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 the 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.
Published: December 18, 2021.
How Much Energy Is Needed To Boil The Water?
In order to boil water, one needs energy in order 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), in order to boil 1 (one) liter (4 cups) of water, one needs:
E = 4.2 * 1 * (100 – 20) = 336 kJ
So, in theory, in order 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 ideal heater, in order to boil 1 liter (4 cups) of water at 20°C (68°F), the boiling time is:
Time = Required Energy / Power of the heater
- 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 (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, in order 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 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 the 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, in order to boil 1 liter of water, 1500W kettle requires ~4 min and 2500W kettle requires 2-3 min.
An electric kettle is not as an efficient 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 the way to go.
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Benefits Of Boiling Water
There are many benefits of boiling water, regardless if 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 such water can kill germs and bacteria, 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 the sea level.
As the height is increased, the water boiling temperature decreases.
For example: in order 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, it is recommended to boil it for 3 minutes (180 seconds).
Energy Sources in Emergencies
In emergencies and similar situations, one of the first things that are gone is - electric energy.
In order 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 source of energy is gas!), a fire pit or anything similar.
But, one can also use a microwave or electric kettle if one has:
- deep discharge battery combined with a power inverter, or
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.
In order to boil a cup of water (250 ml) 1500W electric kettle requires ~1 minute, while 1500W microwave requires 1-1.5 minutes.
Since power stations feature an average energy efficiency of ~85%, boiling a cup of water using 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.
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 microwave oven and/or electric kettle in order to boil water, as long as the output power of the power generator is stronger than the microwave oven and/or electric kettle.