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How Many Amps is 40 Watts? – A Calculation Guide

Written by Edwin Jones / Fact checked by Andrew Wright

how many amps is 40 watts

How many amps is 40 watts? Basically, to answer, you will need to do some simple calculation using the formula:

\text{Amps} = \frac{\text{Watts}}{\text{Volts}}

If a 40-watt device is connected to a 120-volt circuit, you can get the amp rating of 0.3333 amps.

However, as the watt rating increases, the current rating also gets high. In this case, the chart below could be a convenient reference. 

Also, the amount of current may vary depending on the voltage rating and circuit phase.

If you want a better understanding of converting watts to current, the explanation you need is provided below.

Power (Watts) Amount of current in 120V circuit (Amps)
40.00 0.3333
40.01 0.3334
40.02 0.3335
40.03 0.3336
40.04 0.3337
40.05 0.3338
40.06 0.3338
40.07 0.3339
40.08 0.3340
40.09 0.3341
40.10 0.3342
40.11 0.3343

How Many Amps Are There in 40 Watts?


Before anything else, let us first define what Amps, Volts, and Watts are. Recognizing these components of a circuit will help you understand this topic.

Technically, wattage (watts) refers to the power of the appliance, while voltage (volts) is the pressure that drives current (amps) through the circuit.

Now, to determine the amperage of a circuit, you need to consider the amount of pressure or volts you are using for your device, plus its power rating.

As given in the example above, if you want to convert 40 watts to volts and the appliance uses 120V, the amount of current that flows through the circuit is 0.3333 amps.

Additionally, the tool allows for a swift determination of amperage:

However, this calculation is only appropriate for DC electrical connections, where the power loss does not need to be considered, since it is very minimal.

Meanwhile, equipment does not run as efficiently in an AC single-phase or three-phase circuit. This type of connection may only support a power factor of 80 to 90%. As a result, the calculation should account for this.

How to Calculate Amps From Watts?


Now that you understand how to convert watts to amps for a DC electrical connection, let’s look at how to calculate the amperage for a single-phase and three-phase AC connection.

For single-phase setups, we can use this formula to convert 40 watts to amps:

\text{Amps} = \frac{\text{Watts}}{\text{power factor} \times \text{volts}}

Using the same example of 40 watts in a 240 volts circuit with an 80% power factor, we have:

\frac{40 \text{ watts}}{0.8 \times 240 \text{ volts}} = 0.2083 \text{ amps}

However, for a three-phase AC connection, an additional 1.73 factor is required for the equation. As a result, the formula will be:

\text{Amps} = \frac{\text{Watts}}{1.73 \times \text{power factor} \times \text{volts}}

In this case,

\frac{40 \text{ watts}}{1.73 \times 0.8 \times 240 \text{ volts}} = 0.1204 \text{ amps}

If you don’t want to perform this manual calculation, there is a much easier way to compute the current of specific watts. This option is utilizing an online watts to amps calculator. Simply enter the required information into this tool, which will provide an accurate result.

Also, to make it easier for you to determine the amp rating of different watts (with an 80% power factor), I’ve created a simple chart below, which can be your reference.

Wattage Rating Amps Rating on Various Voltage Connections
120V (DC) 120V (single-phase AC) 240V (AC)


480V (AC)


100 0.8 1.0417 0.5208 0.1505
250 2.0833 2.6042 1.3021 0.3763
500 4.1667 5.2083 2.6042 0.7526
750 6.25 7.8125 3.9062 1.1289
1000 8.3333 10.4167 5.2083 1.50529
1500 12.5 15.6250 7.8125 2.2579
2000 16.6667 20.8333 10.4167 3.0105

Frequently Asked Questions

How Many Amps are there in 220 Volts?

The answer may depend on the power rating of the unit and the power factor for that particular circuit. If you use a 2000 watts device at 220v with an 80% power factor, you are likely to get 11.3636 amps on a single-phase circuit.

How Many Battery Amps are there at 12 Volts?

A 12V DC battery usually comes with an amp hours rating (AH). This rating may be an average of 48 Ah, which can draw 1 amp for 48 hours or 48 amps in one hour.


In conclusion, to find out how many amps is 40 watts, you also need to consider the circuit voltage rating. This way, you can determine the amount of current flowing through that particular circuit.

It is very straightforward to use an online watts to amperage calculator. However, knowing how to get the amperage rating manually is also a good idea to have a deep understanding of electrical circuits.

Read more: Tips to convert 3500 watts to amp.

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