Oh, for outdoor use on large items like CARS, a 1600 watt-second, 4-outlet power pack is the place to start. If you want to overpower the sun, an M11 head with an 11.5 inch reflector and only about 400 watt-seconds of flash power will overpower bright, August sunlight with a southern exposure and a large house providing fill,at a distance of about 15 feet. So yeah, for shooting automobiles, 1600 watt-seconds of power is a great amount to begin with....at closer distances you will need depth of field,and you can simply mount the power pack on the bottom of a wheeled hand truck, and bolt the battery/since wave inverter above it. With the 27 pound power pack as ballast, the hand truck will be very stable, and you can mount a couple of simple round "rings" on the hand truck, and then put a monopod "riser", and have a 27 pound + 18 pound Tronix Explorer sine wave inverter as counterweight to a 13 foot telescoping light stand that will mount an M11 and allow you to bring your own sun with you, on wheels. The pack + inverter will give you roughly 45 pounds of weight to counter balance the extension of a light stand shaft--enough to make wind not much of a factor.
YES, for late evening, a 4-outlet 1604 pack would be wonderful for cars. Plenty of power. As I suggest, mount it on a wheeled hand truck,and you've got yourself a system where you can pull it by hand,and have the pack and the inverter be its own ballast and counterweight AND form the anchor point for a telescoping light stand. I hope my idea is clear--it's a hand truck with a light stand's shaft mounted to it, with power pack and inverter as ballast. And yes, compared to Alien Bees, ONE M11 with a full 1600 watt-seconds channeled through it is a LOT of flash power, for only $500.
Pack and head systems are far cheaper when you need ONE very powerful head, but also the option to have say two,or three, or four lesser-powered heads. The D1604 is a huge value for large objects like cars or night-time,light-up-the dark work.