How Long Does It Take For Water To Boil?

In order to boil water, one has to increase the temperature of water to its boiling temperature at normal atmospheric pressure, and this requires energy.

The most common ways of boiling water include heating it on the stove, in the microwave, or in the electric kettle. There are other ways of boiling the water, for example, in the electric oven, but they are rarely used.

Updated: March 17, 2023.

How Much Energy Is Needed To Boil The Water?

In order to boil water, one needs the energy to increase its temperature.

The exact energy requirement is easy to calculate, at least in theory.

Water heat capacity is 4.2 kJ/°Ckg, meaning that it takes 4.2 kJ (kilo-Joules) of energy to raise the temperature of a kilogram of water by 1°C.

If the water temperature is 20°C (68°F), to boil 1 (one) liter (4 cups) of water, one needs:

E = 4.2 * 1 * (100 – 20) = 336 kJ

So, in theory, to raise the temperature of 1 liter (4 cups) of water by 80°C, one needs 336 kJ of energy.

How Long Does It Take to Boil Water?

In the case of an ideal heater, to boil 1 liter (4 cups) of water at 20°C (68°F), the boiling time is:

Time = Required Energy / Power of the heater

For example:

• 0.75 kW heater → T = 336 kJ / 0.75 kW = 448 seconds = 7 min 28 seconds
• 1 kW heater → T = 336 kJ / 1 kW = 336 seconds = 5 min 36 seconds
• 2 kW heater → T = 336 kJ / 2 kW = 168 seconds = 2 min 48 seconds

For 1 cup (250 ml) of water, boiling time is four times shorter (750W heater ~112 seconds, 1 kW heater ~84 seconds, 2 kW heater ~42 seconds, etc.).

However, in real life, plenty of energy is lost because of converting one form of energy (for example, electricity) into another form of energy (thermal energy of water), energy losses because the heater has to heat the water container and the heater itself, etc.

On average, to boil water, it takes:

- 1500W microwave: 1-1.5 min cup (250 ml) of water,

- 700-800W microwave: 2.5-3 min cup (250 ml) of water,

- gas stove: 6-8 min 1 liter (four 250 ml cups) of water,

- electric stove: 8-12 min 1 liter (four 250 ml cups) of water,

- induction cooktop: 5-6 min 1 liter (four 250 ml cups) of water,

- 1500W kettle: ~4 min 1 liter (four 250 ml cups) of water,

- 2500W kettle: 2-3 min 1 liter (four 250 ml cups) of water, etc.

In order to speed up things, one can use a closed water container (it must NOT be a sealed container, or it may blow up due to the pressure inside!), a stronger water heater, a smaller amount of water, etc.

Boil Water in a Microwave

A microwave oven is one of the most efficient means of boiling water, but one has to be very careful.

Note: before boiling water in a microwave, it is highly recommended to read the microwave's Owner's Guide, especially Safety Issues and Boiling Water/Liquids, just in case.

When boiling water in a microwave, always use microwave-safe pots/cups - never use metal ones and never use lids.

Another very interesting but also very dangerous issue is - water overheating.

The microwave heats water very quickly, and usually, one uses cups with very smooth surfaces - steam bubbles usually need some surface imperfection to cling to when forming.

All of this leads to superheated water - water with a temperature above its boiling point but with no steam bubbles.

But, as soon as the user touches the pot/cup with the superheated water, a large number of bubbles may form almost instantly - hot steam bursts, causing burns on the unprotected skin.

For short, be very careful when boiling the water with a microwave.

But, on average, in order to boil water, it takes 1500W microwave 1-1.5 min to boil a cup (250 ml) of water, and 700-800W microwave 2.5-3 min to boil a cup (250 ml) of water.

Boil Water in Electric Kettle

Electric kettles are designed to boil water as quickly as possible.

On average, to boil 1 liter of water, a 1500W kettle requires ~4 min, and a 2500W kettle requires 2-3 min.

An electric kettle is not as efficient a heater as a microwave oven since there is a large "parasite" mass to be heated, but there is no danger of superheating the water.

Personally, if You have the need to quickly boil various amounts of water for coffee, tea, and similar, an electric kettle is a way to go.

Benefits Of Boiling Water

There are many benefits of boiling water, regardless of whether You are at home or outdoors.

Boiling water can be used for making various drinks and for preparing food.

Also, tap water is not always safe to drink - boiling it can kill viruses, bacteria, and other pathogens, making it safe for humans to drink.

How Long to Boil Water to Make It Safe?

The boiling temperature of the water is 100°C (212°F) - at atmospheric pressure of 1 bar (1 kg per 1 cm²), which is more or less atmospheric pressure at sea level.

As the height is increased, the water boiling temperature decreases.

For example: to make water safe for drinking, it is recommended to boil water at sea level for 1 minute (60 seconds), but at an altitude of 2000m (6557 ft) where water boiling temperature is ~199.4°F (~93°C), it is recommended to boil it for 3 minutes (180 seconds).

Water Boiling Temperature vs. Altitude

The following chart lists the boiling temperatures of water (given in °F and °C) according to the altitude (given in feet and meters). As a reference, several cities and well-known places with their altitudes are also shown in the chart.

Thus, if you want to boil water for cooking or drinking at higher altitudes, perhaps it is better to use a pressure cooker and boil water for a shorter period of time.

Energy Sources in Emergencies

In emergencies and similar situations, one of the first things that are gone is - electric energy.

To boil water, one can use a gas stove (if it works properly without electricity - these days, there are even models that don't function properly without electricity, although their main energy source is gas!), a fire pit, or anything similar.

But, one can also use a microwave or electric kettle if one has:

• solar generator/power station, or
• deep discharge battery combined with a power inverter, or
• power generator.

A solar Generator/Power Station can be used to power a microwave oven or electric kettle if the output power of the power station is stronger than the microwave oven/electric kettle's required power.

This means that power stations in the 1500-2000 watts range can be used to power microwaves and electric kettles in the 1200-1500 watts range.

To boil a cup of water (250 ml), a 1500W electric kettle requires ~1 minute, while a 1500W microwave requires 1-1.5 minutes.

Since power stations feature an average energy efficiency of ~85%, boiling a cup of water using a 1500W electric kettle requires ~30 Wh of energy:

E_{(electric kettle)} = 1500W * 60s = 90000 Ws = 25 Wh

E_(battery) = E_{(electric kettle)} / 0.85 = 29.4 Wh (~30 Wh)

Most power stations/solar generators in the 1500-2000W range feature internal batteries being able to store 500 - 2000 Wh of energy.

For example, a power station having a 900Wh internal battery is able to boil 30 cups (250 ml) of water - having warm tea, coffee, soup, or similar drink/food can be extremely important in emergencies or when camping, hiking, and similar. Of course, hot water has other purposes as well, not just for cooking and drinking.

Similarly to power stations, one can use a large, deep discharge battery and power inverter to power a microwave oven or electric kettle to boil water, as long as the output power of the power inverter is stronger than the required power of microwave oven and/or electric kettle.

Portable power generators may be used directly to power the microwave ovens and/or electric kettles to boil water as long as the output power of the power generator is stronger than the microwave oven and/or electric kettle.

Water Boiling Frequently Asked Questions

Here are some of the most popular Frequently Asked Questions (FAQ) about boiling water:

How do you know when water is fully boiled?

When water is boiling, one can see bubbles of water steam exiting the water.

Why does boiling water take so long?

It takes time to transfer heat to the water required for increasing the temperature of the water.

Depending on the heat source and its power, the amount of water, the initial temperature of the water, and altitude, water can boil rather quickly, but also, it can take some time.

How long does it take for water to come to a rolling boil?

When water starts to boil using a very strong heat source, water can start to boil rather quickly.

If that heat source continues to heat the water, a rolling boil occurs - a very strong, vigorous boil with plenty of bubbles and very active and energetic bubbles motion.

Does adding salt make water boil faster?

In real life - no.

Adding salt to the water decreases the water's heat capacity, but it also increases the water's boiling point.

So, in real life, the difference between boiling pure water and the same amount of water with some salt added is a few seconds, even if that.

How do you make water boil faster?

One can make water boil faster by using a stronger heat source, using less water, or using water with a higher initial temperature.

How does a rolling boil work?

When the water is heated using a very strong heat source, which causes a very strong, vigorous boil with plenty of bubbles and very active and energetic bubbles motion - that is a rolling boil.