How many peak amps do I actually need?

Reviewers across the sources we read repeatedly warn that 'peak amps' has no industry standard and is largely a marketing number. Cranking amps and cold-cranking amps are the load-bearing specs. For a typical 4- to 8-cylinder gas engine, 400–500 cranking amps is plenty; for diesels or 24V commercial equipment, look at units rated specifically for those engines.