What is the electrical power draw of a car

Factual Questions
1

The alternator on my Jeep is making noise. Coincidentally the generator on my MG failed a couple of weeks ago. I probably drove over 100 miles with the MG’s ignition light dimly on, and it started right up the next morning so I could take it to the shop. What’s there to run? Power goes to the coil and routed through the (Pertronix electronic ignition) distributor to the plugs. Then there are the incandescent directional and brake indicator lights and the electric tachometer. How much power does it take to run an old style car?

The Jeep not only has the spark plugs and lights to power but also computers, cooling fan, console displays, etc. This is worrisome because I commute 100 miles each way four days a week. Right now the alternator is charging normally. It just has a noise like applying a bent paper clip to a small fan. But it’s definitely going to fail before long. How far can I get on just the battery?

(Notes: I originally thought it was my water pump, so I had it replaced Friday. I and the mechanic were wrong, but there was some play in the water pump pulley so it would have had to have been replaced someday anyway. The sound the alternator is making is not, according to the mechanic, typical of the sound a failing alternator makes.)

The title-GQ is this: How much electrical power does a typical modern, and typical small old, car draw from the battery(ies)?

2

You can probably drive an older car around for a few days with a dead alternator, as long as you don’t need to turn the headlights on. With a newer car, you’ll probably only get an hour or two at best out of it before the battery croaks.

If the alternator completely went, your MG would probably make it the 100 miles home then back to the garage for repair with no problem. I wouldn’t try it with the Jeep, though. I’d get the Jeep to a repair shop ASAP.

The Jeep will probably cycle the cooling fans on and off as needed to maintain the proper engine temp, so it will draw more current on a hot day than a cold day.

3

The Jeep has a new alternator now.

Any idea of the amperage the different cars draw?

4

Johnny A old points type ignition car like your B draws between 5-10 Amps with the engine running. The draw varies with RPM.
As far as your Jeep goes, I am not sure. But on my car it pulls 15A steady with the key on. Start the engine and that goes up to about 20-30amps.