calculating power

hey guys im running a mtx 1501 d and its pushing my Audiobahn alum 10 Q sub which is running at 3 ohms. the specs on the amp say it runs 500 watts @ 4ohms and 1000 watts @ 2 ohms rms 12.5v dc, im wondering how would i calculate the power at 3 ohms..

also, i upgraded my alternator and man its sweet but im still blinkin the lights when the bass hits. should i go ahead and put an extra optima battery or just stick a cap on it?, ( if anyone has pics of an extra battery inside an extended cab s10 let me know)

Use the Ohm's Law formula P = E squared / R  https://www.the12volt.com/ohm/page2.asp

With a DMM, measure the AC output voltage at the speaker terminals, square that number, then divide that by the ohm load that the amp sees from the speaker(s). That will give the RMS going to that speaker load.