What power supply to choose for the LED strip RGB 21 and 25 meters?

No transformer is needed, the tapes are powered by a DC power supply. And the controllers are those who sit on the OTC or release tickets on the tram.

It depends on which LED strip you buy - 12, 24, 36V. If 12V (the most common), then you need a DC power supply with an output voltage of 12V, and the current is 20-30% more than the current consumption of the tape. Judging by the length, you are going to do the illumination of the ceiling around the perimeter?

What is the problem of choosing a controller? You write in Google “RGB-controller 12V”, according to the current, again, consider how much the LED strip consumes.

It is believed so, see the power of the tape per meter, for example 14.4W / m (5050 60 pcs / m), which means that 1 meter LED strip consumes 1.2A.

With your lengths, the total power will be:

21 * 14.4 = 302.4W

25 * 14.4 = 360W

A current:

21 * 1.2 = 25.2A

25 * 1.2 = 30A

Or current per channel (The characteristics of the controllers often indicate not the total current, but the current per channel)

25.2 / 3 = 8.4A

30/3 = 10A

Consider the power of the power supply yourself, since you did not bother to tell how to mount it, no more details. Maybe you will power every 5 meters from a separate power supply unit, maybe one power supply unit per part of the tape (1 power supply unit for 21 and 1 power supply unit for 25 meters), maybe you generally want to supply everything from one unit.

I can only say that for a 21 meter power supply they think so

302.4 + 30% = 393W

And for 25 meters:

360 * + 30% = 468W

Sometimes the output current is indicated on the PSU, not power, then for 21 meters you need at least

393/12 = 33A

And for 25 meters:

468/12 = 39A

The controller, respectively, for the total current is equal to the same value, or divide by three and you will know the current per channel:

For 21 meters:

33/3 = 11A / channel

For 25 meters:

39/3 = 13A / channel

Let me remind you that it is impossible to connect the next 5-meter piece to the end of the 5-meter section of the tape, it is either connected to a separate power supply unit, or a separate cable is laid from the same power supply.