Confussing Loading Computation

Hi Everyone , i have this simple setup of mine and i want to know how can i compute the total loading of this setup:

the siren is rated 12V 15W and i connected it with a 12V 1A Adapter and it works just fine .

i know that P=IV then I=P/V

but im confused if the siren is really consuming 15W knowing that the adapter is only generating 12V x 1A =12Watts, right ?

and from the rating given, 12V 15W, so here the current must be 15/12 = 1.25A , but the adapter is giving only 1A,

how can i compute for the correct wattage and current consumed by the siren when connected with a 12v 1A adapter? tnx!

Just because the power supply is rated for 1 A doesn't mean it can only produce one amp. It may well be able to produce more. It is not recommended though that you run it at a higher current than it's rated for.

In order to figure out the actual wattage, you'll need to measure the voltage an current while the siren is hooked up.

To get the exact wattage you need to measure the exact current and voltage involved.

There is another factor involved. Duty Cycle.

The power supply may be rated 12V 1A continuous, but for short periods it could probably cope with 15W (1¼ Amps) perfectly well.

How long - and how often - do you wish to sound the siren?

Also the siren may work on less current and/or voltage.

Power ratings are often somewhat conservative, so that things work without borderline behaviour. You've just set yourself up for borderline behaviour (PSU overheats slowly, malfunctions, etc etc).

It may be manufacturing spread - your particular siren is less greedy for current...

Without measuring anything its guesswork..