Most packs like this have a 12V "18 amp-hour" battery. You say another has 40 amp-hour? That sounds quite a bit bigger and would likely be heavier (like 25+ pounds versus ~18 pounds for this size), but is it still 12V? I hope that's what it really is so we are comparing similar things. I've seen some things advertised with "300 watt-hours" where 300 sounds much bigger, but converting that to amp-hours it's (300/12=25amp-hours), so not much more than 18. Watch out for such things where they want you to think it's better with a much bigger number, but it may not be.