Hi again,
My overly simplistic "Ohm's Law" answer was designed to stimulate curiosity in it prompting a bit of study. Then you guys HAD to muddy the waters so here comes MY dose of the river bottom, headroom. Now let's say you're the average 6' tall man and you walk into a room with a 6' ceiling, you bump your head on the chandolier, right? That's why the standard is 8' which gives you 3' of headroom.
This must also be considered for the power supply. OK, you total up the load but it doesn't end there, the supply needs headroom so it doesn't strain to provide the proper voltage at full load and not develop excessive heat. Yeah, they rarely provide adequate cooling as it is. Most hams use 13.8VDC regulated supplies rated at 20A continuous and 30A intermittent which gives 100% headroom, the supply being able to deliver 200% of what the load requires. The supply then loafs along, provides more than adequate regulation and barely gets warm, especially since roughly 90% of the load is intermittent.
Please note that we don't consider "watts" because that is confusing, the voltage is understood so amps is the only thought we give to the matter. I have yet to see "watts" on the ratings plate, but some "wall warts" and other related DC supplies may have the AC INPUT rated in watts. I guess they want us to think in terms of the light bulb or something. (;->)
Now if I REALLY wanted to screw you up I would explain why "watts" have nothing to do with AC circuits being it's a DC measurement. The AC measurement is volt-ampere because VAR (volt-ampere reactive) is the AC equivelent of Ohm's Law and all that stuff about efficiency becomes power factor. I have only tantalized you, the next lesson is in polyphase power systems and vector calculations.
"You enter these halls with a mind full of MUSH, and leave thinking like electrical engineers."
Now go chase an electron. (;->)
