how to figure alternator

How do I figure out the power on a new alternator"the hotness"

Place a thermometer on the top of its case. Assume a 1% thermal transference factor less 20% inefficiency (ie, k = 0.008) and using eV (electronVolt) as Planks Constant time Coulomb charge (ie, for say 114.2V) and 2/3rds light velocity caclulate k2 & then divide by the pulley ratio time engine output (Watts obviously; not HP). If you'd like a better or even somewhat sensical & realistic answer, please explain what you mean. Its max current time voltage (14.2V etc), or what power you need, etc. Max alternator output is specific to each alternator and its max output capability is dependent on its RPM which should be found in its specs.

WOW how bout this I have a 2500(two of them), 1000, and 400 watt amps all Rockford fosgate and a second battery (yellow top). after market HID light and a remote start alarm. all in my 07 chevy monte carlo SS. how do I find out what size alternator I need?"the hotness"

Buy the biggest one you can afford. You can never have too much power.

Add all the loads for max power - eg 6400W of amps; say 100W HIDs, 100W ECU etc; then wipers, stop, aircon etc. So say 6600W => 2000/12 = 470A. So if you don't want voltage dips below ~14V with all amps at full blast etc you'll need an alternator (or a few) that put out 470A at whatever lowest RPM you want that dip NOT to occur on. Possibly more practical is a 250A alternator (or larger if you can) with adequate battery capacity - probably you usual cranker plus at least 1 AGM for the audio. Being above a few kW for audio, I'd suggest a cap to extend AGM life, or better still an extra AGM or 2. The final system depends on what current you actually draw & what your average driving & charging profile is (presumably enough to recharge all batteries else you'll need external chargers). Of course a battery isolator for cranking independence and safety, else individual battery monitoring, and some form of long term battery isolation.

PS - there are lots of posting re this type of sizing and issue. I have certainly written heaps - maybe more om mp3car.com than here. I try to emphasise how important it is define what is wanted - eg, full output at engine idle without voltage dips (ie, still 14-14.8V etc) or long life & reliable batteries without external charging and voltage dips to 12.6V or lower. My only change over the years has been (1) that (some) alternators can be paralleled, and (2) capacitors have a use - namely AGM protection in BIG systems larger than a few kw. Otherwise things are much the same - cheap & simple battery isolators; matched batteries only if permanently paralleled; etc.