Hi Stefan , Raymo's explanation summons it all up and your maths is correct.
I believe you would be better off with with the SCA 1900, it is rated at 38 Amp/Hr same as the Jaycar battery, so should last almost as long maybe a bit less as its a lead acid in the sca, as Raymo explained.
Also the battery wont come with any cables / jacks or outlets and you will need a charger for it.
Plus you should have enough juice to jump your car at the end of the night if required
