Not accurate at all..... that doesn't take amplifier efficiency or inefficiency as the case is with cheap brands, in to account.
Multiplying fuse size (max amperage draw) by voltage gives the most power the amp could possibly produce. Actual output will always be less. How much less is unknown.
Now, if an amp has two 30 amp fuses and claims 4000w *cough*quantum audio*cough* then yea, you know its BS.
Again... Plenty accurate for the flea market brands
Efficiency?? C'mon MS. We're looking for a rough estimate here. That's why I said 10-12. It is a given that we're talking about cheap amps. It SHOULD be a given that any cheap class a/b is going to have roughly the same efficiency.
the problem with using fuse ratings is that the fuse rating isn't a brick wall current measurement. To keep this simple, a fuse rating is how long the fuse can last at that particular current throughput, for a specified duration. say, ten seconds. As the current increases, the duration decreases, so say the fuse is a 20A fuse (at 10 seconds) then at 30A, the fuse may last 5 seconds. At 40A it may last 2 seconds.. and so forth. It deosn't mean the fuse trips and blows instantly at 20A.
Most amps use fuse ratings that will sustain current for brief periods beyond the amp's max output so it doesn't pop fuses at brief transient peaks in current draw.. but during a short, the fuses will pop pretty much instantly.