e. how much energy will be ‘lost’ when you are boiling the kettle or pot on a gas hob? I’m not going to try and put a figure on it, but the efficiency of that arrangement would be significantly less than an electric kettle, so gas would not be as cheap as it first appears. Another poster rightly stated that much of this heat loss will be ‘lost’ into the house, which still provides a benefit at least in winter.

Where you can make a simple saving is to put water into a covered jug and let it come up to room temperature.

Assuming the room is 18 degrees, this will save about 15% on heating water directly from the tap which is about 12 degrees at present.

The savings increase as winter comes in as tap water temperature will fall to about 7 degrees.

It's all about thermodynamics - I know nothing about it, but I'm sure someone else does.

But the best way of saving is to boil just what you need.

If the gas was costing €1.00 per litre when bought in cylinders, and a litre gives you 7.1kWh of energy - €1 divided by 7.1 = €0.141 per kWh

I thought it would be useful to be able to compare what 1Kwh of energy cost in gas vs electricity - although note the caveats that extracting that 1kWh of energy from gas can be significantly less efficient (more wasteful) than using an electric element which is generally a very efficient process (little or no waste).

But also that €1.00 per litre would be quite high. Supplies to a bulk tank at a private house would be more like €0.70 per litre at present. Supplies to heavy commercial users much lower again.

In summary, the boiling of a kettle in a domestic enviroment generally costs very little and an electric kettle is likely the most practical way to do it. Heating a house, would be a very different application and you will typically find electricity a much more expensive option over gas.

As has been explained there are many variables when it comes to boiling water on a gas hob that are hard to account for. Notwithstanding that, in my estimation if you carefully boil a kettle on a gas hob it will still be cheaper than using the electric kettle.

If a pretty typical 1.7 litre electric kettle cost €0.06 to boil using electricity, the gas equivalent might be €0.05 - not exactly big money either way.

Yes, ‘Mains gas’ is presumably natural gas and enters a home via a pipeline at street level. It is only available in major cities in Ireland which have the infrastructure.

Natural gas has historically been significantly cheaper than Liquid Petroleum Gas (LPG) which is what you would purchase in a cylinder(drum) of various sizes, or have a bulk storage tank (about the size of an oil tank) at your home which is filled periodically by Calor or Flogas.

The price of LPG can vary greatly depending on how you buy it, a bulk tank is usually the most economical if you are a moderate to heavy user. (Central heating)