I currently have 5 x 20W halogen bulbs attached to two AC 12V rails, and an AC transformer (105VA).

I'd like to replace existing five bulbs with five LEDs, e.g. these: 7.5W (update: 12V AC) (Equivalent halogen 43W).

My initial understanding (also supported by this question) was that the required power for such setup would be 7.5W * 5 = 37.5W, and therefore the existing transformer would work fine. However, the seller's website has this manual which basically states that in my calculations I need to use equivalent halogen power (i.e. 43W) rather than 7.5W. That would mean that the existing transformer wouldn't be sufficient.

I struggle to understand why equivalent halogen power should be used here instead of simply the bulb's power. Can anyone advise on this please?

  • 3
    LEDs use DC. Halogen bulbs generally use AC provided by a simple transformer. LEDs which are substitutes for incandescent bulbs have a power supply in the base to present DC to the LED. Nov 4, 2018 at 23:37
  • Agree with Jim halogen are a totally different different type of load. AC vs dc at the minimum halogen watts are the amount of power needed at that voltage usually LED drivers are rated in this way but the lamps are in equivalent watts not the same, for an example I just replaced 4 1000 lamps MH that actually had a draw of over 1100 wats with 4ea 240w led fixtures that provide more usable or perceived light so the apples to oranges has to be figured out and the AC vs DC.
    – Ed Beal
    Nov 5, 2018 at 0:05
  • How many lumens is your existing halogen setup cranking out? Nov 5, 2018 at 2:05
  • 1
    @JimStewart Yes, LEDs that I'd like to buy are substitutes, and require AC (found specs on Philips website). So (I guess) the existing transformer should work in terms of type of power.
    – AlexW
    Nov 5, 2018 at 14:33
  • @ThreePhaseEel Existing halogens give 5 * 200lm = 1000lm, and it's kind of not enough for the room. That's why I was hoping to replace them with LEDs, which provide more lumens for less W.
    – AlexW
    Nov 5, 2018 at 14:36

1 Answer 1


With the additional information provided these 12v lamps are designed to work with standard halogen 12v fixtures since your wattage of the new lamps is less than the wattage of the halogen yes they will work. It looks like 1/2 the wattage so there is the savings and in my experiance you will have more light or perceived light because light meters read light we cannot see where LED's produce light in a narrow band with so power is not wasted.

  • Thanks, Ed. That was understanding as well, but then I read the calculation manual on the seller website, and it suggested that I should consider halogen equivalent wattage in power calculations, which was quite strange, so I decided to ask.
    – AlexW
    Nov 5, 2018 at 23:23

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