amp total power

How do they rate the power from an amp? To mean it seems misleading?

in what respect?  they usually give an RMS value is is fairly honest, and then the maximum power at 90% THD which is useless...

i saw a (well known brand) amp come through here the other day that boasted 1000 watts on the heat sink, yet was fused with a 30 amp fuse, so i looked up the specs in the service manual...it was 300 watts RMS.... maybe i'm just old ( i am ) but i remember when amplifier companies gave you honest ratings based on a reasonable THD level, like less than 1%.....

Well, actually it IS INTENDED to be misleading, especially in lower-end and mass-market consumer products.  As soon as marketing people figured out that people will buy audio products based on the listed power output, much of the truth in ratings went out the window.  Especially in car audio (but also in home gear and in computer audio) there is a false perception that "more watts = better."  So many manufacturers 'inflate' their ratings.  It is sad.

Not that I think manufacturers lie; undoubtedly the gear will do what they say.  But not necessarily in ways that are listenable.  I have tested car amps listed as "1000 watts" that do, indeed, produce peak output of 1000 watts.  But this is achieved at 20% THD and with severe 50% clipping.  This amp's max output before clipping was a whopping 64 watts!  So did the manufacturer lie?  No, the amp could produce 1000 watts without blowing the internal fuses.  Was the rating misleading?  Yes, very.

Look for ratings at 12 Volts, and less than 1% (0.1% is better) THD into 4 ohms (or higher) and they MIGHT be accurate and reflect what you can expect to get out of the amp.  Ignore "peak" or "music power" ratings.

In general, "Dynamic" is a code word for "peak."  AKA "Peak Music Power" or "Dynamic Headroom."  All made-up words for peak instantaneous (<10ms) output..

RMS refers to voltage and current, by the way.  Power is power.  The so-called RMS ratings are the number of watts the amplifier can sustain based on maximum RMS voltage and current capability.  But it is the best indication of true amplifier output... assuming the distortion rating is also suitably low.  I know of at least one poor-quality manufacturer (think fake German-sounding name) who rate their amps as "RMS Watts" but do not list distortion numbers.  When the amp was tested, it produced the rated power but at 15% distortion!  Recipe for burned voice coils....