If I was planning to add any more electrical stuff in my airplane, the first question I'd ask is how much of a load am I adding and does my system have the capacity to handle it. I believe the 4313 says continuous load shouldn't exceed 80 % of the system capacity. So if it's a 60A alternator, continuous load shouldn't exceed 48A. Do you have a current/valid electrical load analysis for your airplane?
You don't see this topic discussed much, but it's something I'm aware of because I work as an Avionics Engineer for an airline, and it can matter. It took a bit of searching and asking around to discover that the FAA used the ASTM to develop new standards for putting an ELA for part 23 airplanes in 2006.
See ->
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0ahUKEwjZsNiY-KzOAhXpzIMKHeGcDI4QFggeMAA&url=https://www.astm.org/COMMIT/FR%20F2490_05.pdf&usg=AFQjCNFaIMxGORcTAFeROHmLwEwHMhjiKA&cad=rja
I did an ELA on my airplane because I was concerned that all the gear avionics added over the years (about 10A worth) might be too much for my old airplane. I suspect that most GA maintenance people filling out the 337's really don't address the topic with any precision. I personally recognize you cannot cheat the physical laws of nature with ignorance. So I bought the spec ($50.) and did an analysis for my airplane.
I've attached some pdfs so folks (in the interest of safety) can see how I treated the subject. Basically, there are electrical system loads that are drawing current all the time and some loads are intermittent, depending on how you operate your airplane. So I have a continuous loads total that's all the stuff that's always on. I also identify intermittent loads. Recognize for example that a radio draws more power to transmit than to receive. I didn't include intermittent loads in my total of continuous electrical load.
I did take it one step further to evaluate worst case loads for various phases of flight. There the intermittent load is added to my continuous load number, so I can see what my temporary draw is. I have columns for % continuous rating, % total capacity, and actual draw when I'm pulling more than 60A, because that power has to come from the battery.
The worst case loads don't really represent realistic operating numbers because, for example, the PTT and flaps are only operated for seconds at a time. Still, there are times where (on paper) I do temporarily draw power from my battery or risk a voltage drop, which is is a good argument for not ever operating with a dead battery.
Batteries have a finite capacity rated in Amp hours so you can roughly calculate how long it will hold up with a known load, if it's in good shape. Of course all batteries are load tested at annual so that's not a concern. Bottom line, your alternator/generator simply may not be able to produce enough power to cover worst case intermittent loads.
If I wasn't an EE and Avionics Engineer in my day job with an older plane chock full of electrical stuff, I probably wouldn't have done this. I do now see some value in newer more efficient lighting gear. My upper beacon draws 11A, I'd like to cut this in half. My ELA isn't finished and isn't 100% correct (for reasons I won't go into), it's only applicable for my airplane, and it doesn't account for my Aspen display backup battery. It does give me some idea how reasonable the electrical loads are on my airplane.