How do you calculate DC voltage?

I have several lights that operate on AAA batteries. I want to stop using batteries and plug in a DC power supply to power these. Each light uses 3xAAA batteries so that’s 4.5v. These are not equipped to be operated by a power supply, so I'd have to solder wires to battery terminals. 

if I connect these in a way that 2 wires come out of the power supply and distributes to each light and assuming I have 10 lights in total, should my power supply output 4.5v or 45v?

A link to a rough diagram of how the wiring would be below. Note that individual lights can and most likely will connect in different parts of the main wire coming out of the power supply.

https://i.imgur.com/7taFLUW.png

Appreciate all your responses!