What is the most efficient way to connect 20 Lamps for my backyard?
... there is a serious and a parallel way of connecting to an electrical source. - Sandra.





  Wire them in parallel, seriously. – Transistor.


I was being a little naughty with my comment. You meant "series and a parallel way", not "serious". I couldn't resist. 

If they are 110 V lamps and you have a 110 V supply then they have to be wired in parallel. That means you need a cable with live, neutral and, if the lamps require it, an earth. The cable can loop from fitting to fitting so that they appear to be daisy chained but the wiring is in parallel.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Daisy-chained parallel lamp wiring.