Boiling water is one of those tiny everyday jobs that nobody thinks about, yet a household that brews ten cups of tea a day does it more than 3,500 times a year. The method you use matters a little. How much water you heat each time matters far more. This guide puts the kettle, the gas hob, the microwave and the induction hob side by side using current UK energy prices, so you can see where the pennies actually go and stop paying to heat water you tip down the sink.
The short answer. For a kettleful, a gas hob is usually the cheapest in pure pence because gas costs roughly a quarter of what electricity does per unit. For a single mug, an electric kettle filled to just that one mug almost always wins, because it heats only the water you need and wastes very little. At 26.32p per kWh for electricity and 7.97p per kWh for gas (Ofgem cap October to December 2026), a litre boiled in a kettle costs about 3p, on a gas hob about 1.5p, and in a microwave about 4.7p.
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The physics that decides the cost
Every method has to add the same amount of heat to the water. Raising one litre from a cold-tap 15C to a boil at 100C needs about 0.099 kWh of useful energy. That number does not change whether you use a kettle, a hob or a microwave. What changes is how much of the energy you pay for actually ends up in the water, and how much leaks away as heat into the room, the pan or the appliance.
This is what efficiency means in practice:
- Electric kettle: roughly 80 to 90 per cent efficient. The element sits in the water, so almost all the heat goes straight in. Fast, and very little is wasted.
- Induction hob: around 85 per cent efficient. The pan itself becomes the heating element, so it rivals a kettle.
- Gas hob: roughly 40 to 55 per cent efficient. The flame heats the pan but also the air and the pan sides, so a fair chunk escapes.
- Microwave: roughly 50 to 60 per cent efficient. Energy is lost in converting mains power into microwaves before any reaches the water.
So there are two levers: the price per unit of the fuel, and how efficiently the appliance delivers it. Gas is cheap per unit but leaky. Electricity is dear per unit but the kettle and induction hob are tight. Those two facts pull in opposite directions, which is why the answer changes depending on how much water you boil.
Cost per litre, method by method
The table below boils a full litre of water from cold using each method, with the running cost worked out from the useful energy plus a typical efficiency figure. It ignores standing charges, since those are fixed daily costs you pay regardless of how you boil.
| Method | Fuel | Energy used per litre | Cost per litre |
|---|---|---|---|
| Gas hob | Gas | ~0.20 kWh | ~1.5p |
| Electric kettle | Electricity | ~0.11 kWh | ~3.0p |
| Induction hob | Electricity | ~0.12 kWh | ~3.0p |
| Microwave | Electricity | ~0.18 kWh | ~4.7p |
At 26.32p per kWh electricity and 7.97p per kWh gas, Ofgem cap October to December 2026. Figures assume cold water at about 15C and typical appliance efficiencies; your own kettle or hob may differ.
On a per-litre basis the gas hob comes out cheapest, despite being the leakiest method, purely because gas is so much cheaper per unit. The kettle and induction hob land close together. The microwave is the most expensive way to boil a full litre, because it pairs the dear fuel with mediocre efficiency.
Before you switch all your tea-making to the hob, though, read the next section. For the way most people actually use water, the kettle wins handily.
Why the kettle usually wins for a single mug
Almost nobody boils a full litre for one cup of tea. They boil what they pour, plus whatever extra they could not be bothered to measure. This is where the kettle pulls ahead, because it is the easiest method to fill accurately, and because it loses very little heat to the surroundings.
Heat one mug of water, about 250ml, and the picture changes:
| Scenario | Cost per mug |
|---|---|
| Kettle, filled to one mug only | ~0.8p |
| Gas hob, small pan, one mug | ~0.4p |
| Microwave, one mug | ~1.2p |
| Kettle, filled to 1 litre for one mug | ~3.0p |
At 26.32p per kWh electricity and 7.97p per kWh gas, Ofgem cap October to December 2026.
The gas hob is still cheapest in raw pence for a single mug, but it is slow, it heats the kitchen, and reboiling a pan for a second cup is fiddly. The kettle filled to one mug costs under a penny and is ready in under a minute. The real lesson is in the last row: overfill a kettle to a full litre for one cup and you quadruple the cost of that brew. The method matters less than the amount.
If you want to go deeper on getting the most from yours, our kettle energy saving guide covers descaling, fill lines and eco kettles.
The habits that beat the appliance choice
Whichever method you land on, a few simple habits save more than switching appliances ever will.
- Only boil what you need. Fill the kettle to the cup, not to the brim. Most kettles have a minimum line and a cup gauge; use them. This is the single biggest saving on the page.
- Descale regularly. A thick layer of limescale on a kettle element insulates it from the water, so it takes longer and uses more energy. In hard-water areas this matters.
- Match the pan to the ring. On a gas hob, a flame licking up the sides of a small pan is wasted heat. Keep the flame under the base. On any hob, use a lid; uncovered water loses heat to the air the whole time it is heating.
- Do not reboil. Boiling water you boiled twenty minutes ago wastes the lot. Boil fresh, pour all of it, or keep it in a flask.
- Mind the standing charge, not the boil. The cost of boiling water is tiny next to the fixed daily charges on your bill. If you are chasing savings, the kettle is small fry. Our standing charges explained guide shows where the larger fixed costs sit.
A so-called eco kettle with a one-cup setting or a clear gauge helps you fill accurately by default, which is worth more than any efficiency rating printed on the box. The same logic runs through all kitchen appliances; see saving energy in the kitchen for the wider view.
What about a boiling-water tap or a flask?
Instant boiling-water taps and hot-water dispensers are sold on convenience, and they can be efficient per cup because they deliver exactly the volume you draw. The catch is the standby cost: a tank tap keeps a small reservoir hot around the clock, so it draws a trickle of power even when nobody is using it. Whether that standby cost outweighs the per-cup saving depends entirely on how many cups your household draws a day. A household that fills a dozen mugs a day may break even; a one-coffee-a-day flat will not. We work the numbers through in the boiling water tap running cost guide.
A vacuum flask is the quiet winner for anyone who boils once and drinks across the morning. Boil a kettleful, pour what you want now, and tip the rest into a flask. It stays hot for hours with no further energy at all, which beats reboiling every time. For the bigger picture on appliances that sip power while idle, see standby power, the full story.
The bottom line
If you only care about raw pence and you have a gas hob, boiling on the hob is the cheapest method per litre, because gas costs roughly a quarter of what electricity does per unit. For everyday tea and coffee, an electric kettle filled to exactly the number of cups you want is the practical winner: under a penny a mug, fast, and hard to get wrong. An induction hob is nearly as efficient as a kettle if you already have one. The microwave is the most expensive option and the slowest for anything more than a mug.
But the method is a sideshow. The money is in the amount of water. Boil only what you pour, descale the kettle, use a lid on the hob, and keep a flask for the second cup. Do that and it barely matters which appliance you reach for. To see how a kettle stacks up against everything else in your kitchen, our appliance running cost hub puts the whole house in one place, and energy labels explained helps you read the rating on your next appliance.