A 12-V car battery is rated at 120 Ah. Assuming that the terminal voltage remains 12 V, for how long can this battery supply power at 29 W? How much charge flows through this battery in this time? How much energy is initially stored in the battery? I know that an Amp Hour is 3600 C I know that a Watt is a Joule / s I know that 1 J= 1C/V 3600 C * 120 So the initial energy stored in the battery is (3600*120) / 12V=36000 Joules 29 Watts=29 J/s So this battery is delivering 36000 Joules at a rate of 29 J/s 36000/29 = 1241 seconds. This is only about 20 minutes. This seems very short. I use 7Amp hour batteries at work. I've used them to power a light for a few hours. So my intuition tells me this answer can't be right. How much charge flows through the battery during this time? : I=Q/t = 432000 C / 1241 s = 348 Amps. But this assumes I got the seconds correct. Did I do this right?