# How to calculate what wattage transformer to use for LED light strip

I am looking to purchase some LED light strips, and I have seen these:

7.5 Watt RGB LED Tape

They are 7.5w, and have 150 LED lights over a span of 5m and the website states that they are 12V. But I am not sure what transformer/driver to use for these lights. The QA section, below the product description mention:

" To work out what transformer you require this is quite simple. You take the amount of LED tape you have in metres and x this by the wattage of the LED tape you have, again per metre. For example 7m of 4.8w LED tape: 7 x 4.8w = 33.6w Therefore you would require a 60w transformer. "

So for 5 meter of 7.5watts the calculation is 5 x 7.5 = 37.5W The website has power transformers, but at 100W and 150W, and when I rang them they were out of stock and the prices for the transformers were about 3 times what I can see on ebay.

Firstly, can I use a transformer that is producing 40W which is just above the 37.5W that I need, instead of a 150W transformer that they sell and claim is compatible for this LED strip?

Here is one transformer, that looks equivalent to the one they sell, at a relatively affordable price:

This gives me 12v x 12.5A = 150W, but I can see a 15V x 10A, which also gives 150W. Which of these is suitable for me (as they both produce the same wattage, but have different Volts/Amps), and what do the different Amps and Volts mean?

## 2 Answers

Yes, you can use a 40w transformer to power your 37.5w load. However, given the quality of these small power supplies, running at 90%+ all the time will cause heat and premature failure probably.

You will need to drive the LEDs at 12v. Running them at 15-18v (or anything over 12!) is a bad idea for the life of the LED. You will need to put a resistor on an 18v supply to bring it down. I'd recommend simply getting the 150w 12v supply. Wattage/amperage is different than volts, where you can use a higher powered power supply and the connected lights will only draw what they need (example: running a 12v 50w light bulb from a 12v power supply rated at 150w is fine. Running a 6v 50w bulb on the same power supply is not fine).

• Is a 150W transformer not inefficient. I know the LEDs will draw however much they need, but would a 150W transformer not get a lot hotter than say a 60W transformer or a 40W transformer? If I get a 60W to be on the safe side (as my requirements are only for 37.5W load) - that would be a little bit more capacity than needed and less wasteful than 150W - am I right? Commented May 20, 2014 at 13:41
• A power supply (PSU) will only draw as much power from the wall as it needs to power the load. PSU's have a sweet spot for efficiency which could be in the 40-60% of rated load. So, if you need 75w, a 150w would be a good choice. In your example, a 60w running 37w or a 150w running 37w will have a negligible difference of actual consumption. Go for the 60. Commented May 28, 2014 at 14:17

I believe you have misread the listing for these lights. They are 7.5 watts for the whole string. These are incredibly bright and efficient lights that I have used for under-cabinet lighting and for accent lights. For the under cabinet lights, the sections are 1 - 2 meters long and only use a couple of hundred milliamps of power. I use the wall transformer power supplies designed for home CCTV cameras - 12v at 500 milliamps. They work great, they are small and can be hidden easily.

• Are you absolutely sure you have the same lights? the listing does specifically say under the specification tab: "7.2 Watts per metre" and later on says: "Reel length can be 5m or 10m" Commented May 22, 2014 at 14:45
• I'm sorry but I did misread the description, but I still think their specs are not clear. This is the item that I have.tmart.com/… As you can see it's only 20 watts for the whole strip. So calculating for the 7.5 W per meter, it's 7.5 / 12v = .650 amps Commented May 22, 2014 at 15:02