Radars have a heavy power draw compared to other
marine electronics. Especially true of older radars using CRT video displays. The other big draw is autopilots, and so most cruisers use wind-vanes when not running an engine.
I think your radar is probably OK--if you need to run it start your engine or have a gen-set handy. I always had a little
Honda to run my
electric fans and any
electronics if the batteries dropped to 12 volts or less.
Marine fans are expensive and noisy--so I used domestic
electric fans through a 12 DC to 240AC sine-wave
inverter. SO MUCH cheaper. Big inverters so one can use a vacuum cleaner or microwave
oven are far more expensive--but still an excellent idea and worth installing--but you need the good
battery bank plus back-up gen-set. I wired my AC outlets and my DC outlets and feeds in separate PVC ducts.
As for your batteries--you need to have a really good
battery bank on a yacht. Three hundred ampere hours is a good starting point--which equates to a couple of tractor batteries which should set you back a couple of hundred bucks and last about two years or longer. If you go for deep cycle betteries they will last twice as long or longer, but you will need a separate starting battery and a separate
windlass battery if you use one to raise your anchor--and the electronics to separate the
charging and load drain for each.