Relays are properly rated for voltage range and amps (current), rather than wattage (watts being power consumed, the formula for that being Watts = volts X current).
On our side by sides, IMO, it's good to use a relay if the current for an accessory is more than 8 amps or so. Relays are easier on the rocker switches which, though rated at 20 amps, that's for sure an exaggeration looking at the internals. A relay also allows thinner control wires going to the switches, and a shorter "power" run of heavier wire suitable for higher current loads.
If the light bar you are considering is rated at 300 watts, that would be 25 amps at 12 volts. BUT, the mfgs lie about their wattage rating, sort of a ongoing advertising war. Your light bar is probably going to draw more like 15 - 17 amps (based on my experience buying a 312 watt Nilight 54" light bar. Still, very bright, but only draws 16.5 amps or so with the 13.3 lithium battery I was using as a test source.* (No doubt the light bar has an internal voltage regulator as mine is spec'd at 12 to 30 volts DC, so it'll probably draw less current at the upper end of it's allowable voltage range. But for our purposes, figure a couple of hundred watts.)
*I recommend bench testing any LED and similar devices before going to the trouble of installation. That way you can return with minimal hassle if defective out of the box.
