...unless there's a secondary locking mechanism for when the premises is empty. 

If a door just needs to be held closed to stop free access, then a 300kg mini-mag will suffice. 

But 300kg holding force is not enough to rely on for security purposes. If you're fitting the maglock to an external door with no other locking, or an internal door where it's required to keep assets secure, then 500kg minimum is recommended. 

If your maglock is on an external door and only designed to hold it closed when the building is occupied, then 300kg is enough. In this case, we recommend a secondary locking mechanism (like a mechanical lock and key) to secure the door at night or when the building is empty.