Easy Power Usage Formula?

One needs to know the power specifications for the bulb first. ie.12v .25watt

Not an easy one solution fits.  Basically take the bulb rating convert to watts and add them.  If using 12V bulbs at .2 amps they use 12V X .2A =2.4Watts each.  Watts in DC is Voltage X Amps.  I would not worry about AC Versus DC for this calculation.

40 2.4W bulbs need 96Watts of power.  So if you had a 12V transformer rated at 100W it would work.  The problem is safety factor, 18V transformer run at 12V etc..

So in your case these bulbs in parallel will consume 40 times .2amps or 8 amps.  So you need a transformer that can deliver 8 amps at 12V safely.  G

Does the instruction sheet say anything about the replacement bulb...what part number or current it draws...?

It's just that there are lots of different bulbs that draw different amounts of current and are meant for different voltage sources.

If I were to guess, assuming the bulbs are incandescent, I'd say that it takes maybe 100ma per bulb at 12vac. I measured a small grain of wheat type bulb and it took 100ma at 12 vac. So that's 4 amps at 12vac or 48 watts

GGG may be right and it takes double that...96 watts. Either way, a 100watt transformer (12vac at 8 amps) would do it.

Sorry, I should have gave you the part number right away on the lights which are MTH 30-1062. I figured that a formula would be like algebra and that you would be solving for x and y after you put the numbers in the right place. So far from what all of you are saying it seems that my ZW with 2 180W bricks should be more that enough. I did replace about 6 of the lamps with LED's but that became a real big hassle. I will contact MTH and see if they can give me the specs on the lights. Once they get back to me I will post it on this web. Thanks again everyone!

Paul,

yes, the formula is simple, it is Volts x Amps = Watts. Then, plugging in the values and doing some basic algebra, one can determine the theoretical or expected power usage.  That's why you need the bulb's rating.

One of the easiest ways is to actually measure the current usage with a clamp meter. You can get an inexpensive one at Harbor Freight that will suffice for train applications. It has a clamp that goes around the hot wire and measures actual current flow(amps). It's also a multi-meter, so good for checking voltages .The powerhouse bricks have about 10amps available.