How to convert amperes to kilowatts and vice versa

Brief about voltage, current and power

Voltage (measured in Volts) is the potential difference between two points or the work performed by moving a unit charge. Potential, in turn, characterizes the energy at a given point. The magnitude of the current (the number of amperes) describes how many charges flowed through the surface per unit time. Power (watts and kilowatts) describes the speed at which this charge was transferred. It follows from this - the more power, the faster and more charge carriers moved through the body. In one kilowatt a thousand watts, this must be remembered for quick calculation and translation.

In theory, it sounds pretty complicated, let's take a look at the practice. The basic formula that calculates the power of electrical appliances is as follows:

P = I * U * cos Φ

Important! For purely active loads, the formula is used P = U * I for which cos равен is equal to one. Active loads are heating devices (electric heating, an electric furnace with heating elements, a water heater, an electric kettle), incandescent lamps. All other electrical appliances have a certain reactive power value, these are usually small values, therefore they are neglected, therefore, the calculation as a result is approximate.

How to translate

D.C

In the field of auto electricians and decorative lighting, 12 V circuits are used. Let's look in practice how to convert amperes to watts using an example of an LED strip. To connect it, you often need a power supply, but you can’t connect it "just like that", it can burn out, or vice versa, you can buy a too powerful and expensive PSU where it is not needed and spend money in vain.

The characteristics of the power supply on the tag indicate such quantities as voltage, power and current. Moreover, the number of Volts is required, but the power or current can be described together, and maybe such that only one of the characteristics is indicated. In the characteristics of the LED strip, the same characteristics are indicated, but power and current per meter.

Imagine that you bought 5 meters of 5050 tape with 60 LEDs per 1 meter. On the package it says “14.4 W / m”, and in the store on the PSU tags only current is indicated. We select the correct power source, for this we multiply the number of meters by the specific power and get the total power.

14.4 * 5 = 72 W - necessary to power the tape.

So you need to translate into amperes according to this formula:

I = P / U

Total: 72/12 = 6 Amps

In total, you need a power supply of at least 6 Amps. Learn more about how to choose a power supply for LED strip, you can find out from our separate article.

Another situation. You installed additional headlights on your car, but the characteristics are indicated on the bulbs, let's say 55 watts. It is better to connect all consumers in the car through a fuse, but which one is needed for these headlights? You need to convert watts into amperes according to the formula above - dividing the power by voltage.

55/12 = 4.58 Amperes, the closest nominal is 5 A.

Single phase network

Most household appliances are designed to connect to a single-phase 220 V network. Recall that, depending on the country in which you live, the voltage can be 110 volts or any other. The value adopted for the standard is precisely 220 V for a single-phase and 380 V for a three-phase network. Most readers most often have to work in such conditions. Most often, the load in such networks is measured in kilowatts, while the circuit breakers contain the markings in Amperes. Let's look at some practical examples.

Let's say that you live in an apartment with an old electric meter, and you have an automatic 16 amp plug installed. To determine what power the plug will “pull”, you need to convert Amps to kilowatts. The same formula is effective here, linking the current strength and voltage to power.

P = I * U * cos Φ

For the convenience of calculations, we take cos за per unit. We know the voltage - 220 V, the current, too, let's translate: 220 * 16 * 1 = 3520 watts or 3.5 kilowatts - just as much you can connect at a time.

Using the table, you can quickly convert amperes to kilowatts when choosing a circuit breaker:

A little more complicated is the situation with electric motors, they have such an indicator as a power factor. To determine how much such an engine will consume kilowatts per hour, you must necessarily take into account the power factor in the formula:

P = U * I * cosФ

It should be noted that cos должен should be indicated on the tag, usually from 0.7 to 0.9. In this case, if the total engine power is 5.5 kilowatts or 5500 watts, then the consumed active power (and we pay, unlike enterprises, only for active):

5.5 * 0.87 = 4.7 kilowatts, or rather 4785 W

It is worth noting that when choosing an automatic machine and cable for an electric motor, you need to take into account the full power, so you need to take the load current, which is indicated in the passport to the motor. And it is also important to consider inrush currents, since they significantly exceed the operating current of the motor.

Another example, how many amperes does a 2 kW kettle consume? We do the calculation, first you need to perform Convert kilowatts to watts: 2 * 1000 = 2000 watts. After that, we translate the watts into Amps, namely: 2000/220 = 9 Amps.

This means that a 16 Amp plug will withstand the kettle, but if you turn on another powerful consumer (for example, a heater) and the total power is higher than 16 Amperes, it will knock out over time. This is also the case with machine guns and fuses.

To select a cable that can withstand a certain number of amperes more often than formulas use the table. Here is an example of one of them, in addition to the current, it also indicates the load power in kilowatts, which is very convenient:

Three phase network

In a three-phase network there are two main schemes for connecting the load, for example, motor windings - a star and a triangle. The formula for determining and converting power to current is slightly different than in the previous versions:

P = √3 * U * I * cosФ

Since the electric motor is the most frequent consumer of a three-phase power supply network, we will consider it as an example. Suppose we have an electric motor with a power of 5 kilowatts, assembled according to a star circuit with a supply voltage of 380 V.

You need to power it through a circuit breaker, but to pick it up, you need to know the motor current, so you need to convert from kilowatts to amperes. The formula for the calculation will look like:

I = P / (√3 * U * cosФ)

In our example, this will be 5000 / (1.73 * 380 * 0.9) = 8.4 A. Thus, we could easily convert kilowatts to amperes in a three-phase network.

Finally, we recommend watching a useful video on the topic:

For operational work, the electrician needs to master the skills of quick translation. Electric motors often indicate current, voltage, power, and its coefficient, but it happens that the plate is lost or the information on it is not fully read. In addition to electric motors, it is often necessary to connect heating elements or a heat gun, where in addition to the supply voltage and power, often nothing is known. For optimal cable selection, you need to know how to quickly convert amperes to kilowatts, respectively. We hope that the formulas and tips provided have helped you understand all the nuances of the translation. If you can’t translate power into amperes or vice versa, write in the comments, we will try to help you!

It will be useful to read:

• How to choose a circuit breaker
• Calculation of the cable cross section for current and power
• How to determine the power consumption of devices