Speaking from personal experience and what I have also seen firsthand, those with vehicles with bigger engines (V8) will find that in very cold climates, these jump packs will not cold start these engines with any sure reliability. This means a boost from another vehicle with good jumper cables or an AC powered battery charger that also has engine start / boost capability.

As for the comments up thread about generator not starting. Like any preparedness item, generators need to be checked on a regular basis.

Also stay away from ethanol blend gas and also use a good gas stabilizer supplement to keep the gas from gumming and varnishing. For my power equipment, lawnmower, weed eater, boat motors, generators, I use the above non Ethanol gas and Stabilizer and never have any trouble starting any of these motors.