How do I calculate watt when transformer is included?

[Teza]

I have 4 reels with 3 ea 20W/12V halogen bulbs on each.
These reels are connected to 240v (have internal trafos)

I would like to think that these reels do not use 60W each on 240V, or do they?

[cheerio]

not sure. If the Transformer is reaaaaally good you should go for 3W on 240V. beat me if i am wrong ^^

[kam]

watts are always added no matter series or parallel connection. So, 3 x 20W bulbs = 60 Watts.

As for transformer, the power at the input and output are (theoretically) same. 60 watts input means 60 watts output. What changes is the current and voltage. So 60 watts at 12V means I=P/V=>I=60/12=5A, which is the trafo output. The trafo input at 220V is I=P/V =60/220=0.272A. You see the difference...

[Teza]

Hm.
I have a dimmer that I would like to use on these reels.
The dimmer is marked with: Max 230V / 250W bulb, trafo max 40W / 180VA. "Fitted for most trafos for halogen bulbs"

If I am thinking 250/230=1,09A. Then 20/230=0,09A. So 1,09/0,09=12 bulbs max on the dimmer.
But have a feeling that the rest of the info tells me that I am wrong in this?

[kam]

40 watts dimmer means it can dim 40 watts... You dont need to go back to I or V and then back to P again, since you already know your limit. For educational purposes you can calculate the max current though (i=p/v = 180mA)

So, you can max dim 2 x 20W lamps

[Teza]

Thank you.
I thougt that the limitation for these products was the amount of ampere they could handle.
Also found it strange that it could handle a 250W bulb but only a 40W trafo, wich was the reason I had to ask.