How long does an electric water heater take to heat water?

One of the main questions we ask ourselves when looking for a water heater is how long it takes to reach the optimum temperature. To know this data, it is necessary to make some numbers, based on different parameters of the product that we are evaluating.

Unlike gas heaters, in which the water is heated almost on the fly, a different system is used in electric water heaters. These equipments have a resistance to heat water, which is activated when the temperature of the liquid is below the level that we have established in the corresponding thermostat, which serves to regulate the temperature of the electric water heater.

One of the great existing doubts in this regard has to do with the time necessary for the water to reach that temperature. A question that has a lot of elementary physics and that can be very useful both when it comes to knowing which electric water heater is best for us and when using programmers to have hot water at the time we want. However, the calculation has its peculiarities, so we will leave you with some clues on how to proceed.

What factors influence heating time

The first thing we need to know are the factors that influence the heating process. These modify the times in a remarkable way, even when we use the same thermos, but in different conditions. In essence, these would be the elements to consider:

Volume of the thermos: The first thing we must know is the volume of water that the electric thermos has. The greater the volume of water, the longer the heating time required . However, the relationship is not linear, so the heating time of a 100 liter electric water heater will be more than double that of a 50 liter electric water heater.

Power of the thermos: The power of the thermos is another important factor in this process. The higher the power of the device, the greater its ability to heat water in less time.

Target temperature: It is obvious, but we cannot forget the temperature at which we have configured the product. The higher the temperature, the longer it takes to reach it.

Inlet temperature: In parallel with the target temperature, the colder the water entering the thermos, the longer the equipment will need to reach the desired temperature.

Efficiency of the thermos: The last important aspect is the construction of the thermos in its different elements. Something that includes aspects such as the type of electrical resistance mounted or the level of thermal insulation included. The greater the thermal loss of the thermos, the more amount of heat and time will be necessary to achieve the target temperature.

The formula

When calculating the amount of heat needed to increase the temperature of the water, we have a fairly simple formula and an element such as the calorie. This unit is a must have when it comes to heating and cooling. The calorie is defined as the amount of heat required to raise the temperature of one gram of water by one degree . So to heat a liter of water, one kilocalorie would be enough, equivalent to 1,000 calories.

The formula that we are going to use is the following:

Where Q would be the amount of heat needed for the procedure, m the amount of water we have inside the thermos, in liters. C sub e is the number of calories needed to raise the water temperature by one degree, which in this case is 1 kilocalorie per liter, and T f and T i are the target and inlet temperatures of the water. Therefore, this term would be the difference between the initial and final temperatures. 

Let’s see an example: if we have a 100-liter capacity electric water heater, the inlet temperature is 8 degrees and the target is 65 degrees, then the calculations would be as follows:

Q= 100 x 1 x (65-8) = 5,700 kilocalories

That said, we only have to convert that figure to kilowatts, which is the reference unit for electric water heaters. This ratio is 1000 Kilocalories to 1,163 Kilowatts. So we only have to check the power of the thermos and complete the calculation. If said power is 1.5 kilowatts per hour, for example, the calculation would be:

T= 5,700 kilocalories / 1,163 KW / 1.5 KW hour = 3.26 hours of time, or 3 hours and 15 minutes of heating, approximately.

what reality says

In the calculations that we have made before there are details that are not included, such as the level of thermal insulation of the product or the efficiency of the heater when transferring heat. The example above estimates an efficiency of 100%, which unfortunately does not correspond to reality. 

We have the proof in the times offered by some models on the market. For example, in the case of a 50-liter capacity electric water heater equipped with a resistance of 1.2 kilowatts, it takes about 2 and a half hours to heat the water from 10 to 60 degrees . In the case of a 150-liter thermos with a power of 2.2 kilowatts, the time is around 4 hours, for the same temperatures. 

By the way, in this heating system those who take the prize for speed are the models with heat exchangers, whose operation is more similar to that of a conventional heater. A team of this type with a power of 2,000 watts barely needs 20 minutes to heat 80 liters of water from 15 to 60 degrees.

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