I'm surprised this hasn't come up before but I did a search here and didn't find anything, so here goes.
I need to power a dozen 12VDC LED deck post lights, each drawing 1.6 watts. I want to place the transformer in a basement utility room where I have an outlet and run 20' of 12/2 stranded wire to the first light. From there, the rest of the lights are spaced out along 160' of the same wire. I know I need a transformer capable of providing at least 19.2 watts (12 lights x 1.6 watts) but I've seen some references to 300 watt transformers because of the wire length regardless of the number of lights. How do you calculate the current draw of the lights plus the resistance of the wire to determine the required transformer size?