How to Calculate LED Power Supply

How to Calculate LED Power Supply

There are various LED spots, spotlights and other light sources, which are operated with low-voltage. Usual operating voltages are 12V and 24V. An LED power supply is required to operate the low-voltage lamps on the 120V mains-varied to different volt in different country. This transforms the mains voltage down to the required LED voltage. Instead of the term power supply, the following terms are also common:

✔ Transformer
✔ Low-voltage transformer
✔ LED power supply

LED transformers are available in a wide variety of power classes. However, it makes no sense to simply buy an oversized transformer without calculating the actual power required in advance. Many transformers have a minimum load and would not provide any voltage at all if the load is below this limit. You should therefore dimension the transformer exactly for your application.

The required power of the LED power supply can be calculated quite easily. For most light sources and LED spots the power consumption in watts (W) is given. You will find this information both on the packaging and directly printed on the lamp. For example, if you only want to operate a single low-voltage lamp with 10W, the transformer must also be able to supply at least 10W + power reserve.

The operation of several lamps on one transformer is also very common and very economical. Here, the power consumption of all LED’s must be added together.

Example: Power supply calculation for multiple spotlights

Six 12V LED spots with 6 watts each shall be operated:

6 watts · 6 (quantity) = 36 watts

Add 20% power reserve:

36 watts + (0.2 · 36) = 43.2 watts

→ A 45 watts power supply would be a good choice here.

Calculate power via current consumption

In some special cases, the power consumption of the LED’s is unknown. Instead, the current consumption is given in amperes (A). Then the power can be determined by multiplying voltage and current. The result can then be used to calculate the transformer as described above. Examples for power calculation from voltage and current:

✔ 12V · 2.5A = 30W

✔ 24V · 0.8A = 19.2W

Calculate power supply for LED strips

LED stripes are often sold by the foot. This leads to the following specialty. The power consumption in the shop or in the data sheet is therefore usually given in watts per foot (W/ft). For example, if you want to operate a 5ft LED strip, the transformer can be calculated as follows:

Example: Power supply calculation for LED strips

A 12V LED stripe with a length of five feet and 14.4 W/ft shall be operated:

14.4 watts · 5 (feet) = 72 watts

Add 20% power reserve:

72 watts + (0.2 · 72) = 86.4 watts

→ A 90 watts transformer would be a good choice here.

Calculate power reserve

It is not good idea to permanently operate an LED transformer with 100% load. On the one hand, the power supplies fuse could be triggered by the inrush current of the lamps. Furthermore, the power supply unit could heat up above average, which probably would reduce its service life. A reasonable reserve also provides scope for an extension of the lighting installation.

A power reserve of 20% is recommended in most cases. If additional spotlights are to be added later, the reserve should be increased accordingly. The power calculation with reserve usually results in crooked values. It is advisable then to choose the next larger power supply unit.

Post time: Sep-23-2021